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Fundamentals of Corrosion
Related effects can occur when process chemicals are absorbed that may later
react, decompose, or solidify within the structure of the polymer. Prolonged
retention of the chemicals may lead to their decomposition within the polymer.
Although it is unusual, it is possible for absorbed monomers to polymerize.
Several steps can be taken to reduce absorption. Thermal insulation of the
substrate will reduce the temperature gradient across the vessel, thereby preventing condensation and subsequent expansion of the absorbed fluids. This
also reduces the rate and magnitude of temperature changes, keeping blisters to a minimum. The use of operating procedures or devices that limit the
ratio of process pressure reductions or temperature increases will provide
added protection.
5.4 Thermoplasts (Thermoplastic Polymers)
A general rule as to the differences in the corrosion resistance of the thermoplasts can be derived from the periodic table. In the periodic table, the basic
elements of nature are organized by atomic structure as well as by chemical
nature. The elements are placed into classes with similar properties, that is,
elements and compounds that exhibit similar behavior. These classes are the
alkali metals, alkaline earth metals, transition metals, rare earth series, other
metals, nonmetals, and noble (inert) gases.
The category known as halogens is of particular importance and interest in the case of thermoplasts. These elements include fluorine, chlorine,
bromine, and iodine. They are the most electronegative elements in the periodic table, making them 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 predominent
in PVDF and is responsible for the important properties of these materials.
These are among the strongest known organic compounds. The fluorine acts
like a protective shield for other bonds of lesser strength within the main
chain of the polymer. The carbon–hydrogen bond, of which plastics such as
PE and PP are composed, is considerably weaker. The carbon–chlorine bond,
a key bond in PVC, is even weaker.
The arrangement of the elements in the molecule, the symmetry of the
structure, and the degree of branching of the polymer chain are as important
as the specific elements combined in the molecule. Plastics containing the
carbon–hydrogen bonds such as PP and PE, and carbon–chlorine bonds such
as PVC, ECTFE, and CTFE, are different in the important property of chemical resistance from fully fluorinated plastics such as PTFE.
The fluoroplastic materials are divided into two groups: (1) fully fluorinated
fluorocarbon polymers such as PTFE, FEP, and PFA, called perfluoropolymers;
Fundamentals of Corrosion
Related effects can occur when process chemicals are absorbed that may later
react, decompose, or solidify within the structure of the polymer. Prolonged
retention of the chemicals may lead to their decomposition within the polymer.
Although it is unusual, it is possible for absorbed monomers to polymerize.
Several steps can be taken to reduce absorption. Thermal insulation of the
substrate will reduce the temperature gradient across the vessel, thereby preventing condensation and subsequent expansion of the absorbed fluids. This
also reduces the rate and magnitude of temperature changes, keeping blisters to a minimum. The use of operating procedures or devices that limit the
ratio of process pressure reductions or temperature increases will provide
added protection.
5.4 Thermoplasts (Thermoplastic Polymers)
A general rule as to the differences in the corrosion resistance of the thermoplasts can be derived from the periodic table. In the periodic table, the basic
elements of nature are organized by atomic structure as well as by chemical
nature. The elements are placed into classes with similar properties, that is,
elements and compounds that exhibit similar behavior. These classes are the
alkali metals, alkaline earth metals, transition metals, rare earth series, other
metals, nonmetals, and noble (inert) gases.
The category known as halogens is of particular importance and interest in the case of thermoplasts. These elements include fluorine, chlorine,
bromine, and iodine. They are the most electronegative elements in the periodic table, making them 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 predominent
in PVDF and is responsible for the important properties of these materials.
These are among the strongest known organic compounds. The fluorine acts
like a protective shield for other bonds of lesser strength within the main
chain of the polymer. The carbon–hydrogen bond, of which plastics such as
PE and PP are composed, is considerably weaker. The carbon–chlorine bond,
a key bond in PVC, is even weaker.
The arrangement of the elements in the molecule, the symmetry of the
structure, and the degree of branching of the polymer chain are as important
as the specific elements combined in the molecule. Plastics containing the
carbon–hydrogen bonds such as PP and PE, and carbon–chlorine bonds such
as PVC, ECTFE, and CTFE, are different in the important property of chemical resistance from fully fluorinated plastics such as PTFE.
The fluoroplastic materials are divided into two groups: (1) fully fluorinated
fluorocarbon polymers such as PTFE, FEP, and PFA, called perfluoropolymers;