Polystyrene and Styrene Copolymers - ACS Symposium Series (ACS

17 Polystyrene a n d Styrene C o p o l y m e r s WILLIAM DAVID WATSON and TED C. WALLACE

Downloaded by CORNELL UNIV on September 7, 2016 | http://pubs.acs.org Publication Date: September 25, 1985 | doi: 10.1021/bk-1985-0285.ch017

Dow Chemical Company, Freeport, TX 77541

History Chemistry of Styrenic Polymers Manufacturing Processes Applications Future Conclusion

Styrenic-based polymers have evolved over the last 40 years into one of the major thermoplastics. The polystyrene family of polymers consists of polystyrene (PS), styrene-acrylonitrile copolymer (SAN), rubber modified PS (HIPS for high-impact polystyrene), and rubber modified SAN (ABS for acrylonitrile-butadiene-styrene). SAN and ABS contain approximately 25% by weight acrylonitrile. HIPS and ABS contain polybutadiene rubbers in amounts ranging from a few to 20% by weight. Compounding halogenated organics and inorganic oxides into rubber-modified styrenics produces a product with reduced potential to burn. A large number of copolymers of styrene with divinylbenzene, methyl methacrylate, α-methylstyrene, and maleic anhydride, for example, have been commercially produced. In addition, there are a large number of styrene butadiene block copolymers (SB blocks) that have found commercial use. This article w i l l concentrate primarily on PS, SAN, HIPS, ABS, and SB block copolymers. Polystyrene plastics have a wide range of properties as listed in Table I. PS has a glass transition temperature of 100 °C and is stable to thermal decomposition to 250 °C. Therefore, PS is easily fabricated and a versatile product as well as transparent. SAN has improved tensile yield, heat distortion, and solvent resistance because of the incorporation of acrylonitrile. However, both PS and SAN are brittle as shown by the low notched izod impact strength and the inability to elongate under stress as shown in Figure 1. Adding rubber produces products with dramatically improved impact strength and elongation. The properties of HIPS and ABS are very dependent 0097 6156/85/0285-0363506.00/0 © 1985 American Chemical Society

Tess and Poehlein; Applied Polymer Science ACS Symposium Series; American Chemical Society: Washington, DC, 1985.


z

110

excellent

very low

'To convert l b f / i n to MPa, d i v i d e by 145. To convert f t l b f / i n to J/m, d i v i d e by 0.0187.

ASTM methods.

excellent

Relative ease of f a b r i c a t i o n

a

very low

Gardner impact

0.25

Izod impact, f t l b f / i n of notch 0.30

4.9

4.7

9500

Tensile modulus x 10 , l b f / i n

6400 2.0

2

1.5

lbf/in

108

25

—

1.08

SAN

Ultimate elongation, %

Tensile y i e l d ,

V i c a t s o f t e n i n g p o i n t , °C

—

A c r y l o n i t r i l e , weight %

PS

—

b

Butadiene rubber, weight %

'

1.04

a

102

excellent

medium

1.3

3.2

30

3650

—

4.85

1.05

Medium IPS

excellent

high

1.5

2.4

40

2600

100

excellent

high

1.5

3.2

40

3250

102

very high good good

8.0

2.6

8

4800

103

23.5

19.0

1.04

Std. ABS

very high

4.5

2.9

12

5500

106

23.5

17

—

14.5

6.5

7.0

1.05

1.05

Medium ABS Type 1 Type 2

1.05

High IPS

Polymer

M e c h a n i c a l P r o p e r t i e s f o r Main C l a s s e s of Styrene-Based P l a s t i c s (Compression Molded Specimens)

Specific gravity

Property

Table I.



17. WATSON AND WALLACE

Polystyrene and Styrene Copolymers

365

on the amount and type of rubber as w e l l as many other v a r i a b l e s . ABS has improved impact s t r e n g t h , g l o s s , and s o l v e n t r e s i s t a n c e compared to HIPS. Because of the differences i n r e f r a c t i v e indices between the rubber and the p o l y s t y r e n e phases, HIPS and ABS are opaque. The properties of styrene-butadiene block copolymers depend on t h e i r c o m p o s i t i o n w i t h h i g h l e v e l s of butadiene g i v i n g an elastomeric product and low l e v e l s of butadiene producing an impact p o l y s t y r e n e . Because t h e r e i s o n l y one phase these products are c l e a r . The r e l a t i v e c o s t of manufacture i s PS < HIPS < SB b l o c k < SAN < ABS because of the c o s t of raw m a t e r i a l s and manufacturing differences. The phenomenal growth of s t y r e n e polymers i n the U n i t e d S t a t e s i s shown i n T a b l e I I . Examples of uses of these products are PS for disposable tumblers, SAN for c r i s p e r trays, HIPS f o r yogurt c o n t a i n e r s and t o y s , ABS f o r r e f r i g e r a t o r l i n e r s and telephones, and SB block copolymers for c l e a r l i d s . History Styrene was f i r s t reported by Neuman i n the l a t e eighteenth century from s t o r a x . Storax i s a balsam d e r i v e d from the t r e e s of L i q u a m b e r o r i e n t a l i s w h i c h a r e n a t i v e to A s i a M i n o r . His experiments were confirmed years l a t e r . In 1839 Simon named the product " s t y r o l and noted t h a t a f t e r a few months i t became jellylike. In 1841 Gerhardt and Cahours a r r i v e d at the c o r r e c t formula, CgHg, and prepared the dibromide d e r i v a t i v e . In 1845 B l y t h and Hofmann confirmed t h a t a s o l i d mass r e s u l t e d when s t y r e n e was heated. A d d i t i o n a l work was done i n which styrene was prepared from cinnamic a c i d by d e c a r b o x y l a t i o n . In 1866 B e r t h e l o t prepared s t y r e n e from the r e a c t i o n of benzene and e t h y l e n e i n a hot tube. Thus, at the turn of the century the f o l l o w i n g chemistry was known: 1 1

In 1925 Naugatuck C h e m i c a l Company b u i l t a c o m m e r c i a l s t y r e n e / p o l y s t y r e n e p l a n t , but i t o n l y operated f o r a s h o r t time. From t h i s p o i n t I . G. Farben I n d u s t r i e i n Germany and Dow Chemical i n the United S t a t e s pursued c o m m e r c i a l i z a t i o n of s t y r e n e and polystyrene. Both companies independently developed along s i m i l a r routes. Dow became i n v o l v e d i n making s t y r e n e and p o l y s t y r e n e i n the mid-1930s. Two e x c e l l e n t h i s t o r i c a l accounts are a v a i l a b l e (1, 2). Styron 666 general purpose polystyrene was introduced i n 1938, and the f i r s t impact p o l y s t y r e n e , S t y r o n 475, was i n t r o d u c e d i n 1948. These e a r l y processes were batch, but a continuous process was i n t r o d u c e d i n 1952. World War I I i n c r e a s e d the a v a i l a b i l i t y of i n f o r m a t i o n c o n c e r n i n g s t y r e n e and p o l y s t y r e n e because of the c o o p e r a t i v e U.S. e f f o r t to make s t y r e n e butadiene rubbers (SBR)


366

APPLIED POLYMER SCIENCE


8000

0

2

4

6

8

10

12

14

16

18

20

22

Elongation, %

polystyrene family.

Figure 1. Relative stress - s t r a i n curve for Table I I

U.S. Production of Styrene Polymers ( m i l l i o n s of l b )

Year

General Purpose Polystyrene

1945

23

1950

3

RubberModified Polystyrene

SAN

b

ABS

C

Total

—

—

—

23

250

15

—

—

265

1955

320

115

3

8

446

1960

465

260

23

55

803

1965

830

642

38

240

1750

1970

1280

1215

58

510

3063

1975

1550

1630

102

840

4122

1980

1756

1765

111

920

4552

Includes prime, off-grade, export, and polymer used for Styrofoam brand p l a s t i c foam as w e l l as expandable polystyrene beads p l u s miscellaneous end uses. S t y r e n e - A c r y l o n i t r i l e c o p o l y m e r s m o s t l y o f c a . 25% acrylonitrile. A c r y l o n i t r i l e - B u t a d i e n e - S t y r e n e copolymers of a l l types.

b

c


17.

WATSON A N D W A L L A C E

367


rubbers. This s i t u a t i o n was a l s o true i n Germany. Currently there are over 30 p o l y s t y r e n e manufacturers i n the U n i t e d S t a t e s and Europe.


Chemistry of Styrenic Polymers The purity of styrene i s c r i t i c a l i n producing high molecular weight p o l y s t y r e n e free of g e l s . Styrene manufacture begins w i t h the a l k y l a t i o n of benzene w i t h e t h y l e n e by u s i n g a F r e i d e l C r a f t s c a t a l y s t to produce ethylbenzene. This a l k y l a t i o n i s done at low c o n v e r s i o n s because of the a c t i v a t i n g e f f e c t of the e t h y l group. The a l k y l a t i o n r e q u i r e s c o n s i d e r a b l e r e c y c l e of benzene and d i s p r o p o r t i o n a t i o n of the d i e t h y l b e n z e n e s . The ethylbenzene i s c a t a l y t i c a l l y dehydrogenated i n the presence of superheated steam at elevated temperatures and pressures: CH CH 2

3

CH=CH

2

I m p u r i t i e s , c h a i n t r a n s f e r agents such as p h e n y l a c e t y l e n e , and c r o s s - l i n k i n g agents such as d i v i n y l benzene need to be kept at extremely low l e v e l s . Styrene can be p o l y m e r i z e d r a d i c a l l y e i t h e r t h e r m a l l y or by u s i n g f r e e - r a d i c a l i n i t i a t o r s , a n i o n i c a l l y or c a t i o n i c a l l y . Thermally the reaction i s i n i t i a t e d by a D i e l s Alder adduct i n the f o l l o w i n g manner (3):

These two r a d i c a l s i n i t i a t e c h a i n growth. Only the stereoisomer w i t h the p h e n y l group i n t h e a x i a l p o s i t i o n i n i t i a t e s the polymerization. F r e e - r a d i c a l i n i t i a t e d polymerization i s normally done by using peroxy-type i n i t i a t o r s such as b e n z o y l p e r o x i d e , t e r t - b u t y 1 perbenzoate, or d i f u n c t i o n a l i n i t i a t o r s . The mechanism of these r a d i c a l polymerizations proceeds v i a the f o l l o w i n g sequence:



368 I

2R-

Initiation

>

V

Propagation

R - + XH -—

>

ps

—

>

•—

>

R- + S

—

>

~—

n

V

+ V

+ x-

n

Termination by Chain Transfer Termination by Disproportionation

PS

n m

Termination by Combination

+

The symbols used are I for i n i t i a t o r , R for the r a d i c a l derived from the i n i t i a t o r , S f o r s t y r e n e , R * and R * f o r growing p o l y s t y r e n e r a d i c a l s , XH f o r a source of hydrogen r a d i c a l , and PS for polystyrene. Thus, polystyrene can be formed i n the termination step by c h a i n t r a n s f e r , d i s p r o p o r t i o n a t i o n , and combination. Temperature and c h a i n t r a n s f e r agents can be used to c o n t r o l molecular weight and m o l e c u l a r weight d i s t r i b u t i o n . Polystyrene r e s u l t i n g from f r e e - r a d i c a l processes i s amorphous. Anionic polystyrene can be prepared by polymerizing styrene with butyl l i t h i u m , a l k a l i metals, or s o l u b l e a l k a l i metal complexes such as sodium naphthalene:


n

m

Initiation

The propagation step f o l l o w s as expected, but there i s no inherent termination step. Therefore, very high m o l e c u l a r weight products with narrow molecular weight d i s t r i b u t i o n can be made a n i o n i c a l l y . SB block copolymers are made a n i o n i c a l l y . Cationic polymerization i s i n i t i a t e d by acids such as p e r c h l o r i c a c i d , boron t r i f l u o r i d e , or aluminum t r i c h l o r i d e . High molecular weight p o l y s t y r e n e i s d i f f i c u l t to make c a t i o n i c a l l y because of c h a i n t r a n s f e r r e a c t i o n s t h a t occur w i t h the monomer and w i t h the commonly used s o l v e n t s . Thus, molecular weight and molecular weight d i s t r i b u t i o n s can be c o n t r o l l e d by the polymerization conditions and the method of polymerization. Examples of desired molecular weights are shown i n Figure 2. I s o t a c t i c p o l y s t y r e n e can be prepared by using a Z i e g l e r - t y p e c a t a l y s t . I t i s of i n t e r e s t because of i t s high m e l t i n g p o i n t (240 °C). Syndiotactic polystyrene i s unknown. The r e a c t i v i t y r a t i o s for the f r e e - r a d i c a l copolymerization of s t y r e n e (r^ = 0.4) and a c r y l o n i t r i l e ( ^ = 0.04) r e s u l t i n uneven incorporation of each monomer into the copolymer as seen i n Figure 3. Thus, most SAN and ABS polymers are made at the crossover point (A i n Figure 3) to avoid composition d r i f t . The a d d i t i o n of rubber s i g n i f i c a n t l y complicates the picture. Rubbers commonly used for HIPS and ABS have various microstructures depending on the method of manufacture (4):


WATSON A N D W A L L A C E



5.0


369

370

APPLIED POLYMER


cis

trans

SCIENCE

1,2-vinyl

The glass t r a n s i t i o n temperature of the rubbers used as impact modifiers must be below -50 °C to g i v e good impact strength over a broad temperature range. In a d d i t i o n , SB block copolymers can a l s o be used as impact modifiers. These rubbers are most e f f e c t i v e as impact modifiers i f they are grafted to the polystyrene r i g i d phase. Hydrogen abstraction at the a l l y l i e s i t e by an alkoxy r a d i c a l from the peroxide i n i t i a t o r and subsequent reaction with a growing polystyrene or SAN chain produces the g r a f t (.5-8). G r a f t i n g i s important f o r p a r t i c l e s i z i n g , but f i r s t , phase i n v e r s i o n must be discussed. P o l y b u t a d i e n e rubbers are s o l u b l e i n s t y r e n e . As t h e polymerization proceeds phase s e p a r a t i o n o c c u r s , and the s o l u t i o n turns opaque because of the difference i n r e f r a c t i v e indices between the two phases. I n i t i a l l y p o l y b u t a d i e n e i n s t y r e n e i s the continuous phase, and p o l y s t y r e n e i n styrene i s the discontinuous phase. When the phase volumes are e q u a l and s u f f i c i e n t s h e a r i n g a g i t a t i o n e x i s t s , phase i n v e r s i o n o c c u r s . After t h i s point polystyrene i n styrene i s the continuous phase, and polybutadiene i n styrene i s the discontinuous phase. Phase i n v e r s i o n i s represented i n F i g u r e 4. A change i n v i s c o s i t y i s a l s o observed at phase i n v e r s i o n (6). Rubber p a r t i c l e s i z e i s extremely important to make an optimized impact product. P a r t i c l e s t h a t are both too s m a l l and too l a r g e cause a l o s s of impact s t r e n g t h . The a b i l i t y to form s t a b l e p a r t i c l e s of optimum s i z e depends on the graft that functions as an o i l i n o i l emulsion. This might better be referred to as an emulsion of two incompatible organic phases. To s i z e the rubber p a r t i c l e s , shearing a g i t a t i o n must be provided. I f i t i s not provided, phase i n v e r s i o n does not occur, and a c r o s s - l i n k e d continuous phase that produces g e l i s the r e s u l t . Measurements of p a r t i c l e s i z e i n the f i n a l product can be done w i t h a C o u l t e r Counter or e l e c t r o n microscopy by u s i n g osmium tetraoxide to s t a i n the rubber p a r t i c l e s . This s t a i n i n g a l s o a l l o w s v i s u a l observation of the rubber morphology as shown for HIPS and ABS i n F i g u r e 5. The p o l y b u t a d i e n e rubber i s s t a i n e d dark w i t h osmium tetroxide and the continuous polystyrene phase i s the l i g h t background. What i s observed i s somewhat c i r c u l a r rubber p a r t i c l e s with polystyrene occlusions. These occlusions extend the e f f e c t i v e rubber phase volume f o r b e t t e r use of the rubber. P a r t i c l e s i z e v a r i e s as w e l l as the density of the rubber p a r t i c l e . A l l of these v a r i a b l e s are important to designing a product for a s p e c i f i c use. For example, a glossy HIPS product needs to balance the s m a l l p a r t i c l e s i z e against impact strength (10). Resistance to e n v i r o n m e n t a l s t r e s s c r a c k agents i s improved by h i g h rubber content, large p a r t i c l e s i z e , high matrix molecular weight, and the choice of p l a s t i c i z e r (11).



WATSON AND WALLACE


371

Figure 4. Formation of rubber p a r t i c l e s by phase inversion of polymeric o i l - i n - o i l emulsion. Phase contrast micrograph (rubber phase i s l i g h t ) according to G. E . Molare and H. Keskkula.




372

Figure 5. Transmission electron micrograph of osmium tetroxide stand—HIPS and ABS.


17.

WATSON AND WALLACE


373

SB block copolymers are made a n i o n i c a l l y . These copolymers can be d i b l o c k s , t r i b l o c k s , and r a d i a l block copolymers with different degrees of t a p e r i n g . Kraton was i n t r o d u c e d i n the mid-1960s by S h e l l . Another major manufacturer i s P h i l l i p s with Solprene and KResins. These products can be used as thermoplastic elastomers or as impact modifiers. One of the most i n t e r e s t i n g aspects of these resins i s the different types of morphologies that can be obtained as shown i n F i g u r e 6 (12-15).


Manufacturing Processes Economics of making p o l y s t y r e n e d i c t a t e s l a r g e , c o s t - e f f i c i e n t , continuous processes. Batch p o l y m e r i z a t i o n i s important from a h i s t o r i c a l p o i n t of view and i s s t i l l used i n SB l a t e x e m u l s i o n polymerization, anionic SB block copolymerizations, and suspension p o l y m e r i z a t i o n t o make s t y r e n e - d i v i n y l b e n z e n e copolymer or polystyrene expandable beads. In fact suspension polymerization of s t y r e n e made some of the best q u a l i t y p o l y s t y r e n e a l t h o u g h the process i s no longer economically f e a s i b l e . Examples of continuous processes are shown i n Figures 7 and 8. These reactors are designed to remove the heat of reaction (-17.4 c a l / m o l ) from a h i g h l y viscous medium. Reactor designs for polystyrene have recently been reviewed (16). Reactors such as the one shown i n F i g u r e 7 are l o n g and c y l i n d r i c a l w i t h m u l t i p l e heat t r a n s f e r tubes and an a g i t a t o r . Styrene, i n i t i a t o r , and d i l u e n t s such as toluene or ethylbenzene are fed i n t o the r e a c t o r s . The d i l u e n t s are used to reduce the v i s c o s i t y i n the t h i r d stage. The temperatures i n the reactors are g r a d u a l l y increased from approximately 100 °C to 180 °C during which time the s o l i d s i n c r e a s e . The polymer i s fed to a d e v o l a t i l i z e r where the d i l u e n t and any r e s i d u a l styrene i s removed for r e c y c l e . The polystyrene i s stranded, cooled, and cut i n t o p e l l e t s for s a l e . A d d i t i v e s are u s u a l l y added to improve product p r o p e r t i e s and p r o c e s s a b i l i t y . Such a d d i t i v e s are p l a s t i c i z e r s such as mineral o i l to c o n t r o l flow properties, antioxidants such as hindered phenols to p r e v e n t y e l l o w i n g , and mold r e l e a s e agents such as s t e a r a t e s to prevent mold s t i c k i n g . C e r t a i n of these a d d i t i v e s t h a t do not i n t e r f e r e with the reactor such as mineral o i l can be added i n the feed or post added w h i l e a d d i t i v e s t h a t do i n t e r f e r e such as antioxidants are u s u a l l y post added. The use of rubber again c o m p l i c a t e s the p i c t u r e . F i r s t the rubber must be d i s s o l v e d i n styrene. This process i s slow and must be done as d i s c u s s e d e a r l i e r w i t h enough s h e a r i n g a g i t a t i o n to effect phase i n v e r s i o n and to adequately s i z e the rubber p a r t i c l e s . In HIPS manufacture phase i n v e r s i o n n o r m a l l y occurs i n the f i r s t stage. Because of the h i g h e r amounts of rubber i n ABS, phase i n v e r s i o n occurs i n the second stage. The type and the amount of graft depend on the microstructure of the polybutadiene as w e l l as the conditions of the thermal or i n i t i a t e d polymerization of styrene (17, 18). C a r e f u l c o n t r o l of the amount of c o n v e r s i o n and d e v o l a t i l i z a t i o n c o n d i t i o n s produces a product w i t h the d e s i r e d amount of c r o s s - l i n k i n g (19) which occurs p r i m a r i l y between the 1,2v i n y l p o r t i o n of the p o l y b u t a d i e n e and the p o l y s t y r e n e phase. Because of differences i n r e a c t i v i t y r a t i o s , the c r o s s - l i n k i n g does not occur u n t i l almost complete styrene conversion. Figure 8 shows a continuous s t i r r e d tank reactor that uses evaporative c o o l i n g to achieve uniform temperature c o n t r o l . Tess and Poehlein; Applied Polymer Science ACS Symposium Series; American Chemical Society: Washington, DC, 1985.


.•'.•;..v\\; .


v

Droplets

Cellular

Entanglements

Fiber Aggregates

Spherical Aggregates

1|im

Shells

ure 6. Various p a r t i c l e structures o f rubber i n impact polystyrene.




375

RECOVERED STYRENE 8 SOLVENT SOLVENT' STYRENE

rx rx REACTOR

REACTOR


)EV0LATILIZER

EXTRUDER

" a

XJ

POLYSTYRENE PELLETS

COOLER CUTTER

TJ

Figure 7. Diagram of a continuous process for styrene polymerization. Control

Figure 8. Diagram of a process to manufacture polystyrene that uses a continuous s t i r r e d tank (CSTR) reactor.



376

By changing v a r i a b l e s numerous products can be manufactured. C o n t r o l of m o l e c u l a r weight, c o m p o s i t i o n , a d d i t i v e s , g r a f t i n g , p a r t i c l e s i z i n g , morphology, and c r o s s - l i n k i n g produces products with a wide v a r i e t y of p h y s i c a l properties.


Applications One of the major a t t r i b u t e s of the polystyrene family of resins i s i t s ease of f a b r i c a t i o n . Ease of p l a s t i c i z a t i o n , good m e l t s t r e n g t h , and low c o e f f i c i e n t of thermal expansion make these p o l y m e r s i d e a l f o r the r e l a t i v e l y l o w - c o s t s i n g l e s c r e w extrusion/therraoforming process. This processing technology c o n t i n u e s to grow, and 40% of a l l s t y r e n e polymers and copolymers are processed by t h i s technique (20). The advent of l o w - c o s t computers has made c l o s e d l o o p c o n t r o l of the e x t r u s i o n system commonplace and thus has y i e l d e d more uniform gauge c o n t r o l at higher output rates (21). The other most common f a b r i c a t i o n technique for the polystyrene f a m i l y i s i n j e c t i o n m o l d i n g , most f r e q u e n t l y accomplished by reciprocating screw and screw p r e p l a s t i c i a t o r machines (22). Major a p p l i c a t i o n s for styrene p l a s t i c s are summarized i n Table I I I (23). The packaging and s e r v i c e w a r e ( d i s p o s a b l e s ) markets predominate, and account for approximately 50% of the t o t a l . One of the most r a p i d l y growing portions of these markets i s i n low-density ( u s u a l l y 1-10 l b / f t ^ ) p o l y s t y r e n e foams, e i t h e r i n the form of e x t r u d e d foam s h e e t or expanded p o l y s t y r e n e beads ( E P S ) . Projections indicate that production of these foams w i l l be greater than 2000 metric tons (24). The use of c e l l u l a r s t y r e n e p l a s t i c s f o r i n s u l a t i o n has been widespread f o r many years (25). More r e c e n t l y , polystyrene " s t r u c t u r a l " foams have been used, e s p e c i a l l y i n wood replacement a p p l i c a t i o n s . Such use i s expected to grow i n the f u t u r e , p a r t i c u l a r l y as wood becomes l e s s a v a i l a b l e and g r e a t e r demands are placed on more e f f i c i e n t use of "short" p l a s t i c materials. Future Although the styrene p l a s t i c s industry i s r e l a t i v e l y mature, i t i s s t i l l an area for f r u i t f u l research and development. For example, an area of major current concern i s the development of products that are d i f f i c u l t to i g n i t e and y i e l d l i t t l e smoke when burned. E x i s t i n g approaches to the flammability problem i n v o l v e the use of halogen-containing a d d i t i v e s p l u s antimony oxide s y n e r g i s t s (26). In the long term, products may be made with decreased flammability p o t e n t i a l v i a polymerization with halogen-containing comonomers, by a l l o y i n g w i t h f i r e - r e s i s t a n t e n g i n e e r i n g t h e r m o p l a s t i c s , or by copolymerizing with monomers that induce charring at the flame front by c r o s s - l i n k i n g . Although impact modifiers have been investigated since the b i r t h of the r u b b e r - m o d i f i e d p l a s t i c s i n d u s t r y , they s t i l l remain an a c t i v e area of i n v e s t i g a t i o n . The e a r l i e r impact polymers were modified with polybutadiene and p o l y s t y r e n e - c o - b u t a d i e n e made v i a e m u l s i o n p o l y m e r i z a t i o n . As the i n d u s t r y matured, " s o l u t i o n " p o l y b u t a d i e n e and to a minor e x t e n t s t y r e n e / b u t a d i e n e block



Table I I I .

377


WATSON AND WALLACE

Major Applications for Polystyrene ( m i l l i o n s of l b ) a

Market

1960

1965

1970

1975

1980

1985

Packaging

100

345

600

750

1,055

1,350

Appliances

93

121

150

140

110

115

Radio, TV, Electronics

32

55

138

200

235

275

Housewares, Furnishings, Furniture

95

184

338

325

320

365

Recreation

75

212

285

170

210

235

Serviceware

10

40

150

210

290

360

150

263

334

330

530

560

555

1,220

1,995

2,125

2,750

3,260

Miscellaneous Total a

estimated



378


copolymers became widely used as r e i n f o r c i n g agents. The s o l u t i o n or "diene" type rubbers, prepared by a l k y l l i t h i u m i n i t i a t e d polymerization of butadiene (27), tend to be "cleaner" than emulsion elastomers and have d i s t i n c t l y lower g l a s s t r a n s i t i o n temperatures than t h e i r emulsion-prepared counterparts. These properties lead to b e t t e r low-temperature impact p r o p e r t i e s i n the r u b b e r - m o d i f i e d styrene p l a s t i c s . L i k e w i s e , s t e r e o s p e c i f i c or h i g h c i s - 1 , 4 polybutadiene-modified p l a s t i c s e x h i b i t s i m i l a r d e s i r a b l e properties (28). Another example i s the blending of s p e c i a l block copolymers and GP p o l y s t y r e n e (29-31). C o n s i d e r a b l e r e s e a r c h i s being done with s p e c i f i c thermoplastic elastomers used as c o m p a t i b i l i z e r s for a l l o y s of normally incompatible polymers such as p o l y s t y r e n e and polypropylene (32). A d d i t i o n a l opportunities i n the styrene p l a s t i c s industry e x i s t f o r the development of products h a v i n g such unique p r o p e r t i e s as high heat r e s i s t a n c e and o p t i c a l transparency. Arco produces the D y l a r k f a m i l y of h e a t - r e s i s t a n t s t y r e n e p l a s t i c s , which are copolymers of styrene-maleic anhydride. These products have a good balance of mechanical properties and have a heat d i s t o r t i o n (under ASTM D68) of 234 °F (33) which i s markedly h i g h e r than homo polystyrene. Perhaps the best known member of the " h e a t - r e s i s t a n t " s t y r e n e p l a s t i c s family i s General E l e c t r i c ' s Noryl (34). Noryl i s an a l l o y of p o l y ( p h e n y l e n e oxide) and h i g h - i m p a c t p o l y s t y r e n e . Heat d e f l e c t i o n temperatures f o r N o r y l range as h i g h as 300 °F. The balance of mechanical properties i s e x c e l l e n t , although processab i l i t y i s more d i f f i c u l t than for conventional styrene p l a s t i c s . P h i l l i p s Petroleum Company now manufactures the K-Resin family of o p t i c a l l y t r a n s p a r e n t impact p o l y s t y r e n e (35). These r e s i n s , made v i a anionic polymerization techniques, owe t h e i r transparency to the e x t r e m e l y s m a l l s i z e of the d i s p e r s e d rubber phase. The p r i n c i p a l use of K-Resin i s i n packaging a p p l i c a t i o n s . The development of unique techniques for f a b r i c a t i o n of styrene p l a s t i c s into end-use items i s a l s o a f r u i t f u l area of research and development. The use of coextrusion, for example, i s now becoming widespread. C o e x t r u s i o n a l l o w s one to combine the p r o p e r t i e s of various polymers i n t o a s i n g l e layered structure (36). For example, the m u l t i l a y e r e d packaging s t r u c t u r e i n F i g u r e 9 combines the s t r u c t u r a l p r o p e r t i e s of h i g h - i m p a c t p o l y s t y r e n e and the oxygen b a r r i e r properties of Saran with the moisture b a r r i e r , food contact s u r f a c e , and heat s e a l p r o p e r t i e s of p o l y e t h y l e n e . Such f i l m i s made v i a coextrusion through a feedblock of the type i l l u s t r a t e d i n F i g u r e 10. One can make f i l m h a v i n g hundreds of l a y e r s v i a t h i s technique. I t i s a l s o p o s s i b l e to use "scrap" i n c o e x t r u s i o n processes by " h i d i n g " i t i n a c e n t r a l l a y e r . The 1980s s h o u l d see widespread use of the a l l - p l a s t i c can based on complex structures t h a t i n c l u d e p o l y s t y r e n e (37). An example of such a can i s an aseptic packaging system that was developed j o i n t l y by Erca SA and Cobelplast of France and i s t y p i c a l l y a thermoformable seven-layer sheet: polyethylene/polypropylene/glue/poly(vinylidene c h l o r i d e ) / glue/PS/pigmented PS (38).




WATSON AND WALLACE

HZ Impact Polystyrene

—

Figure 9. M u l t i l a y e r structure made v i a coextrusion.

Transition Channel

Direction Of Flow

Feed ports Meter Layers Of Two Or More Polymers

Layered Sheet Or Film (Number of layers is equal to number of feed ports)

Figure 10. Coextrusion sheet and container.


380


An emerging and extremely s i g n i f i c a n t f a b r i c a t i o n technique for p l a s t i c m a t e r i a l s , i n c l u d i n g styrene p l a s t i c s , i s "scrapless thermoforming" (39). As i l l u s t r a t e d i n Figure 11, i t i s possible to extrude (or coextrude) sheet, cut b l a n k s from t h i s sheet, and then forge the b l a n k i n t o a c o n t a i n e r w i t h o u t g e n e r a t i n g s c r a p . An a d d i t i o n a l b e n e f i t of t h i s f a b r i c a t i o n technique i s the improved toughness and s t r e s s crack r e s i s t a n c e t h a t i s d e r i v e d from the o r i e n t a t i o n induced i n the f o r g i n g p r o c e s s . Continued use of f a b r i c a t i o n techniques that minimize or eliminate scrap generation i s expected i n the future.


Conclusion The styrene p l a s t i c s industry has emerged over the past 30 years to become a major worldwide business. The industry has grown because the e x c e l l e n t balance of mechanical properties and p r o c e s s a b i l i t y of styrene p l a s t i c s a l l o w i t to f i l l diverse market needs. The advent of workable i n d u s t r i a l processes for both monomer and polymer and the fact that styrene p l a s t i c s were made from once inexpensive raw materials have l i k e w i s e contributed to the growth of the industry. In s p i t e of the r e l a t i v e maturity of the science and the industry, styrene p l a s t i c s remain a f r u i t f u l area for research. For example, the development of new materials having unique properties, such as f i r e and heat r e s i s t a n c e , and the development of e f f i c i e n t energy and m a t e r i a l - s a v i n g f a b r i c a t i o n processes are expected to be the subject of extensive study i n the future.




381

Acknowledgments The authors would like to thank A. E. Piatt of the Dow Chemical Company for preparing Figures 1-8 and for reading and commenting on the manuscript. Literature Cited 1. 2. 3. Downloaded by CORNELL UNIV on September 7, 2016 | http://pubs.acs.org Publication Date: September 25, 1985 | doi: 10.1021/bk-1985-0285.ch017

4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

17. 18. 19. 20. 21. 22. 23. 24. 25.

26. 27. 28.

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29.

Durst, R. R. U.S. Patents: 3 906 057, 3 906 058, 3 907 929, 3 907 931, 1975. Platzer, N. Chem Tech., October 1977, 634. Aggarwal, S.; Livigni, R. Polym. Eng. and Sci. 1977, 17(8), 498. Holden, G.; Govw, L. U.S. Patent 4 188 432, 1980. Plastics Tech. 1981, Manuf. Handbook, 27(7), 557. "Noryl Thermoplastic Resins"; Technical Bulletin, General Electric Company: Selkirk, N.Y. "K-Resin Butadiene Styrene Polymers"; Technical Bulletin, Phillips Chemical Company, Bulletin 19430, 1980. Schrenk, W.; Alfrey, T. "Polymer Blends"; Academic Press, 1978; Vol. 2, p. 129. Terwilliger, F. Food and Drug Pkg. 1981, 46(1), 54. Sneller, J. Modern Plastics 1981, 58(12), 54. "Scrapless Forming Process"; Technical Bulletin, The Dow Chemical Co., Bulletin S-303-74-76, 1976.

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General References 1.

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Polystyrene and Styrene Copolymers - ACS Symposium Series (ACS

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