AN INTRODUCTION TO PLASTICS - Plastics and Environmental Sustainability - page 112

Figure 3.14 Left : An electron micrograph of a thin section of HIPS
showing the rubber microdomains. Right : An electron micrograph of a thin
section of SBR copolymer.
Source: Courtesy of Styrolutions, Styrolux, BASF.

Table 3.5 Properties of Different Grades of Polystyrene

HIPS a SPS b GPSS

Specific gravity 1.04 1.04 1.02

Melt flow (g/10 min) 1-20 2-15
Tensile strength (psi) 5-7 3-7 1.04
Flexural strengths (psi) × 1000 17-15.0 2-8 11.6
Tensile elongation to break (%) 2-3 15-65 30
Flexural modulus (psi) × 1000 450-500 270-420 465
Vicat softening temperature (°F) 195-228 185-225

a Properties depend on rubber content.
b SPS is syndiotactic polystyrene.

The variety of PS that is ubiquitous is expanded polystyrene (EPS) or PS
foam commonly used in cups and food service applications. The expanded
grade is manufactured as beads of PS containing pentane, a liquid
hydrocarbon at room temperature. When the beads are heated in steam
(or “expanded”), the pentane volatilizes to expand the beads into a
thermoplastic foam. These foam beads are subsequently packed into molds
and fused by heat into the shape desired. The EPS can also be extruded into
sheets and thermoformed into products such as meat trays and plates. EPS
has good thermal insulation and shock-absorbing properties.