128 Fundamentals of Corrosion
table, the basic elements of nature are placed into classes with similar prop-
erties, that is, elements and compounds that exhibit similar behavior. These
classes are alkali metals, alkaline earth metals, transition metals, rare earth
series, nonmetals, and noble (inert) gases.
Of particular importance and interest, in the case of thermoplasts, is the
category known as halogens. The elements included in this category are uo-
rine, bromine, chlorine, and iodine. Because these are the most electrone-
gative elements in the periodic table, they are the most likely to attract an
electron from another element and become a stable structure. Of all the halo-
gens, uorine is the most electronegative, permitting it to bond strongly with
carbon and hydrogen atoms, but not well with itself. The carbon–uorine
bond is predominant in polyvinylidene uoride (PVDF) and is responsible
for the important properties of these materials. These are among the stron-
gest known organic compounds. The uorine acts as a protective shield for
other bonds of lesser strength within the main chain of the polymer. The car-
bon–hydrogen bond, of which such plastics as polyethylene (PE) and poly-
propylene (PP) are composed, is considerably weaker. The carbon–chlorine
bond, a key bond in polyvinyl chloride (PVC), is still weaker.
The arrangement of the elements in the molecule, the symmetry of the
structure, and the degree of branching of the polymer chains are as impor-
tant as the specic elements contained in the molecule. Plastics containing
carbon–hydrogen bonds (such as PE and PP) and the carbon–chlorine bonds
(such as PVC, ethylenechlorotriuorethylene (ECTFE), and chlorotriuor-
ethylene (CTFE)) are different in the important property of chemical resis-
tance from a fully uorinated plastic such as polytetrauorethylene (PTFE).
Table 4.7 lists the abbreviations used for the thermoplasts. In considering
these polymers for outdoor use, the surrounding atmospheric temperatures
must be considered, as well as the operating temperatures. Certain geo-
graphic areas may have ambient temperatures at times that will be outside
the allowable range of some of the polymers. Table 4.8 provides the allowable
operating temperatures for the basic polymers.
It must be remembered that all thermoplastic polymers are compounded
and in many instances other materials are added to enhance specic proper-
ties. In almost all cases, this enhancement of a specic property results in
a corresponding reduction in another property. One of the properties most
often affected is that of corrosion resistance. Consequently, care must be
exercised in specifying thermoplastic polymers. Check with the manufac-
turer to verify the specic application.
Table 4.9 lists the atmospheric resistance of the basic polymers. Some addi-
tives enhance resistance to atmospheric corrosion, resulting in enhanced
protection from UV degradation and increased moisture resistance.