128
Fundamentals of Corrosion
table, the basic elements of nature are placed into classes with similar properties, 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 fluorine, bromine, chlorine, and iodine. Because these are the most electronegative 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 halogens, fluorine is the most electronegative, permitting it to bond strongly with
carbon and hydrogen atoms, but not well with itself. The carbon–fluorine
bond is predominant in polyvinylidene fluoride (PVDF) and is responsible
for the important properties of these materials. These are among the strongest known organic compounds. The fluorine acts as a protective shield for
other bonds of lesser strength within the main chain of the polymer. The carbon–hydrogen bond, of which such plastics as polyethylene (PE) and polypropylene (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 important as the specific elements contained in the molecule. Plastics containing
carbon–hydrogen bonds (such as PE and PP) and the carbon–chlorine bonds
(such as PVC, ethylenechlorotrifluorethylene (ECTFE), and chlorotrifluorethylene (CTFE)) are different in the important property of chemical resistance from a fully fluorinated plastic such as polytetrafluorethylene (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 geographic 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 specific properties. In almost all cases, this enhancement of a specific 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 manufacturer to verify the specific application.
Table 4.9 lists the atmospheric resistance of the basic polymers. Some additives enhance resistance to atmospheric corrosion, resulting in enhanced
protection from UV degradation and increased moisture resistance.
Fundamentals of Corrosion
table, the basic elements of nature are placed into classes with similar properties, 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 fluorine, bromine, chlorine, and iodine. Because these are the most electronegative 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 halogens, fluorine is the most electronegative, permitting it to bond strongly with
carbon and hydrogen atoms, but not well with itself. The carbon–fluorine
bond is predominant in polyvinylidene fluoride (PVDF) and is responsible
for the important properties of these materials. These are among the strongest known organic compounds. The fluorine acts as a protective shield for
other bonds of lesser strength within the main chain of the polymer. The carbon–hydrogen bond, of which such plastics as polyethylene (PE) and polypropylene (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 important as the specific elements contained in the molecule. Plastics containing
carbon–hydrogen bonds (such as PE and PP) and the carbon–chlorine bonds
(such as PVC, ethylenechlorotrifluorethylene (ECTFE), and chlorotrifluorethylene (CTFE)) are different in the important property of chemical resistance from a fully fluorinated plastic such as polytetrafluorethylene (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 geographic 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 specific properties. In almost all cases, this enhancement of a specific 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 manufacturer to verify the specific application.
Table 4.9 lists the atmospheric resistance of the basic polymers. Some additives enhance resistance to atmospheric corrosion, resulting in enhanced
protection from UV degradation and increased moisture resistance.