Understanding Polystyrene: History, Production, and Applications

Posted on Dec 14, 2025 in Chemical Engineering

Historical Background and Production

Polystyrene is an aromatic thermoplastic resin made by the polymerization of the monomer, styrene. Styrene readily polymerizes in air, so long before the concept of polymerization was defined, it was known that it has the capacity to convert under certain conditions into a clear resinous solid that is almost odor-free.

Polystyrene (PS) was first synthesized in 1839 by Eduard Simon. Unknowingly, this was the first recorded instance of polymerization [3]. However, the conclusion that styrene had polymerized was not reached until 75 years later. Hermann Staudinger won the Nobel Prize in Chemistry for his research in 1953. By 1941, Germany had already established an industrial styrene monomer production process, a styrene-butadiene elastomer process, and a mass styrene polymerization process [3].

1. Reaction kettles in the BASF polystyrene production plant, 1940 (3)

2. Photograph of a styrene polymerization vessel inside the I.G. Farben BASF plant in Ludwigshafen, Germany, 1940 [3]

In 1930, Dow started to produce styrene monomer by cracking its ethylbenzene precursor. In 1938, Dow began manufacturing commercial quantities of PS. After this time, the polystyrene (polymerized to approximately 99% conversion) was removed from the can and crushed to a free-flowing powder (Figure 3). Early photograph of the ‘can’ process for the commercial production of PS [3].

Styrofoam was commercialized in the United States in 1954 by the Dow Chemical Company [4].

Nowadays, the plastics industry is by far the largest of the main polymer-based industries, as it can be observed in Figure 4.

4. U.S. consumption volume in billions of pounds for the three largest polymer-based industries. [4]

As of 2000 (Figure 5), polystyrene occupies the fourth place in global plastics consumption.

5.

There are three major types of PS that are produced:

• General Purpose Polystyrene (GPPS) is the result of styrene monomer polymerization. It is a transparent solid product manufactured in the form of 2-5 mm pellets.
• High Impact Polystyrene (HIPS) has enhanced impact resistance and toughness due to its content of polybutadiene.
• Polystyrene Foam: There are two types of PS foams: expanded polystyrene (EPS) and extruded polystyrene (XPS). XPS is much denser and more durable, and it is used in larger applications such as architectural models. Table 1 shows some of the principal properties of pure styrene.

Properties of Styrene Monomer

Relative Molar Mass

104.14

Density at 25°C

901 kg m^-3

Refractive Index at 25°C

1.5439

Boiling Point

145.2 °C

Melting Temperature

-30.6°C

Volume Shrinkage on Polymerization

17%

Table 1. Properties of Styrene [9][4]

Polystyrene (PS) is a clear, amorphous, nonpolar commodity thermoplastic. Depending on the way it is polymerized, it can be hard and transparent like glass or foamed and expanded into a soft, white insulating material. Characteristic reactions of a phenyl group such as chlorination, hydrogenation, nitration, and sulfonation can all be performed with PS [3]. Chain rupture and discoloration are frequently additional effects of such reactions. Both GPPS and HIPS can be colored with colorants.

Properties of Polystyrene

Property

Units

Conditions

Value

Density

g cm^-3

Amorphous

1.04 – 1.065

Glass Transition Temperature

°C

100

Thermal Decomposition

°C

Initial Temperature

300

Half Decomposition Temperature

366

Upper Use Thermal Decomposition Temperature

°C

61

Vicat Softening Point

101

Thermal Conductivity

W m^-1 K^-1

0.17

Dielectric Constant

At 1 kHz

2.49-2.55

Dissipation Factor

^-4

Dielectric Strength

V mm^-1

2.36 ·10⁴

Dielectric Loss

At 1 kHz

15·10^-4

Heat Capacity

kJ K^-1 mol^-1

T=100K

0.04737

Heat Conductivity

J s^-1 m^-1 K^-1

0.13

Refraction Index

l=589.3 nm

1.59 – 1.60

Optical Dispersion

l=486.1 nm

1.92·10^-2

Tensile Strength at Break

MPa

30 – 60

Elongation at Break

%

1 – 4

Flexural Strength

MPa

95

Compressive Strength

MPa

95

Table 2. Properties of PS [7]

PS has a high refractive index of 1.59 and “brilliance” in appearance. PS has good electrical insulation properties as it has a low dissipation factor.

PS has good resistance to water absorption (0.03%–0.1%/24 h). Above 100°C, PS softens easily but it does not exhibit a sharp melting point because it is amorphous.

PS exhibits pseudoplastic behavior. PS chemical resistance is presented in Table 1.

Polystyrene Chemical Resistance

High Resistance

Moderate Resistance

Not Resistant

• Higher-molecular-weight alcohols such as butyl alcohol, n-amyl alcohol, n-octyl alcohol

• Soft drinks such as colas, ginger ale, etc.

• Low-molecular-weight alcohols such as methanol, ethanol, allyl alcohol

• Aliphatic amine compounds such as ethylene diamine, triethylene tetramine

• Milk, etc.

• Aromatic and aliphatic hydrocarbons such as toluene, xylene, butane, gasoline, diesel oil, fuel oil, turpentine, kerosene
• Halogenated compounds such as carbon tetrachloride, chloroform, bromine liquid, bromoform, refrigerants
• Oxygenated materials such as esters, ketones, ethers, anhydrides, aldehydes (butyl acetate, acetone—nail polish remover), tetrahydrofuran, acetic anhydride, phenol, propylene oxide, acetaldehyde
• Unsaturated oils such as citronella oil, lemon oil, anise seed oil, boiled linseed oil, almond oil, wintergreen oil
• Concentrated mineral and organic acids such as sulfuric, nitric, glacial acetic
• Nitriles such as acrylonitrile, acetonitrile
• Aromatic amines such as aniline, etc.

Table 3. PS Chemical Resistance [4]

PS’s good chemical resistance makes it sterilizable by gas and chemical disinfectants. PS is not sterilizable by gamma irradiation and dry heat.

PS has high surface hardness and gloss but is easily scratched. Most commercially available PS qualifies for FDA approval for food contact applications.

The main problems regarding PS’s properties are:

• It is brittle (amorphous).
• It has poor impact resistance.
• Its inability to withstand the temperature of boiling water.

These weaknesses can be overcome by copolymerization with different monomers. ESCR of HIPS can be enhanced by increasing its rubber particle size, percentage grafting, and percentage of gel. This problem can be corrected by compounding with UV stabilizers such as 2-hydroxybenzophenones (octabenzone) [4]. PS is not autoclavable. Fire Spreading Properties of PVC and Other Selected Thermoplastics [4]

Figure 3 shows the timeline of discovery of various styrenic polymers and copolymers:

6. Timeline of the development of styrenic polymers (BASF document by Franz Haaf, entitled ’50 Jahre Polystyrol – Entwicklung’, BASF, Ludwigshafen)

Chemistry of Polymerization

Styrene Synthesis

Styrene is widely produced starting from ethylbenzene followed by dehydrogenation over ferrum oxide catalysts [7]:

A different pathway is to start from toluene and ethene:

Another way to obtain styrene is by the oxidation of ethylbenzene:

Another possibility is to synthesize styrene from butadiene:

Polystyrene is formed by chain-growth polymerization. This is due to the low polarity of the styrene molecule and to the resonance stabilization of the growing polystyryl species in the transition state.

Radical Polymerization

Styrene can undergo spontaneous polymerization by thermal initiation. Diffusion of the radicals from the cage and subsequent polymerization to polystyrene is, in fact, only a side reaction in this scheme (Figure 7).

Figure 7. Reactions during styrene oligomer formation in the chain initiation phase [3]

Initiators In addition to the thermal initiation, there are a lot of initiators that can be used for the radical polymerization of styrene.

An example of a peroxide initiator is benzoyl peroxide:

Azobisisobutyronitrile (AIBN) is an example of an azo compound initiator:

Recently, multifunctional peroxides have also been used in order to obtain products with special molecular weight distributions [7].

Chain Transfer The chain transfer agent can be the solvent, monomer, initiator, polymer, or an added chemical agent. Chain transfer is an important aspect regarding the control of molecular weight, so an adequate chain transfer agent should be chosen. The transfer to monomer has a value of 10^-5 which can be neglected [7].

Inhibitors To prevent premature polymerization of styrene monomer, special inhibitors have to be added so that it could be stored until needed. t-Butyl-catechol at 15 to 50 ppm is the most common inhibitor for commercial styrene. The inhibitors have to be removed before polymerization in order to avoid an induction period [7].

The Mechanism of the free radical polymerization of styrene has three steps:

Initiation – radical initiator attaches to the styrene monomer double bond to form an active polymer radical.

Propagation – the polymer radical grows into long polymer chains by successive addition to the styrene double bonds.

Termination – two active polymer chains react with each other to form a stable carbon–carbon bond.

The overall reaction is:

The main disadvantage of radical polymerization reactions is their low selectivity; neither the molecular weight, the molecular weight distribution, nor the molecular structure can be precisely controlled.

Controlled Radical Polymerization

Controlled Radical Polymerization attempts to avoid the bimolecular, irreversible termination reactions, typically obtained in free radical polymerization (combination, disproportionation, etc.) by decreasing the number of growing radical chains.

All these substances react with the growing macromolecular radicals by forming temporarily dormant species, minimizing termination by recombination or disproportionation. This results in a low overall polymerization rate, but the molecular mass can be very well controlled and very narrow molecular weight distributions can be obtained [7].

Anionic and Cationic Polymerization

The phenyl group of styrene is able to act as an electron-donating or an electron-withdrawing center. This allows the growing end of the polymer to be either a carbenium ion or a carbanion.

Coordination Polymerization

Styrene can be polymerized to stereoregular structures by coordination catalysts. Highly isotactic polystyrene is prepared using Ziegler–Natta-type catalysts.

Syndiotactic polystyrene is obtained using a mixture of methylaluminoxane (MAO) and cyclopentadienyltitanium(III) chloride as catalyst. The stereocontrol in this catalyst is induced by the phenyl groups of the growing polymer chain and not by the symmetry of the catalyst as in most types of coordination catalysts.

Manufacturing Methods

PS may be prepared by bulk, solution, emulsion, or suspension techniques.

Styrene polymerization to polystyrene is a highly exothermic reaction. Therefore, temperature control is highly important when considering the process because polystyrene degrades rapidly at temperatures above 250 °C and the molecular weight of the polystyrene produced decreases rapidly with temperature [3].

Bulk

Bulk polymerization of PS can be either batch or continuous. Bulk polymerization is the most used process for making crystal clear PS, mostly due to its purity advantage and high conversion efficiency [4]. Heat can only be taken away from the points of higher temperature by conduction because of the very high viscosity of the reacting material, and also the low thermal conductivities of both styrene and PS [9].

Solution

In solution polymerization, styrene is diluted with solvents, which reduces the problem of heat transfer and the physical movement of viscous masses. The main disadvantage consists of the solvent recovery and the possibility of chain transfer reactions [9].

Suspension

Suspension polymerization is most suitable for large-scale production of polymers of high average molecular weight. Suspension polymerization can be used to produce crystal PS, IPS, and EPS beads. Contamination with stabilizing agents is considered a disadvantage [3].

Emulsion

Emulsion polymerization requires water as a carrier with emulsifying agents. The particles of rubber are finely dispersed in the rigid phase; the impact-modified styrenic copolymers thus have a multiphase structure.

Figure 8. Impact modification of styrene copolymer via emulsion core/shell [3]

Applications

PS can be processed by all techniques used in the processing of thermoplastics, but most of the PS products are made mainly by injection molding, extrusion, and thermoforming.

Packaging is the largest application area for PS. Globally, packaging accounts for 36% of all PS usage [4]. The PS used in packaging is atactic, so it cannot crystallize.

PS is a very versatile protective food packaging material. The clarity of PS packaging enables customers to see exactly what they are buying, and this is a unique selling point, particularly for supermarket shopping. 4; Dow Product Safety Assessment, General-Purpose Polystyrene, http://www.dow.com.cn/productsafety/pdfs/233-00520.pdf)

Injection-Molding Applications of Polystyrene

GPPS and HIPS are both used in the making of injection-molded products. Injection Molding [10]

Some injection-molded applications are:

1. Crystal and impact-modified PS are used for tumblers, lids, toys, cutlery, air conditioner and fan grills, refrigerator parts, medical vials, syringes, culture dishes, and test tubes, lighting shields, etc.

Figure 10. Extrusion [10]

Extruded foams are used mainly for low-temperature insulation situations such as:

• Liquefied natural gas (LNG) storage tanks
• Truck bodies
• Refrigerated pipelines
• Railroad cars, and others

Extruded PS boards are used for residential insulation in styrofoam form and as core materials for structural sandwich panels.

Thermoformed Polystyrene Applications

Extruded PS sheets are used in thermoformed packaging materials such as:

• Egg cartons
• Meat and poultry trays
• Fast-food containers, etc. The formed product is cooled and trimmed to create a finished and usable product.

Figure 11. Extruded impact PS sheets are most suitable for products such as cold cups, dinnerware, portion containers, and lids. Extruded PS foam products include dinnerware, hinged trays, clamshells that are often used in take-out, restaurant fast-food items, plates, and bowls.

PS is very popularly used in the making of blister packs, which are more cost-effective of the two. Clamshells are used for packaging and effective display of products such as fishing lures, thumbtacks, and others [4].

Health and Environmental Impact

Health Effects

Ingestion of styrene-contaminated foods is the main way styrene can enter the body. Studies show that the amount of styrene migrating from the polymers is proportional to the square root of the time of exposure [12]. The health effects for people exposed to styrene for longer periods of time are not known [11].

Recycling

Incineration is the most common method for PS disposal. The burn temperature must be maintained over 1000⁰C, and it must be burned with excess oxygen to break down chemicals. If it is burnt below 900°C, up to 90 dangerous compounds can be released such as alkyl benzenes and carbon monoxide [13]. The National Bureau of Standards Center for Fire Research identified 57 chemical byproducts released during the combustion of polystyrene foam [14].

The byproducts of incineration are:

• CO₂ (as exhaust gas)
• Water vapor
• Soot (biochar – easily biodegradable)
• Heat

Combustion is an effective method for reducing the volume of waste; however, the gaseous emissions are a grave environmental concern.

ReprocessingThe high thermal stability of polystyrene makes it possible for it to be reprocessed to give a material that can be equivalent to the starting material.

Polystyrene foam can be recycled and reused by different methods. Because chlorofluorocarbons are a source of concern due to the fact that they contribute to ozone depletion, extensive research has been conducted to find alternative materials [3]. Table 5 lists a number of blowing agents and their respective properties for flammability and environmental consideration.

Table 5. Flammability and environmental considerations for blowing agents [3]

While the technology for recycling polystyrene is available, the market for recycling PS is very small. There are currently very few products for which material recycling is worthwhile from the point of view of energy consumption and recycling costs. The two most commonly used methods for reducing PS waste are landfilling and combustion. A viable option for minimization of waste can be source reduction and reuse of containers and packaging.

References

[1] Simon, E., 1839. Annals of Chemistry 31, 265

[2] Blyth, J. Chem., 53, 289 (1845).

[3] JOHN SCHEIRS and DUANE B. Kricheldorf, Graham Swift, Oskar Nuyken, Handbook of Polymer Synthesis, Second Edition, 2005

[8] http://courses.chem.psu.edu/chem36/SynFa06Web/Expt44.pdf

[9] Brydson

[10] Bill Fry, Working with Polystyrene, 1999

[11] TOXICOLOGICAL PROFILE FOR STYRENE, Agency for Toxic Substances and Disease Registry U.S. Public Health Service, 1992

[12] Johannes Karl Fink, Handbook of Engineering and Specialty Thermoplastics Volume

[13] https://wmich.edu/mfe/mrc/greenmanufacturing/pdf/Polystyrene%20Recycling.pdf

[14] http://www.earthresource.org/campaigns/capp/capp-styrofoam.html

[15] http://www.intcorecycling.com/How-To-Recycle-Polystyrene.html

[16] De J. Wünsch, Polystyrene: Synthesis, Production and Applications, 2000