on their chemical composition, polyethylene, polypropylene, polyvinyl chloride, and poly-
styrene are the most commonly produced plastics. A grocery bag is an example of a product
made from high-density polyethylene (HDPE). However, note that in a broader sense poly-
ethylene and polystyrene, for example, are thermoplastics. In general, the way molecules of a
plastic are arranged will influence its mechanical and thermal properties.
Plastics have relatively small thermal and electrical conductivity values. Some plastic mate-
rials such as Styrofoam cups are designed to have air trapped in them to reduce the heat con-
duction even more. Plastics are easily colored by using various metal oxides. For example,
titanium oxide and zinc oxide are used to give a plastic sheet its white color. Carbon is used to
give plastic sheets their black color, as is the case in black trash bags. Depending on the appli-
cation, other additives are also added to the polymers to obtain specific characteristics such as
rigidity, flexibility, enhanced strength, or a longer life span that excludes any change in the
appearance or mechanical properties of the plastic over time. As with other materials, research
is being performed every day to make plastics stronger and more durable and to control the
aging process, to make plastics less susceptible to sun damage, and to control water and gas dif-
fusion through them. The latter is especially important when the goal is to add shelf life to
food that is wrapped in plastics. Those of you who are planning to study chemical engineering
will take semester-long classes that will explore polymers in much more detail.Silicon
Silicon is a nonmetallic chemical element that is used quite extensively in the manufacturing of
transistors and various electronic and computer chips. Pure silicon is not found in nature; it is
found in the form of silicon dioxide in sands and rocks, or found combined with other elements
such as aluminum, calcium, sodium, or magnesium in the form that is commonly referred to as
silicates. Silicon, because of its atomic structure, is an excellent semiconductor, a material whose
electrical conductivity properties can be changed to act either as a conductor of electricity or as
an insulator (preventor of electricity flow). Silicon is also used as an alloying element with other
elements such as iron and copper to give steel and brass certain desired characteristics.
Be sure not to confuse silicon withsilicones, which are synthetic compounds consisting of
silicon, oxygen, carbon, and hydrogen. You find silicones in lubricants, varnishes, and water-
proofing products.568 Chapter 17 Engineering Materials
A computer chip.
Source:Péter Gudella /
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