Addition and Condensation Polymerization Processes

33 Expandable Polystryrene Processes A. R. INGRAM

Downloaded by FUDAN UNIV on December 11, 2016 | http://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0091.ch033

Koppers Company, Inc., Research Department, Monroeville, Pa.

Various processes are reviewed for making expandable polystyrene which can be molded or extruded into foam. Most processes employ aqueous suspension systems in which pentanes are introduced before, during, or after polymeri zation. Water-free systems may also be used. The require ments for particle size and shape are critical. In suspension polymerization, bead size is controlled by the agitation and dispersing agents. To reduce off-size particles, prescreened polystyrene beads or chopped filaments may be impreg nated. Since pentane is not a solvent for polystyrene, its diffusion is hastened by raising the temperature and/or by adding solvents. Chemical and physical modifications of expandable polystyrene processes are considered in terms of expandability, cell structure, heat resistance, flammability, and solvent resistance of foams.

Τ η 1967 the total sales of polystyrene foam in the United States were •*· about 193 million pounds (78). Foams molded or extruded from expandable polystyrene amounted to 145 million pounds. The recent growth rate of polystyrene foam in the United States is depicted in Figure 1. The lower section represents expandable polystyrene. Expandable polystyrene consists of particles of styrene polymers with volatile organic liquids trapped among the rigid chains. The most significant property responsible for the commercial acceptance of ex pandable polystyrene is its ability to be steam-molded into such useful, light-weight, low-cost, closed-cell foams as beverage cups, packages, picnic chests, and ice buckets, as well as insulation board (121). The molding of foams is done in two steps. First, the particles are expanded in a continuous upward flow of steam to yield free-flowing individual pieces of foam (94). Then, after an aging period of several hours during which air diffuses into the expanded particles, they are placed in a slightly vented mold. The particles are then heated by steam at super513 Platzer; Addition and Condensation Polymerization Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

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atmospheric pressure injected into the mold so they expand to fill the empty spaces and fuse together. The molding operation is completed by circulating or spraying cold water in the jacket of the mold until the foam no longer exerts pressure. Expandable polystyrene may also be extruded into films, sheets, and boards.

'64

Figure I.

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'66

'67

Four-year sales record of polystyrene foams (million pounds)

Preparing Expandable Polystyrene Diffusion of Blowing Agents into Polystyrene. I N C O N T A C T W I T H I N T H E A B S E N C E O F W A T E R . Particles Plus Excess Liquid, The first patent on the preparation of expandable polystyrene containing volatile hydrocarbons, applied for in Great Britain in 1944, covers the soaking of polystyrene particles, held in a cheesecloth bag, in a solution comprising a nonsolvent, petroleum ether (90-99 volume %), and a solvent (10-1%) such as acetone, ethyl acetate, or benzene (126). At 2 0 ° - 2 5 ° C . , impregnation was complete within 0.5 to 12 hours, depending on particle size and composition. A mixture of diethyl ether and ethanol is also claimed as rendering polystyrene expandable after three days of soaking therein (127). Nonvolatile additives may be incorporated in the polystyrene, hastening its impregnation by n-pentane, and avoiding the ORGANIC LIQUIDS

Platzer; Addition and Condensation Polymerization Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1969.


33.

INGRAM

Expandable Polystyrene Processes

515

use of solvents as disclosed above. Cited additives are rubbery isobutylene polymers, rubbery diene polymers (e.g., butadiene-styrene elastomers), fatty acid salts or esters ( e.g., stéarates ), organic insoluble pigments, and inorganic pigments smaller than 5μ in diameter (22, 87). Ground poly styrene, screened through an 8-mesh sieve and onto a 20-mesh sieve, was stirred with an equal weight of n-pentane. In 24 hours the particles absorbed 4.7% by weight; in 48 hours, 7.7%. However, the presence within the polystyrene of intimately mixed 1% calcium carbonate of a particle size of Ο.ΐμ increased the 24-hour absorption of n-pentane to 7.8% (87). While the above modifications of the polymer decrease ab sorption time effectively, they also increase the rate at which n-pentane diffuses out of the polymer, thereby shortening shelf life. A difficulty with the above steeping processes is that the particles become softened and tend to agglomerate. Therefore, they are stirred vigorously and kept at relatively low temperatures. The addition of finely divided solids to the dispersion avoids agglomeration and permits the use of higher temperatures to hasten impregnation. Thus, a slurry of 24% polystyrene in pentane was stabilized by 5% (polystyrene basis) calcium carbonate and 2% calcium phosphate. After stirring at 90°C. for 3 hours, the beads contained 10% pentane and were not stuck to gether (120). By another process a 50% slurry in n-pentane was sta bilized by 1% of calcium silicate and stirred for 0.5 hour at 40 °C. and 40 p.s.i.g. under nitrogen (43). This product contained 8% n-pentane. A mechanical device for impregnating polystyrene placed in the annular space between two cylinders has been disclosed (30). Particles Plus the Minimum Amount of Liquid. A process without a dilution medium or antiagglomeration agent would be desirable. The first such process involves the continual rolling for 32 hours at 70 °C. of a closed vessel containing 1000 parts of polystyrene particles with a solution comprising 65 parts of hexane, 5 parts of benzene, and 15 parts of methanol (122). Another process involves impregnation under carbon dioxide well above the softening point of the polymer: 900 parts of poly styrene and 80 parts of hexane under carbon dioxide at 10 atm. for 4 hours at 140 °C. (125). This product was broken apart after cooling. The absorption of butane into polystyrene proceeds more rapidly than pentane or hexane. Thus, the addition of the required amount (7% ), plus an excess only for air-purging, to polystyrene ( or to rubber-modified polystyrene) particles in a sealed container provides useful expandable polystyrene after agitation for 24 hours at room temperature (27). If the butane is mixed with a noncombustible gas of lower density, the explosion hazard is avoided (50). Molded Objects Immersed in Liquids. Molded, non-foamed articles, such as drinking cups from rubber-modified polystyrene, are immersed


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in absorbable liquid blowing agents. The molding is then heated, creating an insulating surface according to the location and amount of heat, as well as the extent of absorption of liquid (3, 107).


I N C O N T A C T W I T H O R G A N I C V A P O R S . Finely divided polystyrene beads ( 2 2 - 4 8 mesh) supported on a screen suspended above the surface of pentane in a closed vessel absorb as much as 9 . 2 % in 2 days at 3 0 ° C . In the same way, a styrene-acrylonitrile copolymer is rendered expand able by exposure to vapors of a 9 0 / 1 0 mixture of pentane and methylene chloride ( 5 ) .

A graft copolymer sheet of 5 parts of rubber ( 2 5 / 7 5 styrene/butadiene) and 9 5 parts styrene may be kept for a week at — 7 0 ° C . among "dry ice" (carbon dioxide) to make it expandable (47). I N A Q U E O U S SLURRIES. Patent applications on the diffusion of pentane fractions of petroleum into slurries of polystyrene beads in water at elevated temperatures were made in both the United States and Germany in 1953. In the U.S. application the suspension was stabilized by a finely divided calcium phosphate and an anionic surfactant, and impregnation was carried out at 9 0 ° C . (24). In the German application, the suspension was stabilized by an emulsifying agent alone, and the impregnation was conducted at 8 0 ° C . (19). To permit the introduction of butanes rather than pentanes as blowing agent without creating excessive pressure in the reactor, methylene chloride or ethyl acetate (strong solvents) were first added to the slurry, then the polystyrene beads readily absorbed butane within 5 hours at room temperature (20). Reactor pressure was 3 0 p.s.i.g. The impregnation of polystyrene in aqueous suspension can be performed without a pressure vessel if a mixture of equal parts of petroleum ether and ethyl acetate is added slowly to the bead slurry (72). Addition time is 6 hours; soaking time, 1 8 hours. The use of poly (vinyl alcohol) as the sole or partial suspending agent is claimed to permit the rapid impregnation by pentane of polystyrene slurries at rather high temperatures—115° and 1 2 0 ° C . (49, 114). Instead of charging beads to be impregnated with pentane, one may charge ground or chopped strands of polystyrene. When the impregnation is conducted at very high tem peratures, 1 3 0 ° - 1 3 5 ° C . , these particles become rounded, and the sus pension is stable provided prescribed suspending agents (a mixture of carboxymethyl cellulose and the ammonium salt of sulfonated poly (vinyltoluene) are used. Pentane is added under pressure at the elevated temperature, preferably in a slow stream or by several small additions (128). In another modification of this process, the length/diameter dimensions of the cylindrical particles—e.g., about 0.1-inch long X 0.02inch diameter—are such that they become spherical as a result of being heated in a suspension stabilized by basic magnesium carbonate at



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INGRAM

517


130°-135°C. The temperature of the suspension is then reduced, and pentane is added at a lower temperature, 118°C. (65). In a process for obtaining simultaneously a rapid rate of diffusion of pentane and good foam-forming qualities, the autogenous pressure of the vessel is maintained constant by increasing temperature in the range 9 0 ° - 1 2 0 ° C . within four hours (93). Polymerization of Styrene Solutions of Volatile Hydrocarbons. ADDITION O F H Y D R O C A R B O N B E F O R E P O L Y M E R I Z A T I O N . Bulk Polymerization. Expandable polystyrene was prepared inadvertently in 1945 in an at tempt to bulk copolymerize 10% isobutylene with styrene. The product formed a low density foam when heated (96). An early method (1950) for rendering polystyrene expandable by petroleum ether was to dissolve 6 parts of petroleum ether in a 40% solution of polystyrene in benzoyl peroxide-catalyzed styrene and to hold the mass for 28 days at 32°C. (124). In a recent version of this process, the monomer (chlorostyrene) and blowing agent ( trichlorofluoromethane ) in a poly (vinyl fluoride) bag were irradiated with γ-rays (105). Suspension Polymerization. The first patent application on poly merizing a suspended styrene solution of petroleum ether (7%) was made in Germany in 1951. The suspending agent was poly (vinyl 2-pyrrolidone), the initiator was benzoyl peroxide (0.7% ), and the tempera ture of polymerization was 82°C. (123). These products were used to develop the popular process for steam molding expanded beads (121). This system was improved by using special initiators to reduce the re sidual monomer content, in one case with a mixture of azobisisobutyronitrile and di-tert-buty\ peroxide, and in the other by using a difunctional peroxide (17, 81). An objectionable feature of these processes is the formation of blisters, craters, and pock marks by pentane escaping from the particles at a critical viscosity during the latter stage of polymerization. This effect can be reduced greatly by increasing the pressure in the polymerization vessel by at least one-fifth of the original pressure by introducing an inert gas such as nitrogen, when the polymerization has reached the bead identity point—i.e., where bead growth by coalescence has ceased (138). ADDITION

O F B L O W I N G

AGENTS

T O S T Y R E N E

SOLUTIONS

O F

P O L Y S T Y

If the pentane is added to a suspension polymerization of styrene after the bead identity point has been reached, the formation of blisters is avoided and the diffusion of pentane into the bead is rapid. Thus, the two objections to the "pentane-in-monomer" process and the "post-poly merization impregnation" processes are avoided (31, 119). The same system has also been used to introduce normally gaseous blowing agents, such as butane, propane, st/m-dichlorotetrafluoroethane, propylene, bu tène, and butadiene (51, 91,115).

R E N E .



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The styrene monomer first may be polymerized to about 65% con version in the absence of water, and then the blowing agent (cyclopentane) may be added with additional peroxide. This solution is sus pended in water in the presence of potato starch, and the polymerization is finished (74). By another modification a styrene solution of "waste" polystyrene and peroxide is suspended by poly (vinyl alcohol) in water, and pentane is added to the suspension after the solids content of the oil phase is greater than 70% (133). Polystyrene particles may also be suspended in water by a mixture of poly (vinyl alcohol) and a phenyl sulfonate and then a mixture of equal parts of pentane and catalyzed styrene (8% each on polymer) is diffused into the polymer. The tem perature is elevated, and the polymerization is completed (23). Suspension Polymerization Systems for Controlling Particle Size. SUSPENDING AGENTS. Both types of suspension stabilizers—the finely divided water-insoluble solids and the soluble film formers—have been used extensively to prepare styrene polymers of a particle size between about 10 and 40 mesh. Examples of such materials are listed below: Finely Divided Solids: Tricalcium phosphate with anionic surfactants (48) Tricalcium phosphate with sodium /^-naphthalene sulfonate and sodium polyacrylate (35) Zinc oxide without or with ammonia (33, 34) Silica plus a diethanolamine adipic acid polymer (56) Bentonite plus gelatin (4) Water-Soluble High Molecular Weight Polymers: Poly (vinyl 2-pyrrolidone) (123) Vinyl 2-pyrrolidone copolymers with alkyl acrylates (16) Poly (vinyl 2-pyrrolidone) (123) alkylnaphthalene sulfonic acid (97) Methyl cellulose (100) Hydroxyethyl cellulose (58) Ammonium poly(vinyltoluene sulfonate) plus anionic surfactant (66) AGITATION SYSTEM. The particle size of polystyrene made in a sus pension polymerization is influenced not only by the type of suspending agent but also is subject to mechanical factors such as shape and size of the vessel and speed of agitation. A review of such mechanical factors in large reactors (50 and 500 liters) was published recently (137). Deposition of Expandable Polystyrene from Solution. Polystyrene is dissolved in a miscible blend of two liquids (80/20 pentane/dimethylformamide), and this solution is precipitated by addition to a non-solvent (methanol), whereby particles of polystyrene containing pentane are precipitated out (116). The polymer may be dissolved in a water-immiscible liquid, then suspended into droplets, and the resulting suspension is partially stripped until a suspension of expandable particles is obtained. For example, a



33.

INGRAM


519

solution of chlorinated polystyrene in methylene chloride is suspended by poly (vinyl 2-pyrrolidone ), and the resulting suspension is simultane ously sparged and agitated by nitrogen (70). Quenched-Pellet Process. This process consists of extruding a mix ture of polystyrene and pentanes, chilling the extrudate to avoid foaming, and chopping the strand into particles of the desired size. The following features were disclosed in the first patent applications: (1) extrusion into a water bath at 50 p.s.i.g. pressure; (2) extrusion into a water bath to cool the strand instantly to below 50°C. at atmospheric pressure; (3) to hold the quenched pellets at an elevated temperature (50°C. to the plasticizing temperature )to relieve orientation strains (45, 77, 104). In the above processes pentane was injected into a molten stream of polystyrene. Several improvements and modifications of the apparatus to make the quenched pellets have been made—extrusion through a die made of or lined with polytetrafluoroethylene to broaden the permissible temperature range of extrusion (6); a conveyor system for passing con tinually the freshly chopped pellets through a normalizing bath to relieve orientation strains at 6 5 ° - 8 5 ° C . (32); an extruder to facilitate mixing of pentane with polystyrene (109); replacement of the water bath by a chilled roll containing grooves in which the extrudate is cooled (21). Orientation strains were relieved by short exposure (0.5-15 sec.) above the heat plasticizing temperature ( 7 7 ° - 9 9 ° C . ) (28). The cell size of foam made from quenched pellets was reduced substantially by institut ing cyclic shock waves in the unfoamed extrudate (68). In another method the pellets were treated to reduce cell size either by storing them at —10°C. or by agitating them until they appeared cloudy (73). The charge to the quenched pellet extruder may be in the form of expandable beads (69). However, because spherical beads tend to slip around the screw in the feed section, they do not extrude smoothly without "stuffing." This problem was overcome by feeding expandable disks obtained from a modified suspension polymerization process (62). Considerable quanti ties (2-40%) of a light weight, finely divided, inexpensive filler, espe cially Wollastonite, may be incorporated without harming the foaming properties substantially (129). The quenched-pellet operation may be incorporated as the finishing step to a bulk polymerization of a styrene solution of pentane (83). The feed to the extruder also may be a mixture of equal parts of polystyrene particles and granulated polystyrene con taining about 12% petroleum ether, impregnated with the assistance of methylene chloride (85). A particular feature of the quenched pellet process is its facility for incorporating colorants and nucleating agents. The latter additives are discussed later. Water-in-Monomer Polymerizations for Open-Celled Foams. Porous polymers or mixed polymers of styrene are prepared by polymerizing the


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monomer with water emulsified therein. These foams are open-celled rather than thç typical closed-cell structure of the foams discussed above (14,131).


Composition of Expandable Polystyrene The Blowing Agent. The blowing agent for most commercial ex pandable polystyrene has consisted of hydrocarbons obtained from the pentane boiling range of petroleum. Expandable polystyrene typically contains about 6.3 to 7.3% pentane. For each composition there is an optimum amount of blowing agent for complete foaming. Only slightly more (i.e., to 1 part per 100 of polymer) than this amount leads to such undesirable properties as sweating, foam collapse, and excessive cell size (63). The tolerable amount of blowing agent decreases with the molecu lar weight of the polymer. A practicable volume of n-pentane (6.5% by weight) for foaming is identical with the free volume determined by the compressibility of amorphous polystyrene (60). Expandable beads made in water suspensions must be dried at rela tively low temperatures (e.g., 5 0 ° - 6 0 ° C . maximum) to avoid excessive evaporation of n-pentane (b.p., 3 6 ° C ) . The loss of n-pentane in one hour is related to temperature and n-pentane content of the beads as shown in Table I (63). Table I.

Loss of w-Pentane in 1 Hour from Expandable Beads at Varying Température

Temperature, Loss (wt. % ) from Beads with an n-Pentane Content of: °C. 7.0% 6.0% 5.0% 4.0% 35 45 52.5 65

0.0 0.0 0.0 0.0

0.0

Addition and Condensation Polymerization Processes

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