Urethane Chemistry and Applications - ACS Publications - American

37 The Role of Release Agents in Urethane Molding DAVID B. COX Downloaded by CALIFORNIA INST OF TECHNOLOGY on January 26, 2017 | http://pubs.acs.org Publication Date: November 30, 1981 | doi: 10.1021/bk-1981-0172.ch037

Chem-Trend Incorporated, 3205 East Grand River, Howell, MI 48843

Background. The process of molding urethane products requires the use of a mold release agent in order to prevent the adhesive bonding of isocyanates to the mold. Without a release agent, the reaction of isocyanate with active hydrogens on the mold surface would produce such strong chemical bonds that the molded part could not be removed intact. This is reason enough for considering release agents an important element of molded urethane production, but release alone is usually not enough. The release agent must often produce, or permit production of, a desired surface. The release agent must also not introduce (ideally) other problems, such as excessive buildup on the mold or problems with painting or glueing the part. The purpose of this paper is to present a brief review of the state of the art in urethane mold releases, with some comments on the chemical and physical nature of successful products, and some further comments on future possibilities. S c i e n t i f i c p u b l i c a t i o n s on the t o p i c o f urethane mold r e l e a s e are n e a r l y n o n e x i s t e n t . There i s no coherent body of b a s i c research p u b l i c a t i o n . The patent l i t e r a t u r e i s i l l u s t r a t e d by a few e n t r i e s i n a review of r e l e a s e agents published i n 1972 (1). The f a c t that a r e l e a s e agent i s needed f o r molding urethanes i s a problem, i f only because production would be s i m p l e r without the need. Once one accepts the r e a l i t y of the need, one can a r r i v e a t four options f o r p r o v i d i n g r e l e a s e . These are: 1) a permanent r e l e a s e c o a t i n g on the mold, 2) an i n t e r n a l r e l e a s e agent added t o the r e a c t i o n mixture, 3) a comb i n a t i o n o f a permanent base coat w i t h an o c c a s i o n a l l y renewable top coat, and 4) a renewable r e l e a s e coat a p p l i e d every c y c l e or every few c y c l e s . The options a r e l i s t e d i n order o f d e s i r a b i l i t y from a production p o i n t of view. Option 4 i s i n f a c t what n e a r l y everyone uses, by n e c e s s i t y . Once Option 4 i s accepted, i t i s r e a s s u r i n g t o know that e x c e l l e n t r e l e a s e agents are a v a i l a b l e which can provide smooth, automatic p r o d u c t i o n . On the other hand, a c h i e v i n g that smoothness, and keeping i t , 0097-615 6/ 81 / 0172-05 65$05.00/0 © 1981 American Chemical Society

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

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Downloaded by CALIFORNIA INST OF TECHNOLOGY on January 26, 2017 | http://pubs.acs.org Publication Date: November 30, 1981 | doi: 10.1021/bk-1981-0172.ch037

r e q u i r e s a c a r e f u l matching of production v a r i a b l e s to r e l e a s e agent formula. These g e n e r a l i t i e s w i l l be i l l u s t r a t e d through a d i s c u s s i o n of the r e l e a s e agent technology i n two d i f f e r e n t and important types of molded urethane product. These are automotive seat foams and RIM urethanes. Before that d i s c u s s i o n , however, i t seems u s e f u l to d i s c u s s why Options 1, 2, and 3 f o r r e l e a s i n g urethanes have not been s u c c e s s f u l so f a r . Permanent Coatings. The usual f i r s t candidates one t h i n k s of are m a t e r i a l s w i t h low s u r f a c e energy, such as p e r f l u o r o polymers or s i l i c o n e s . In f a c t , these m a t e r i a l s are normally s u c c e s s f u l a t f i r s t t r y . Even sheet polyethylene may be s u c c e s s f u l at f i r s t . The l i m i t a t i o n appears when a l a r g e number of m u l t i p l e r e l e a s e s i s r e q u i r e d . I f one wishes to use a mold f o r making a thousand p a r t s or more, one f i n d s that the "permanent, low-energy surfaces begin to f a i l i n r e l e a s e , u s u a l l y long before one thousand p a r t s have been molded. How soon the f a i l u r e occurs depends on the urethane type, the comp o s i t i o n of the s u r f a c e , and the manner i n which i t was coated on the mold, but the mechanism i n v o l v e s a gradual a l t e r a t i o n of the o r i g i n a l s u r f a c e . Spotwise adherence of the urethane begins, probably due to d e p o s i t i o n of urethane components i n microscopic depressions i n the s u r f a c e . Subsequent contact w i t h f r e s h urethane allows chemical bonding to the deposited m a t e r i a l , and the area tends to grow r a p i d l y . 11

I n t e r n a l Release Agents. I n c o r p o r a t i o n of a r e l e a s e agent i n the urethane mix would at l e a s t remove the need f o r a separate pre-molding process s t e p . Once a g a i n , i t can be shown that i n t e r n a l r e l e a s e agents do work to some e x t e n t . The drawbacks, n e v e r t h e l e s s , are severe enough to outweigh the advantages. The i n t e r n a l r e l e a s e must be mixed i n t o the urethane, but i t can only a c t at the s u r f a c e of the p a r t , so the mass that must be used i s p a r t l y wasted unless i t can e i t h e r migrate r a p i d l y to the surface at the r i g h t time or e l s e c o n t r i b u t e some other usef u l property to the urethane. At the l e a s t , the i n t e r n a l r e l e a s e should not make the bulk p r o p e r t i e s of the urethane p a r t l e s s s a t i s f a c t o r y . In p r a c t i c e , so f a r , i n t e r n a l r e l e a s e agents have t y p i c a l l y been found to f a i l a f t e r the molding of ten or twenty p a r t s because they cannot p r o t e c t the mold surface against urethane buildup d e p o s i t s . They a l s o may continue to migrate to the surface a f t e r the p a r t i s made, causing problems w i t h p a i n t i n g or surface appearance. The b a s i c i d e a i s a t t r a c t i v e enough that there w i l l be c o n t i n u i n g pressure to f i n d a way around the problems, but f o r the immediate f u t u r e , use of i n t e r n a l r e l e a s e agents i s l i k e l y to be q u i t e l i m i t e d . Permanent Base Coat and Renewable Top Coat. This concept has an advantage (over renewing the r e l e a s e coat each c y c l e )


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only when the top coat i s r e q u i r e d a t i n f r e q u e n t i n t e r v a l s . Therefore, the problems tend to be l i k e those of the simple permanent coat o p t i o n . Nevertheless, some experimentation i s going on w i t h t h i s concept. With c a r e f u l matching of p r o d u c t i o n c y c l e needs and r e l e a s e agent p r o p e r t i e s , there i s some promise to the concept.


D i s c u s s i o n of Current

Technology

Automobile Seat Foams. Let us s t a r t by c o n s i d e r i n g how auto seats are manufactured, which w i l l d e f i n e the performance requirements f o r the mold r e l e a s e agent used. The t y p i c a l prod u c t i o n machine i s a c a r o u s e l w i t h a l a r g e number of molds being moved through a c y c l e w i t h the f o l l o w i n g steps: spraying and d r y i n g of mold r e l e a s e , pouring of urethane, c u r i n g o f the foam, and demolding. This c y c l e i s o v e r - s i m p l i f i e d but contains the major steps from the r e l e a s e agent p o i n t of view. The b a s i c requirements f o r the mold r e l e a s e agent are that i t be i n a form f o r easy a p p l i c a t i o n to the mold i n the c o r r e c t amount ( u s u a l l y sprayed), that i t dry to form a f i l m before the time of urethane pour, that i t r e s i s t p e n e t r a t i o n of the urethane to the mold s u r f a c e , and that i t provide easy and complete r e l e a s e at the time of demold. There are u s u a l l y a d d i t i o n a l demands made, which i n c l u d e production of a d e s i r e d s u r f a c e . Some companies p r e f e r a closed or " s k i n " s u r f a c e . Others r e q u i r e an open c e l l s u r f a c e . At the l e a s t , the surface should be f r e e of c o l l a p s e d foam areas. Other secondary, but important, c o n s i d e r a t i o n s are e f f e c t s of the evaporating s o l v e n t on a i r q u a l i t y and f i r e hazard. Buildup of e i t h e r mold r e l e a s e agent or urethane on the mold i s a problem i n that i t r e q u i r e s stopping production f o r c l e a n i n g , so minimum b u i l d u p i s d e s i r a b l e . On the other hand, the e f f e c t of buildup on the dimensions or cosmetics of the p a r t surface may not be critical. H i s t o r i c a l l y , the s u c c e s s f u l mold r e l e a s e agents used f o r seat foam have been based on waxes. The f i r s t seat foams were s o - c a l l e d one-shot, or "hot" foams. The term "hot" u s u a l l y meant a cure temperature between 200°F and 300°F. This i n t u r n meant that the mold s u r f a c e temperature at demold was both hot enough to melt most waxes, and that i t was a l s o hot enough to evaporate water. Therefore, i t was p o s s i b l e to use a r e l e a s e agent a p p l i e d w i t h e i t h e r a high f l a s h p o i n t m i n e r a l s p i r i t s or w i t h water as a c a r r i e r . I t i s p o s s i b l e to make d i s p e r s i o n s of waxes i n m i n e r a l s p i r i t s by d i s s o l v i n g the wax mixture i n hot s o l v e n t and c o o l i n g i n a manner to p r e c i p i t a t e the waxes i n a p a r t i c l e s i z e range of about one micron average. Such d i s p e r sions can be sprayed to produce a t h i n , continuous f i l m when d r i e d . In order to use water as a c a r r i e r s o l v e n t , i t i s necessary to emulsify the wax mixture i n the melted s t a t e , and then c o o l the emulsion to produce a d i s p e r s i o n of s o l i d wax p a r t i c l e s . Once again, these can be produced i n a p a r t i c l e s i z e


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range o f one micron or s m a l l e r , so that a t h i n sprayed f i l m can be d e p o s i t e d . In the 1970 s, there had been a s t r o n g trend toward use o f High R e s i l i e n t (HR) foams, which are cured a t a lower temperature than hot foam. The o r i g i n a l attempts were c a l l e d " c o l d - c u r e " foams, but f o r v a r i o u s reasons, the s u r v i v i n g foam systems have come to have a range of cure temperatures from about 110°F to 170°F. The main s i g n i f i c a n c e o f t h i s drop i n temperature from the hot foam range i s that use of water-dispersed r e l e a s e agents was not p r a c t i c a l a t f i r s t . I n f a c t , i n some cases, high f l a s h m i n e r a l s p i r i t s have been replaced w i t h lower f l a s h types, which i s more hazardous, or by c h l o r i n a t e d s o l v e n t s , which i s more expensive and which causes some odor problem. The p o s s i b i l i t y for use of water d i s p e r s i o n s i s g e t t i n g renewed a t t e n t i o n because of the r i s i n g cost of petroleum-based s o l v e n t and because o f a i r p o l l u t i o n c o n t r o l laws. There i s no doubt that water-dispersed r e l e a s e agents can be s u c c e s s f u l l y developed and used, but t h e i r use may r e q u i r e some s p e c i a l e f f o r t s to handle the problem of e v a p o r a t i o n o f water from the sprayed f i l m a t system temperatures. That i s , foam producers may have to i n s t a l l a i r blow-off d e v i c e s , use r a d i a n t h e a t e r s , or some such energy-using device.


f

RIM Urethanes. The use o f R e a c t i o n I n j e c t i o n Molding f o r producing urethane p a r t s i s growing, p a r t i c u l a r l y i n auto p a r t s such as bumpers and f a s c i a . There are many p o t e n t i a l and a c t u a l uses f o r RIM, i n c l u d i n g r i g i d housings f o r o f f i c e equipment and instruments. One of the economic a t t r a c t i o n s of RIM i n autos i s t h a t , i f p a r t s can be made r a p i d l y enough, the cost f o r molds and machinery can be h e l d low enough f o r o v e r a l l c o m p e t i t i o n w i t h stamped s t e e l . Cycle times have now been brought down to a few minutes, and the need to apply and dry a mold r e l e a s e f i l m i n each c y c l e has become r e l a t i v e l y more of a time l i m i t . On the other hand, a major need f o r auto p a r t s i s a Class A s u r f a c e f i n i s h . This must be produced no matter what e l s e , so keeping the mold surface smooth and b l e m i s h - f r e e i s mandatory. Internal mold r e l e a s e agents have not been a b l e t o prevent buildup f o r enough c y c l e s before c l e a n i n g i s necessary, and they a l s o f a i l to give adequate r e l e a s e w i t h i n a few c y c l e s , probably f o r the same reason. Therefore, e x t e r n a l mold r e l e a s e agents a r e s t i l l necessary a t present. I n a d d i t i o n to the s u r f a c e f i n i s h r e q u i r e ment, the r e l e a s e agent must s t i l l do i t s primary r e l e a s e f u n c t i o n very w e l l . RIM urethanes tend t o be l e s s a g g r e s s i v e and adhesive than seat foam urethanes, i n p a r t because they r e a c t so r a p i d l y that much o f the i s o c y a n a t e i s gone by the time they cont a c t the mold s u r f a c e , but there are s t i l l problems. Sometimes the p a r t shape i s complex, as i n the case o f g r i l l w o r k on an auto f r o n t f a s c i a . The p a r t must r e l e a s e e a s i l y a t those l o c a t i o n s where there i s s l i d i n g motion between f i n i s h e d p a r t and the mold s u r f a c e . Another problem may occur i n the gate and runner areas



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because the urethane i s i n j e c t e d very r a p i d l y ; i t can a c t u a l l y abrade or erode away a r e l e a s e f i l m . RIM molds are t y p i c a l l y at temperatures of 165°-180°F at the time of s p r a y i n g mold r e l e a s e agent, so the choice of c a r r i e r solvent i s s i m i l a r to that of HR type seat foam. Both aqueous and non-aqueous s o l v e n t systems are i n use, u s u a l l y depending on the chemistry of the a c t i v e i n g r e d i e n t s of the r e l e a s e agent. Some s u c c e s s f u l RIM r e l e a s e agents are based on waxes, and these are u s u a l l y c a r r i e d i n m i n e r a l s p i r i t s or other non-aqueous, non-polar s o l v e n t s . Other formulas are based on amphiphatic s o a p - l i k e i n g r e d i e n t s , which can be c a r r i e d by water. In these formulas, the evaporation of water i s u s u a l l y promoted by i n c l u s i o n of low molecular weight a l c o h o l s . Water-based r e l e a s e agents can be e a s i e r to remove from the p a r t s u r f a c e when c l e a n i n g i s c r i t i c a l , but wax-solvent r e l e a s e agents are a l s o s u c c e s s f u l w i t h proper c l e a n i n g . Formulations and Mechanisms. The above d i s c u s s i o n has s e t f o r t h some of the d e t a i l s which must be considered i n s e l e c t i o n and use of mold r e l e a s e agents f o r urethanes, although there are a l s o many s p e c i a l problems which have not been covered. In t h i s s e c t i o n , the d i s c u s s i o n i s d i r e c t e d to some probable elements of the mechanism of f u n c t i o n . Mold r e l e a s e technology, l i k e many o t h e r s , has developed i n advance of the d e t a i l e d b a s i c s c i e n t i f i c understanding. Nevertheless, progress i s being made, and more science i s emerging from the a r t . One may d i v i d e the requirements f o r a mold r e l e a s e agent i n t o two aspects. The f i r s t i s p r e v e n t i o n of contact between the urethane and the mold s u r f a c e , and the second i s the r e l e a s e i t s e l f . I f we consider the f i r s t requirement, i t seems l o g i c a l to suppose that a t minimum, one must have a continuous f i l m , or a t l e a s t one w i t h gaps which are very s m a l l so that the urethane cannot wet w e l l enough to get i n t o them. Next, i t appears that the f i l m must u s u a l l y be a s o l i d i n nature. Success can be achieved i n some cases w i t h f i l m s of l i q u i d s i l i c o n e , f o r example, but l i q u i d f i l m s have not been at a l l adequate f o r seat foam, and not good enough f o r RIM. The f i l m should probably not be h i g h l y s o l u b l e i n urethane components, a t l e a s t at the temperatures encountered. Many attempts have been made to c r e a t e r e l e a s e agent f i l m s i n which some components p l a y an a c t i v e chemical r o l e . That i s , the p r i n c i p l e was that an a c t i v e chemical i n g r e d i e n t r e acted w i t h isocyanates to make sure they reacted i n the f i l m and not a t the mold s u r f a c e i t s e l f . Such attempts have not u s u a l l y been s u c c e s s f u l . On the other hand, some experiments to d e t e r mine the r e a c t i v i t y of TDI w i t h waxes and other i n g r e d i e n t s used i n s u c c e s s f u l r e l e a s e agents has shown that a r e a c t i o n takes p l a c e i n some cases. One may summarize by saying that chemical r e a c t i v i t y i s not necessary, but may be present. The problem of producing a f i l m that i s continuous, very t h i n (to minimize cost and b u i l d u p ) , and impervious to urethane has been solved i n most cases by use of waxes. There are many



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w a x - l i k e m a t e r i a l s to choose from, i n c l u d i n g petroleum-based p a r a f f i n and m i c r o c r y s t a l l i n e types, other n a t u r a l waxes from v e g e t a b l e , a n i m a l , and m i n e r a l sources, and s y n t h e t i c waxes such as p o l y e t h y l e n e s . Most w i l l not work by themselves. I t has been necessary t o make m i x t u r e s , use s p e c i a l a d d i t i v e s , and otherwise modify the b a s i c p r o p e r t i e s . There i s not any u s e f u l g e n e r a l i z a t i o n here except to say t h a t i t i s f r e q u e n t l y necessary to t a i l o r - m a k e r e l e a s e agents t o match a p a r t i c u l a r s e t of urethane producing requirements. This i s o f t e n due to the need to combine s p e c i a l requirements such as a p a r t i c u l a r s u r f a c e c h a r a c t e r w i t h a p a r t i c u l a r temperature range o f c u r e , and so f o r t h . Some reasons why t h i s i s so may be seen by c o n s i d e r i n g the second f u n c t i o n , namely the r e l e a s e i t s e l f . Let us examine the case o f hot cure seat foam as one example of how the r e l e a s e i t s e l f i s accomplished. At the time o f pouri n g o f the foam, a mold i s t y p i c a l l y a t a temperature o f about 95° t o 110°F., so that a wax-based f i l m i s a s o l i d . During the e a r l y moments of the foam r i s e , the f i l m i s probably s t i l l s o l i d , but the exotherm of the foam p l u s the imposed heat from the c u r i n g ovens g r a d u a l l y i n c r e a s e s the temperature of the r e l e a s e f i l m . By the time o f demolding, the system i s above 200°F., and the wax f i l m has melted. Presumably, the i s o c y a n a t e content of the s u r f a c e of the foam has d e c l i n e d to zero or near i t , so the foam can no l o n g e r be adhesive. Then, the a c t o f demolding i s a simple p e e l i n g of the foam away from a l i q u i d f i l m , w i t h r e s u l t ing low f o r c e . The a c t u a l p i c t u r e may not be that s i m p l e . For example, i t i s p o s s i b l e t o get extremely easy r e l e a s e from s o l v e n t - c a r r i e d r e l e a s e agents, but i n g e n e r a l , the r e l e a s e ease from water-based r e l e a s e agents has not been as good. I t i s comp l e t e , but the a c t u a l f o r c e may be h i g h e r . The reasons a r e not f u l l y understood y e t . In the case of HR foam, RIM, and many other urethane types which a r e cured a t temperatures under 200°F., the r e l e a s e f i l m i s normally not melted a t the time o f demold. The mechanism of r e l e a s e c o u l d , i n p r i n c i p l e , i n v o l v e s e v e r a l p o s s i b i l i t i e s . The urethane p a r t could adhere t o the r e l e a s e f i l m , w i t h the r e l e a s e o c c u r r i n g between the mold s u r f a c e and the r e l e a s e f i l m . The urethane could f a i l to wet the r e l e a s e f i l m , and t h e r e f o r e never adhere to i t . Or the r e l e a s e f i l m could break i n such a way t h a t some p a r t stayed w i t h the mold and some t r a n s f e r r e d t o the u r e thane p a r t s u r f a c e . I n f a c t , one can f i n d examples of a l l three mechanisms. The knowledge necessary to e l u c i d a t e more of the d e t a i l s of these mechanisms, and the expected g r e a t e r a b i l i t y to c o n t r o l the f u n c t i o n through chemical f o r m u l a t i o n are now being gathered through a b a s i c r e s e a r c h approach. Figures 1-4 i l l u s t r a t e some evidence f o r the t h i r d mechanism j u s t l i s t e d , i n which the r e l e a s e f i l m f a i l s c o h e s i v e l y . The photographs were taken from a scanning e l e c t r o n microscope. The specimens were produced as f o l l o w s . F i g u r e 1 i s the top s u r f a c e of a f r e s h l y sprayed and d r i e d r e l e a s e f i l m . I t shows that the


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Figure 1.

Figure 2.

in

Urethane

Undisturbed

Surface of film remaining

Molding

surface of mold release

571

film.

on the mold after demolding foam.

of HR

urethane



572


Figure 3. Surface offilmtransferred to the urethane foam surface after demold

Figure 4. Same surface as in Figure 3, at lower magnification, showing underl urethane cell structure.



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r e l e a s e agent contained s o l i d wax p a r t i c l e s which s t i l l g i v e a s t r u c t u r e to the f i l m , i n c l u d i n g a r e l a t i v e l y rough s u r f a c e . HR foam was then poured and cured on a specimen of the f i l m , f o l l o w e d by demolding. F i g u r e 2 i s the s u r f a c e of the mold, and F i g u r e 3 i s the s u r f a c e of the foam, both at the same m a g n i f i c a t i o n as F i g u r e 1. The t e x t u r e i s the same, as one would expect from a f a i l u r e w i t h i n the f i l m . F i g u r e 4 i s the same specimen as F i g u r e 3, but at s m a l l e r m a g n i f i c a t i o n . In t h i s view, one can see the u n d e r l y i n g c e l l s t r u c t u r e of the urethane to which the wax from the mold r e l e a s e f i l m has adhered. Future Trends Molded urethane technology i n the immediate f u t u r e should not be l i m i t e d by mold r e l e a s e c o n s i d e r a t i o n s , but the r o l e of the mold r e l e a s e i n new systems should be considered e a r l y i n the p l a n s . Regulations f o r a i r q u a l i t y c o n t r o l and worker s a f e t y are expected to create a c o n t i n u i n g s h i f t to water-based mold r e l e a s e s , or a t l e a s t some means of g r e a t l y reducing the amount of s o l v e n t evaporated. Some progress seems l i k e l y i n use of semi-permanent r e l e a s e coatings which can be renewed by p e r i o d i c a p p l i c a t i o n of a top coat. Acknowledgement : The knowledge expressed i n t h i s paper contains c o n t r i b u t i o n s from the work of C. Peer Lorentzen, Steen Tamstorf, Kenneth C. R e n n e l l s , G. C l a r k e Borgeson, and Roberto Torres.

Literature Cited 1. McDonald, Μ., Ed. "Release Agents"; Noyes Data Corp., Park Ridge, N.J., 1972. RECEIVED April 30,

1981.


Urethane Chemistry and Applications - ACS Publications - American

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