7 Conversion and Composition Profiles in Polyurethane Reaction Molding
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M A T T H E W T I R R E L L , LY JAMES LEE, and C H R I S T O P H E R W .
MACOSKO
Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455
F a b r i c a t i o n o f most a r t i c l e s from polymeric m a t e r i a l s has been done by melt forming o f thermoplastic m a t e r i a l s . Recently, technology has been developed f o r r a p i d in s i t u p o l y m e r i z a t i o n , to form the d e s i r e d articles d i r e c t l y from monomeric l i q u i d s . This process has come t o be known as Reaction I n j e c t i o n Molding (RIM).(1) To date, the major commercial RIM-processed m a t e r i a l s are polyurethanes. The reasons f o r t h i s are that polyurethane chemistry i s able t o provide both 1) f a s t , complete r e a c t i o n with no s i d e products, necessary t o minimize mold c y c l e time and 2) a wide degree o f modulus variability, through the domain -forming p r o p e r t i e s o f segmented polyurethanes and the i n t r o duction o f some c r o s s l i n k i n g , necessary t o give the d e s i r e d mechanical p r o p e r t i e s . Development o f s i m i l a r d e s i r a b l e chara c t e r i s t i c s i n epoxy, s i l i c o n e , p o l y e s t e r , nylon (and perhaps other) polymerizations i s c u r r e n t l y a very a c t i v e field ( 2 ) . P r i n c i p a l a p p l i c a t i o n o f t h i s technology has been i n the automotive i n d u s t r y . For example, polyurethane automotive f a c i a as large as f i f t e e n pounds are p r e s e n t l y being r e a c t i o n i n j e c t i o n molded i n a s i n g l e shot i n l e s s than one minute from l i q u i d components. Nearly fifty m i l l i o n pounds o f polyurethane p a r t s will be produced i n the US by RIM i n 1978.(3) The low temperature and pressure requirements o f RIM lead t o economic advantages o f lower c a p i t a l equipment and energy c o s t s , r e l a t i v e to thermoplastic i n j e c t i o n molding and metal forming. The lower d e n s i t y o f polymers in general, or, more e x a c t l y , t h e i r high s p e c i f i c strength, provides an a d d i t i o n a l energy conservation motivation f o r t r a n s p o r t a t i o n a p p l i c a t i o n o f more polymeric materials.(2) Vital to the continued growth o f RIM p r o c e s s i n g is a b a s i c understanding o f how the process i n f l u e n c e s the s t r u c t u r e and p r o p e r t i e s o f the polymer formed. Fusion o f the skills o f p o l y m e r i z a t i o n r e a c t o r designer and i n j e c t i o n molding designer is required. In t h i s paper, we focus p r i m a r i l y on the polymerizat i o n engineering aspects although n a t u r a l l y the two cannot be e n t i r e l y d i v o r c e d . More s p e c i f i c a l l y , we d e s c r i b e here e x p e r i -
0-8412-0506-x/79/47-104-149$07.75/0
©
1979 American Chemical Society
Henderson and Bouton; Polymerization Reactors and Processes ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
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ments and modelling e f f o r t s aimed at e l u c i d a t i n g the e f f e c t s o f nonuniform s p a t i a l d i s t r i b u t i o n s o f temperature and conversion during l i n e a r polyurethane RIM p o l y m e r i z a t i o n on the molecular weight, molecular weight d i s t r i b u t i o n sequence d i s t r i b u t i o n , and u l t i m a t e l y , morphology and mechanical p r o p e r t i e s o f RIM produced polyurethane.
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B r i e f D e s c r i p t i o n o f RIM Process and Reactant
Chemistry
A b r i e f d e s c r i p t i o n o f the a c t u a l p h y s i c a l c o n d i t i o n s o f the RIM process i s i n order here. Figure 1 shows the key e l e ments o f the RIM p o l y m e r i z a t i o n process s c h e m a t i c a l l y . There are two r e a c t a n t r e s e r v o i r s . One contains a d i i s o c y a n a t e (comm e r c i a l l y o f t e n 4,4-diphenylmethanediisocyanate, MDI) and the other a mixture o f p o l y o l s (one low molecular weight short d i o l , and one, more f l e x i b l e m a c r o d i o l ) . We are thus d e a l i n g with a step-growth c o p o l y m e r i z a t i o n . ( T r i f u n c t i o n a l p o l y o l s are a l s o o f t e n used.) Reactants are metered i n , i n the exact s t o i c h i o metric r a t i o necessary to achieve high molecular weight, from each r e s e r v o i r with a s i n g l e stroke o f the d r i v e c y l i n d e r . An e f f e c t i v e bench top l a b o r a t o r y RIM machine, developed at the U n i v e r s i t y o f Minnesota, ( i ) which u t i l i z e s a pneumatically d r i v e n c y l i n d e r and a movable l e v e r arm f o r s t o i c h i o m e t r y c o n t r o l i s shown i n F i g u r e 2. The two r e a c t a n t streams impinge on one another i n the mixing head, flow through the runner and f i l l the mold i n about two seconds. When the urethane p o l y m e r i z a t i o n i s c a t a l y z e d f o r example by d i b u t y l t i n d i l a u r a t e , the r e a c t i o n i s very f a s t , begins at the moment o f impingement, proceeds somewhat i n the runner, reaches a very high conversion i n the mold and s o l i d i f i e s i n as l i t t l e as ten seconds. The p a r t i s then e j e c t e d from the mold and the mold r e s e a l e d . T h i s completes one c y c l e from the p o i n t o f view o f the machine. Reaction i s u s u a l l y incomplete i n t h i s s o l i d s t a t e and i s d r i v e n to completion by a post-RIM c u r i n g . Of course, the degree o f conversion which has been achieved i n the time o f one c y c l e d i c t a t e s the molecular weight o f the polyurethane formed i n t h i s step growth copolymerization.(5) Moreover, since the urethane-forming r e a c t i o n i s h i g h l y exothermic and RIM-formed p a r t s o f t e n have s u b s t a n t i a l t h i c k n e s s and low thermal c o n d u c t i v i t y , a nonuniform temperature p r o f i l e develops across the p a r t d u r i n g polymerization. Therefore, at any s p e c i f i e d r e a c t i o n time, a higher conv e r s i o n (and t h e r e f o r e higher molecular weight polymer) w i l l be achieved i n the higher temperature regions o f the p o l y m e r i z i n g mixture. Our modelling and experimental e f f o r t s , (to be des c r i b e d below), have provided a q u a n t i t a t i v e b a s i s f o r these q u a l i t a t i v e statements. There i s however another i n f l u e n c e on the course o f urethane RIM p o l y m e r i z a t i o n which i s perhaps not so obvious. I t r e s u l t s from the chemistry and s t r u c t u r e o f the r e a c t a n t s themselves. The s t r u c t u r e s o f the r e a c t a n t s used i n t h i s work are
Henderson and Bouton; Polymerization Reactors and Processes ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
Downloaded by UNIV OF MISSOURI COLUMBIA on April 15, 2018 | https://pubs.acs.org Publication Date: July 31, 1979 | doi: 10.1021/bk-1979-0104.ch007
TERRELL ET AL.
Polyurethane
Figure
Figure
2.
Photograph
1.
Reaction
Schematic
of University
Molding
of RIM
of Minnesota
machine
Laboratory
RIM
machine
Henderson and Bouton; Polymerization Reactors and Processes ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
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shown i n F i g u r e 3. These are t y p i c a l , as noted above, o f those used i n commerical RIM f o r m u l a t i o n s . They copolymerize to form sequence l e n g t h d i s t r i b u t i o n s o f AABB u n i t s (hard segments) and AACC u n i t s ( s o f t segments). I t has long been recognized t h a t , i n the s o l i d s t a t e , such a segmented copolyurethane w i l l form a phase separated s t r u c t u r e , where the hard segments o f sequence length separate to form s e m i - c r y s t a l l i n e domains i n a l e s s c r y s t a l l i n e matrix o f s o f t segment and short hard segment m a t e r i a l (6). T h i s type o f s t r u c t u r e i s r e s p o n s i b l e f o r the d e s i r a b l e mechanical p r o p e r t i e s o f these m a t e r i a l s . Schnieder and coworkers (7) have proposed that the o v e r a l l o r g a n i z a t i o n o f the s o l i d - s t a t e o f a segmented polyurethane may be s p h e r u l i t i c as shown i n Figure 4. Very l i t t l e i s known, on the other hand, about the "morphology" o f the p o l y m e r i z i n g mixtures, or f o r that matter, how the r e a c t o r c o n d i t i o n s i n f l u e n c e the s t r u c t u r e and morphology of urethane polymers. I t i s known from the r e l a t i v e l y few g e l permeation chromatography s t u d i e s o f polyurethanes that have been done t h a t , while the p o l y d i s p e r s i t i e s o f MWD o f p o l y u r e thanes formed i n s o l u t i o n are very c l o s e to 2(8), c h a r a c t e r i s t i c o f the geometric d i s t r i b u t i o n , bulk p o l y m e r i z a t i o n s g i v e products with very broad, o f t e n bimodal, MWD o f p o l y d i s p e r s i t y i n the range o f 6 to 20.(9) We here present evidence t h a t , i n f a c t , phase s e p a r a t i o n occurs at a r e l a t i v e l y e a r l y stage i n the p o l y m e r i z a t i o n and exerts a profound i n f l u e n c e on the course o f the subsequent p o l y m e r i z a t i o n , the molecular weight d i s t r i b u t i o n (and morphology) o f the polymer formed and u l t i m a t e l y the mechanical p r o p e r t i e s o f product formed. Temperature P r o f i l e s During RIM P o l y m e r i z a t i o n A n a l y s i s and Experiment f o r a Nonlinear P o l y m e r i z a t i o n The heat t r a n s f e r problem which must be solved i n order to c a l c u l a t e the temperature p r o f i l e s has been posed by Lee and Macosko(lO) as a coupled unsteady s t a t e heat conduction problem i n the a d j o i n i n g domains o f the r e a c t i o n mixture and o f the nonadiabatic, nonisothermal mold w a l l . Figure 5 shows the geometry o f i n t e r e s t . The f o l l o w i n g assumptions were made: 1) no flow i n the r e a c t i o n mixture ( t y p i c a l molds f i l l i n
