Page 246 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Free Radical Chain Polymerization 209

TABLE 6.8
General Properties of Polystyrene
Density (g/mL) 1.05
Density EPS (g/mL) 0.025–0.200
Dielectric constant 2.4–2.7
Electrical conductivity (S/m) 10 –16
Glass transition temp. (°C) 95
Melting point (°C) 240
Coefficient of linear expansion (1/K) 10 –6

Tensile strength (MPa) 45–60
Young’s modulus (MPa) 3,000–3,600
Elongation at break (%) 3–4

mixture of 90%–95% PS and 5%–10% gas (also referred to as a blowing agent), generally pentane,
nitrogen, or carbon dioxide. The solid plastic is heated generally using steam followed by introduc-
tion of the gas. EPS is produced from PS beads containing the entrapped blowing agents. When
heated, the blowing agent turns to a gas expanding, resulting in the individual beads expanding and
fusing together. XPS is similarly formed except the blowing agents become gaseous as the mixture
emerges from an extruder giving a more continuous product in comparison to EPS.
Extruded PS foam is often known by its trade name Styrofoam. XPS has air inclusions giving
the material some flexibility, low-thermal conductivity, and a low density. It is widely used as com-

mercial and residential insulation. In construction, it is also used to make ornamental pillars that are
subsequently coated with a harder material. Under roads and buildings, it is employed to prevent soil
disturbance due to weathering. Life rafts are generally made from XPS but can be made from EPS.
Styrofoam insulation is estimated to reduce energy costs by more than $10 billion annually.

About 3%–4% of landfill waste comes from old roofs. On the average, roofs need to be replaced
every 7–10 years. Recently, Styrofoam is being employed in forming protected membrane roofs
(PMRs) that insulate as well as waterproofs the roof area. Unlike many roof systems, the Styrofoam
is placed over the PMR. It is believed that such systems increase the life expectancy of the roof to

40 years, thus reducing the landfill component from roofs.
Styrofoam structural insulation sheathing (SIS) has recently been introduced by Dow. SIS is a
wall system that combines structural and water-resistance with insulation properties. It is made with
about 80% recycled material and believed to reduce energy consumption by about 10%.
Expanded PS foam is incorrectly best known to the general public as Styrofoam cups, coolers,
and egg containers. EPS can be generally distinguished from Styrofoam by the presence of small
beads or spheres that are present in the EPS and are missing in the XPS. It is also used to make a
number of other products such as inexpensive surfboards and other water and pool fl oating devices.
It is used to make packing peanuts and molded packing material for cushioning items such as TVs,
computers, and stereo equipment.
Extruded PS is used for objects where a somewhat rigid inexpensive plastic material is needed
such as plastic Petri dishes, plastic test tubes, plastic model kits, CD jewel cases, toys, house wares,
and appliance parts. Medical products are sterilized after the product is made using irradiation or
treatment with ethylene oxide.
Legislation was put in place in some states to insure the recycling of PS. Interestingly, some of
this legislation was written such that all PS had to be recycled within some period of time such as
a year. This legislation was changed to refl ect the real concern of fast-food containers when it was
pointed out that less than 10% PS is used in this manner and that well over twice as much was used
as house insulation that should not be recycled every year or so.

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