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Pipe and Pipe Installation Considerations 155

consequence of creep, failure of the material will occur after load is
applied for a certain amount of time. So time dependency is a major
factor to consider in viscoelastic material behavior. An important fact
is that the time to failure is inversely proportional to the applied
stress. In thermoplastic pressure pipe, it is therefore possible to find
and apply a stress level that is low enough to ensure that the theo-
retical time to failure will surpass the design life of the pipeline.
In thermoplastic pipe applications, creep is prevented because
the deflection of the pipe is kept constant, as is the case in buried PVC
gravity (or pressure) pipe. Consequently, it can be seen from Fig. 4.25
that the initial stress decreases with time, and is referred to as the
relaxation property of thermoplastic piping materials. These basic
properties of viscoelastic materials, such as PVC and PE, enable engi-
neers to design pipelines that ensure both structural integrity and the
long-term design life of their municipal piping systems.

4.7.2 Polyvinyl Chloride Pipe
Polyvinyl chloride (PVC) was discovered almost accidentally in the
nineteenth century when German scientists, observing a newly cre-
ated organic chemical gas, vinyl chloride (C H Cl), discovered that
2 3
when it is exposed to sunlight, a chemical reaction took place, result-
ing in the creation of an off-white accumulation of solid material. Since
then, scientists had observed the first polymerization and creation of a
new plastic material, PVC. In 1839, a technical paper was published
detailing the observations of the process. In 1912, several decades after
its accidental discovery, Fritz Klatte, another German, laid the ground-
work for the technical production of PVC. The oldest known PVC
pipe was manufactured and installed in the 1930s in World War II in
Germany and continues to be in service today. The technology was
brought to the United States following World War II and by the mid-
1950s ASTM groups were organized for plastic pipe standardization.

Manufacturing
A vinyl chloride molecule comprises carbon, hydrogen, and chlorine,
configured as shown in Fig. 4.26a. PVC is obtained by polymerization
of single units of the vinyl chloride molecule, which join to create
long chains, and ultimately form PVC resin, Fig. 4.26b.

H H

C C

H CI

(a) (b)
FIGURE 4.26 (a) and (b) Vinyl chloride molecule and PVC chain.