Fifty Years of Polystyrene - ACS Symposium Series (ACS Publications)

18 Fifty Years of Polystyrene H E I N Z G . P O H L E M A N N and A D O L F

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B A S F Aktiengesellschaft, Ludwigshafen, West Germany

Just as Herman Mark was an important initiator of the scientific development of polymer chemistry and physics in our century and has left his decisive stamp on it, so did he have a particular influence on the early phase of the scientific and industrial development of styrene monomer and polystyrene. In 1980, polystyrene can look back on 50 years of industrial production, which began at the end of 1930 at Badische Anilin& Soda-Fabrik (now BASF) in Ludwigshafen. It is surprising that even on its golden jubilee polystyrene is known only to the specialists, although it has become one of the most commonly used plastics and has had its place in our daily life for many decades. We come across it in household appliances and toys, in furniture and electrical articles, in vehicles, in building and in mechanical engineering, in refrigeration and in packaging, at the workplace and in leisure activities - and scarcely anybody knows it by name. Roots (from the 18th Century to 1930) A c t u a l l y polystyrene or i t s s t a r t i n g m a t e r i a l styrene monomer i s much o l d e r than 50. As e a r l y as the 18th century a chemist named Neuman obtained the styrene monomer as an e s s e n t i a l o i l when he subjected " s t o r a x " to steam d i s t i l l a t i o n 0_). I t s o r i g i n a l name " s t y r o l " , which i s s t i l l used i n German, was coined by E. Simon, a B e r l i n pharmacist, who d i s covered the c o r r e c t elementary composition and observed that " s t y r o l " s o l i d i f i e d i n t o a g e l a f t e r standing f o r some time (2_). J . B l y t h and A.W. Hofmann c o r r e c t l y i n t e r p r e t e d t h i s i n 1845 as a r e a c t i o n without any change i n the composition and c a l l e d the s o l i d r e a c t i o n product "metastyrol" ( 3 ) . Subsequentl y , styrene was described by a number of workers, i n c l u d i n g B e r t h e l o t , who obtained i t by passing a mixture of benzene and ethylene through red-hot tubes ( 4 0 . B e r t h e l o t a l s o found that the polymerization of styrene can be a c c e l e r a t e d by c a t a l y s t s (.5). Closer i n v e s t i g a t i o n s of the polymerization r e a c t i o n

0097-6156/81/0175-0265$5.50/0 © 1981 American Chemical Society

In Polymer Science Overview; Stahl, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.


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were made by K r o n s t e i n (6^) and Stobbe and Posnjak (7_). The f i r s t p r a c t i c e - o r i e n t a t e d work dates back to 1911 when poly styrene was described i n two patents of Matthews (8) f o r the f i r s t time as an engineering m a t e r i a l and " v e h i c l e f o r p a i n t s and varnishes" without there being any p r a c t i c a l consequences. The second e f f o r t was made i n the twenties by Naugatuck Chemi c a l s i n the U.S.A., which t r i e d to manufacture polystyrene on the b a s i s of patents of Ostromislensky, but the t e c h n i c a l and economic d i f f i c u l t i e s proved too great ( 9 ) · However, what polystyrene a c t u a l l y was, i t s c o n s t i t u t i o n as a high molecular weight substance with a c h a i n l i k e s t r u c t u r e , the mechanism of i t s formation from the monomer i n a c h a i n r e a c t i o n , - these f a c t s had come to l i g h t i n u n i v e r s i t y and i n d u s t r i a l l a b o r a t o r i e s s i n c e 1920. H. Staudinger, K.H. Meyer and H. Mark a r e the most important names on t h i s path. They are the true founders of polymer chemistry as an independent d i s c i p l i n e . In h i s famous work of 1920 Hermann Staudinger f i r s t des c r i b e d the c o r r e c t s t r u c t u r e of polystyrene ( 1Ό ). I t was Staudinger, too, who gave polystyrene i t s name and e l u c i d a t e d the mechanism of i t s formation (11 ). The p o l y m e r i z a t i o n of styrene provided access to a b i g c l a s s of substances and made a s i g n i f i c a n t c o n t r i b u t i o n to the understanding of n a t u r a l poly mers and to the synthesis of i n d u s t r i a l p l a s t i c s . A whole new branch of the chemical i n d u s t r y i s based on the key substance polystyrene. Three f u r t h e r c o n d i t i o n s , however, had to be f u l f i l l e d before polystyrene could be manufactured on a l a r g e s c a l e as an engineering p l a s t i c : the monomer had to be a c c e s s i b l e a t a reasonable p r i c e and i n s u f f i c i e n t q u a n t i t i e s ; polymerization on an i n d u s t r i a l s c a l e had to be mastered; and there had to be an e f f i c i e n t method of f a b r i c a t i o n a v a i l a b l e . The f i r s t of these o b s t a c l e s was surmounted by H. Mark and C. Wulff i n 1929 a t BASF Ludwigshafen. They succeeded i n dehydrogenating ethylbenzene to styrene monomer over metal oxides (12 ). T h i s i s the process which i s s t i l l used today. The second c o n d i t i o n was a l s o f u l f i l l e d i n Ludwigshafen: C. Wulff and E. Dorrer (13) developed a continuous p o l y m e r i z a t i o n pro cess that was ready f o r operation i n 1930. I t was a happy c o i n c i d e n c e that a t the same time i t became p o s s i b l e , i n c o l l a b o r a t i o n with Dynamit AG, T r o i s d o r f , to t r a n s f e r the i n j e c t i o n molding process f o r metals to p l a s t i c s p r o c e s s i n g . This i s described i n a patent of Deutsche L e g r i t - G e s e l l s c h a f t , Berlin U 4 ) . Thus the stage was s e t f o r the production of polystyrene. The F i r s t Steps (1930-1945) Polystyrene as an I n d u s t r i a l Product. The beginnings modest. The f i r s t Ludwigshafen polystyrene plant produced


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6 tons per month. Up to the beginning of 1932, however, only 62 tons of polystyrene ("Polystyrene I") had been manufactured. The f i n i s h e d a r t i c l e s produced from i t were b r i t t l e and deve loped cracks because they contained too much r e s i d u a l monomer and s o l v e n t . The removal of these c o n s t i t u e n t s w i t h steam a t subatmospheric pressure ("Polystyrene I I " ) and a higher molecu l a r weight ("Polystyrene I I I " ) remedied t h i s . New grades were added with the advent of emulsion polymerization which provided polystyrenes with a s u b s t a n t i a l l y higher molecular weight ( p o l y styrene Ε and polystyrene E F ) . Output i n Germany soon increased a p p r e c i a b l y . In 1936, 845 tons of ethylbenzene, 714 tons of styrene and about 550 tons of polystyrene were produced. By 1942 output had r i s e n to almost 5000 t . At t h i s time the f i r s t copolymers were being produced a t Ludwigshafen by emulsion p o l y m e r i z a t i o n : p o l y styrene EN, a copolymer of 3 parts of styrene and 1 part of a c r y l o n i t r i l e as a c h e m i c a l - r e s i s t a n t brand and polystyrene EH (2 parts of s t y r e n e , 1 part of a c r y l o n i t r i l e and 1 part of v i n y l c a r b a z o l e ) as a b o i l i n g - w a t e r - r e s i s t a n t brand. Poly styrene EN became a precursor of the present SAN copolymers. I n d u s t r i a l production of styrene monomer i n the U.S.A. d i d not begin u n t i l 1935, polystyrene f o l l o w i n g i n 1938. In Great B r i t a i n polystyrene was produced from 1941 onward; other coun t r i e s d i d not f o l l o w u n t i l a f t e r 1945. Foamed polystyrene, too, dates from the t h i r t i e s . The method of adding blowing agents to polystyrene i n an extruder so that the extrudate expands was developed by Dow ( 1_5 ). I t i s s t i l l used today on a l a r g e s c a l e . Whereas, a t the beginning of the t h i r t i e s , polystyrene had been the d r i v i n g f o r c e i n the styrene monomer and polystyrene f i e l d s , t h i s development was soon reversed. Under Germany's e f f o r t s to become s e l f - s u f f i c i e n t there was a much bigger demand f o r styrene monomer f o r the manufacture of s y n t h e t i c rubber than f o r p o l y s t y r e n e . As e a r l y as 1938 approximately 2500 t of styrene monomer was produced i n Ludwigshafen f o r the newly commissioned rubber p l a n t s . When the A l l i e s were cut o f f from t h e i r A s i a n rubber p l a n t a t i o n s i n the Second World War, the U.S.A. followed s u i t with l a r g e styrene monomer c a p a c i t i e s f o r the manufacture of rubber. Thus there were b i g c a p a c i t i e s f o r styrene monomer a v a i l a b l e by 1945 f o r other uses. Manufacturing Processes. By 1945 three processes were competing with each other f o r the f u r t h e r expansion of poly styrene c a p a c i t i e s : a) the continuous mass polymerization process b) the batch suspension process, and c) the emulsion p o l y m e r i z a t i o n process. A l l three were developed a t BASF Ludwigshafen and operated i n v a r i o u s I.G. Farben p l a n t s .



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The BASF continuous mass p o l y m e r i z a t i o n process employed a tower r e a c t o r with an upstream continuous s t i r r e d tank r e a c t o r (16) (Figure 1). The conversion was 30-35% i n the prepolymerization tank r e a c t o r and about 98% at the discharge end of the tower. The polymer was then fed by means of an extruder to a p e r f o r a t e d d i e , extruded as strands and granulated. This p r i n c i p l e i s s t i l l being used today by many companies, even i f i n modified form. The suspension process, developed i n the U.S.A. (17) and a l s o tested during the war i n Ludwigshafen f o r polymerizing styrene (JJO, d i d not f u l l y come i n t o i t s own u n t i l a f t e r 1945. The emulsion process, however, competed s t r o n g l y i n the i n i t i a l phase w i t h the continuous mass p o l y m e r i z a t i o n process, one reason being the e a s i e r heat removal but the main reason being that high molecular weights were obtained i n a simple manner. The process f i r s t appeared i n the patent l i t e r a t u r e (19, 20) i n 1927 and was f u r t h e r improved by H. Fikentscher (21), f i n d i n g wide a p p l i c a t i o n i n the whole f i e l d of polymer chemistry. Processing and Uses. Polystyrene i s an outstandingly good i n j e c t i o n molding m a t e r i a l . In t h i s process the m a t e r i a l i s melted and f o r c e d through d i e s i n t o a mold where i t cools and s o l i d i f i e s . The process i s very adaptable and i s p a r t i c u l a r l y s u i t a b l e f o r the mass-production of s m a l l a r t i c l e s . Components f o r the e l e c t r i c a l i n d u s t r y , objects f o r everyday use (buttons, combs, tube tops, f o u n t a i n pens, e t c . ) and a l s o a r t i c l e s f o r the m i l i t a r y f i e l d (components f o r detonators i n grenades, p a r t s of gas-masks e t c . ) were manufactured i n t h i s way at that time. Beside the i n j e c t i o n molding process, extruder processing was introduced. Extruded, b i a x i a l l y s t r e t c h e d blown f i l m of p o l y s t y r e n e was used as i n s u l a t i n g m a t e r i a l f o r deep-sea cables and as condensor f i l m . It was here t h a t p a r t i c u l a r l y the high molecular weight emulsion polymers were used. Polystyrene EN c o n t a i n i n g a c r y l o n i t r i l e was used f o r making p r i n t i n g type and b o i l i n g - w a t e r - r e s i s t a n t polystyrene EH plays a r o l e i n highfrequency technology. Research. We have already pointed out the c o n t r i b u t i o n s made to macromolecular chemistry by H. Staudinger, K.H. Meyer and H. Mark. I t was not u n t i l 1930 t h a t a b a s i c consensus of opinion was reached on the molecular s t r u c t u r e of h i g h p o l y mers. 1932 saw the p u b l i c a t i o n of Staudinger's monograph on "High molecular weight organic compounds", i n which he was the f i r s t to t r e a t p o l y m e r i z a t i o n as a f r e e - r a d i c a l c h a i n r e a c t i o n (11). For h i s fundamental work he was awarded the Nobel P r i z e i n 1953. Work was now c a r r i e d out on a broad f r o n t i n t h i s f i e l d . Staudinger demonstrated the r e l a t i o n s h i p between v i s c o s i t y and molecular weight; i n 1932 Fikentscher published h i s


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famous v i s c o s i t y / c o n c e n t r a t i o n f u n c t i o n with i t s constant which i s s t i l l used today as K value" f o r e s t i m a t i n g the molecular weight (22). P. F l o r y (23) and G.V. Schulz (24) succeeded, independently of each other, i n deducing an expression f o r molecular weight d i s t r i b u t i o n based on k i n e t i c c o n s i d e r a t i o n s . The t r a n s f e r r e a c t i o n was f i r s t described by Mayo (25), the theory of emulsion polymerization by Smith and Ewart f o l l o w ing a l i t t l e l a t e r (26, 27) a f t e r p r e l i m i n a r y work by H. F i k e n t scher (21). During t h i s p e r i o d i t a l s o proved p o s s i b l e to desc r i b e copolymerization using k i n e t i c models ( 2 8 ) .


l!

Polystyrene's Development to a Major I n d u s t r i a l Product (1945-73) The Polystyrene Family. A f t e r 1945 there was a d r a s t i c drop i n the demand f o r styrene monomer because the need f o r s y n t h e t i c rubber suddenly d e c l i n e d . On the other hand, a b i g demand e x i s t e d f o r goods i n the c i v i l i a n s e c t o r and t h i s provided a strong impetus t o the expansion of p o l y s t y r e n e . In many cases, however, the mechanical s t r e n g t h of polystyrene was inadequate and t h i s i n i t i a t e d numerous research e f f o r t s , esp e c i a l l y i n the immediate postwar p e r i o d . One of the most important outcomes of these e f f o r t s was impact-resistant p o l y s t y r e n e , which was obtained by modifying the b r i t t l e m a t e r i a l with rubber. The f i r s t products were blends of polystyrene and s y n t h e t i c rubbers; recourse was soon made, however, to a p r i n c i p l e that Ostromislensky (29) had suggested as e a r l y as 1927: styrene monomer was polymerized i n the presence of rubber d i s s o l v e d i n i t . Copolymers of s t y r e n e , e s p e c i a l l y with a c r y l o n i t r i l e , a l s o a t t a i n e d i n c r e a s i n g importance both i n the unmodified form (30) and modified with rubber as ABS copolymers. The f i r s t products of t h i s k i n d were blends of n i t r i l e rubber and SAN (31 ). However, these o n l y had mediocre mechanical p r o p e r t i e s because the i n t e r f a c i a l c o m p a t i b i l i t y was i n s u f f i c i e n t . The breakthrough came when n i t r i l e rubber was replaced by a polybutadiene rubber which was g r a f t e d i n emulsion with styrene and a c r y l o n i t r i l e (32).

It was p o s s i b l e to cover a d d i t i o n a l a p p l i c a t i o n s w i t h these new types. In 1951 BASF found i t was p o s s i b l e to impregnate p o l y s t y r e n e d i r e c t w i t h expanding agents i n the suspens i o n process. This opened up great new p o s s i b i l i t i e s f o r p o l y styrene foam (STYR0P0R \ BASF, 33^), f o r example i n the packaging and b u i l d i n g f i e l d s . Manufacturing Processes. The three manufacturing processes a l r e a d y mentioned (continuous mass p o l y m e r i z a t i o n , batch suspension and emulsion polymerization) continued t o compete with each other a f t e r 1945. Whereas the t h i r d one g r a d u a l l y decreased i n importance, the other two were g i v e n preference i n



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f u r t h e r development. Up to t h i s time, the production of polystyrene had been mainly c a r r i e d out i n the various works of I.G. Farben, but now the l e a d i n g r o l e i n process development was taken over by the U.S.A. Union Carbide (34) and i n p a r t i c u l a r Dow adopted the continuous mass p o l y m e r i z a t i o n process. C r e d i t goes to Dow (35) f o r improving the o l d BASF process i n such a way t h a t good q u a l i t y i m p a c t - r e s i s t a n t polystyrenes became a c c e s s i b l e . The r e s u l t was that i m p a c t - r e s i s t a n t polystyrene o u t s t r i p p e d unmodified c r y s t a l p o l y s t y r e n e . Today, some 60% of polystyrene i s of the i m p a c t - r e s i s t a n t type. The t e c h n i c a l improvement i n volved numerous d e t a i l s ; i t was necessary to l e a r n how to handle h i g h l y viscous polymer melts, how to c o n s t r u c t r e a c t o r s f o r optimum removal of the r e a c t i o n heat, how to remove r e s i dual monomer and s o l v e n t s , and how to convey and meter melts and mix them w i t h a u x i l i a r i e s ( a n t i o x i d a n t s , a n t i s t a t i c s , mold-release agents and c o l o r a n t s ) . A l l t h i s was necessary to o b t a i n not only an e f f i c i e n t l y operating process but a l s o uniform q u a l i t y products d i f f e r e n t i a t e d to meet the r e q u i r e ments of various f i e l d s of a p p l i c a t i o n . In the meantime t h i s process has a t t a i n e d t e c h n i c a l maturity; over the years i t has been modified a number of times ( S h e l l i n 1966 (36), BASF i n 1968 (37), Granada P l a s t i c s i n 1970 (38) and Monsanto i n 1975 (39)) but the b a s i c concept has been r e t a i n e d . The continuous mass process i s d i v i d e d i n t o 4 s t e p s : rubber s o l u t i o n i n styrene monomer, polymerization, d e v o l a t i l i z a t i o n and compounding. In 1970 N. P l a t z e r (40) drew up a survey of the s t a t e of the a r t . Polymerization i s d i v i d e d i n t o p r e p o l y m e r i z a t i o n and main polymerization; f o r both steps r e a c t o r designs other than the tower r e a c t o r s shown i n Figure 2 have been proposed. Main p o l y m e r i z a t i o n i s taken to a convers i o n of 75 t o 85%; r e s i d u a l monomer and any solvent are separated under vacuum. The copolymer then passes t o g r a n u l a t ing equipment, f r e q u e n t l y through one o r more intermediate extruders i n which c o l o r a n t and other a u x i l i a r i e s are added. The second l a r g e - s c a l e process was the batch mass suspens i o n process. Monsanto d i d the pioneer work on t h i s (41 ). In t h i s process, prepolymerization i s c a r r i e d out i n bulk and main p o l y m e r i z a t i o n i n suspension; the l a t t e r i s taken t o conversions of over 99%. In c o n t r a s t to the continuous mass process, peroxide s t a r t e r s are used i n order t o achieve a high conv e r s i o n a t t o l e r a b l e r e a c t i o n times. Figure 3 shows a b a s i c flow diagram of such a p l a n t . A d e t a i l e d d i s c u s s i o n of advantages and disadvantages of the two processes can be found i n R. Bishop's monograph published i n 1971 (42), and i t i s continued i n a paper by Simon and Chappelear i n 1979 (43). I t was a d e c i s i v e f a c t o r f o r the economic success of impact polystyrene that these processes had been completely developed and mastered i n theory and p r a c t i c e .


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J—^-Vacuum Solvent & Monomer Recycling


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Continuous mass process (40) for crystal and high impact polystyrene.

Rubber!_ Chopper"

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High impact polystyrene by mass-suspension polymerization (40).



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Economic Development . In terms of output, polystyrene experienced a dramatic development i n the postwar years. Whereas i n 1940 world production was under 10,000 tons per year, by 1950 consumption i n the western world had r i s e n to 138,000 tons. The U.S.A. alone accounted f o r 85% o f t h i s . In 1960 550,000 tons and i n 1970 2.1 m i l l i o n tons were used. C a p a c i t i e s increased p a r a l l e l with the increase i n consumption up t o 1970 (Figure 4) a f t e r which they forged ahead of consumpt i o n . Petrochemistry on the one hand and p l a s t i c s f a b r i c a t i o n machinery on the other made enormous c o n t r i b u t i o n s to t h i s r a p i d expansion. For example, as the r e s u l t of a great e f f o r t i n Europe i n the f i f t i e s , there was a switch from c o a l to o i l as the f e e d stock. Only the petrochemical industry was a b l e to supply the i n c r e a s i n g l y l a r g e amounts of raw m a t e r i a l s f o r p l a s t i c s . This i s p a r t i c u l a r l y true of benzene and ethylene, the two s t a r t i n g m a t e r i a l s f o r styrene monomer and p o l y s t y r e n e . Ever-growing c a p a c i t i e s and p l a n t s s u p p l i e d these chemicals up t o 1973 a t ever lower p r i c e s which brought down the p r i c e of polystyrene and opened up f u r t h e r f i e l d s of a p p l i c a t i o n s . Processing and A p p l i c a t i o n . Polystyrene and the other thermoplastics would not have been able to enter on t h e i r triumphal march i f the manufacturers of processing equipment had not been able to provide e f f i c i e n t f a b r i c a t i o n methods. A d e c i s i v e f a c t o r was that between 1950 and 1960 the i n j e c t i o n molding machine with a screw p r e p l a s t i c i z e r was developed and e x t r u s i o n through s l i t d i e s to give sheeting o r f i l m followed by thermoforming was evolved. The screw i n j e c t i o n molding machine was given a very cons i d e r a b l e impetus by H. Beck's design (44); t h i s was followed by the well-known Windsor machine with a double-screw prep l a s t i c i z e r (45), which found p r a c t i c a l a p p l i c a t i o n e a r l i e r than the single-screw system. I n j e c t i o n molding techniques then progressed to greater and greater shot weights and l o c k i n g pressures u n t i l i t f i n a l l y became p o s s i b l e to make the inner boxes f o r r e f r i g e r a t o r s from easy-flow high-impact polystyrene by i n j e c t i o n molding. Machines f o r shot weights of over 10 kg and l o c k i n g pressures of up to 3000 tons were needed f o r t h i s (46). E x t r u s i o n techniques f o r sheet and f i l m were brought t o a h i g h degree of p e r f e c t i o n , p a r t i c u l a r l y i n the e a r l y postwar years (47 ). They formed the b a s i s f o r the use of impact p o l y styrene i n the manufacture of l a r g e parts ( r e f r i g e r a t o r inner boxes and door l i n e r s ) and l a t e r f o r the mass production o f packaging a r t i c l e s ( e s p e c i a l l y d r i n k i n g cups). I t has r e t a i n e d i t s important p o s i t i o n as a f a b r i c a t i o n method up to the present day; about 25 - 30% of impact polystyrene has been and s t i l l i s going i n t o e x t r u s i o n .



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Production of styrene polymers in the western world from 1950-80.

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Fifty Years of Polystyrene - ACS Symposium Series (ACS Publications)

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