Page 39 - Materials Science and Engineering An Introduction

1.4 Classification of Materials • 11

C A S E S T U D Y

Carbonated Beverage Containers

ne common item that presents some interesting beverages. In addition, each material has its pros and
Omaterial property requirements is the container cons. For example, the aluminum alloy is relatively
for carbonated beverages. The material used for this strong (but easily dented), is a very good barrier to
application must satisfy the following constraints: (1) the diffusion of carbon dioxide, is easily recycled,
provide a barrier to the passage of carbon dioxide, cools beverages rapidly, and allows labels to be
which is under pressure in the container; (2) be non- painted onto its surface. However, the cans are op-
toxic, unreactive with the beverage, and, preferably, tically opaque and relatively expensive to produce.
recyclable; (3) be relatively strong and capable of Glass is impervious to the passage of carbon dioxide,
surviving a drop from a height of several feet when is a relatively inexpensive material, and may be recy-
containing the beverage; (4) be inexpensive, includ- cled, but it cracks and fractures easily, and glass bot-
ing the cost to fabricate the final shape; (5) if opti- tles are relatively heavy. Whereas plastic is relatively
cally transparent, retain its optical clarity; and (6) be strong, may be made optically transparent, is inex-
capable of being produced in different colors and/or pensive and lightweight, and is recyclable, it is not
adorned with decorative labels. as impervious to the passage of carbon dioxide as
All three of the basic material types—metal aluminum and glass. For example, you may have no-
(aluminum), ceramic (glass), and polymer (polyes- ticed that beverages in aluminum and glass contain-
ter plastic)—are used for carbonated beverage con- ers retain their carbonization (i.e., “fizz”) for several
tainers (per the chapter-opening photographs). All years, whereas those in two-liter plastic bottles “go
of these materials are nontoxic and unreactive with flat” within a few months.

Composites
A composite is composed of two (or more) individual materials that come from the
categories previously discussed—metals, ceramics, and polymers. The design goal of a
composite is to achieve a combination of properties that is not displayed by any single
material and also to incorporate the best characteristics of each of the component ma-
terials. A large number of composite types are represented by different combinations
of metals, ceramics, and polymers. Furthermore, some naturally occurring materials are
composites—for example, wood and bone. However, most of those we consider in our
discussions are synthetic (or human-made) composites.
One of the most common and familiar composites is fiberglass, in which small glass
9
fibers are embedded within a polymeric material (normally an epoxy or polyester). The
glass fibers are relatively strong and stiff (but also brittle), whereas the polymer is more
flexible. Thus, fiberglass is relatively stiff, strong (Figures 1.5 and 1.6), and flexible. In
addition, it has a low density (Figure 1.4).
Another technologically important material is the carbon fiber–reinforced polymer
(CFRP) composite—carbon fibers that are embedded within a polymer. These materials
Tutorial Video:
Composites are stiffer and stronger than glass fiber–reinforced materials (Figures 1.5 and 1.6) but
more expensive. CFRP composites are used in some aircraft and aerospace applications,
as well as in high-tech sporting equipment (e.g., bicycles, golf clubs, tennis rackets, skis/
snowboards) and recently in automobile bumpers. The new Boeing 787 fuselage is pri-
marily made from such CFRP composites.
Chapter 16 is devoted to a discussion of these interesting composite materials.

9 Fiberglass is sometimes also termed a glass fiber–reinforced polymer composite (GFRP).