AN INTRODUCTION TO PLASTICS - Plastics and Environmental Sustainability - page 91

the entire ensemble of chains is a single giant molecule (this actually does
occur in natural rubber when vulcanized or cross-linked.) An automobile
or aircraft tire 8 is an example of a fully cross-linked polymer. On heating,
cross-linked polymers do not convert into a viscous liquid “melt” as the
molecules are chemically linked to one another and cannot flow
independently.
From an engineering perspective, plastics fall into two broad categories:
thermoplastics and the thermosets . Thermoplastics refer to those plastics
that soften and flow on heating, allowing them to be molded or formed into
differentshapes.Thechangeonheatingisaphysicalchange(aswithheating
candle wax), and the chemical nature of the polymers remains unaltered.
Thermoplastics can therefore be recycled as they can be melted and
reformed into different products (Subramanian, 2000). Thermoset plastics
on the other hand are cross-linked polymers such as vulcanized rubber,
polyurethanes, glass-reinforced polyester, or epoxy resins that do not melt
or flow on heating and cannot therefore be remolded into a different shape.
Whenheatedtohightemperatures,thematerialsimplydegradeschemically
into small molecular products.
3.2.5 Glass Transition Temperature
Another interesting phenomenon that is unique to polymer materials is the
phenomenon of glass transition. In amorphous glassy plastics, the chains
are frozen in place and unable to move around (translational movement)
within the bulk of the material at ambient temperatures. On gradual
heating, thethermal orvibrational energy absorbed bythemolecules results
in increased motion of sections (short segments that are only 6-10 carbons
long) of the polymer chains. With segments of this length moving or flexing
as a single unit, additional free volume is created in the bulk, and the
polymer characteristics abruptly change. The temperature at which this
change occurs is the glass temperature 9 Tg of the polymer. As the name
suggests, the hard glass-like microstructure of the plastic is converted into
a softer rubbery material (it is also called glass-rubber transition) at this
temperature. Other modes of mobility of even shorter segments of the
chains or just the side chains can occur at specific lower (sub-Tg)
temperatures in the amorphous glassy state.
The Tg of the polymer is not a characteristic property of the material but
depends on the rate of heating during its measurement. With polymers that