204
Fundamentals of Corrosion
between the components is necessary for the formation of a continuous film.
The interaction with the solvent should decrease to enable the solvent to
evaporate from the curing film. To achieve optimum storage and application
properties, a correct choice of additives is vital. Correct material selection for
coating formulation is often a complicated operation where elaborate practical experience is a requirement.
Organic solvents (water is considered either a solvent or an emulsifier)
usually are required only to apply the coating and, after the application,
are designed to evaporate from the wet paint film. The rate at which the
solvents evaporate strongly influences the application characteristics of the
coating; and if the solvents are partially retained and do not completely
evaporate, quite often the coating will prematurely fail due to blistering
and pinholing. As a general rule, the synthetic resins (vinyls, epoxies, chlorinated rubbers, etc.) are more polar and therefore more readily dissolve
in polar solvents. However, polar solvents are more apt to be retained by
a polar resin system and therefore, when using such resins, particularly in
immersion service, it is imperative that sufficient time is allowed for the
coating to dry. Because these resins depend more on solvents for penetration and flow, they require a greater degree of surface preparation than do
oleoresinous or oil-modified coatings.
Coatings usually are formulated to be applied at ambient conditions of
approximately 75°F (24°C) and 50% relative humidity. If ambient conditions
are considerably higher or lower than these optimum ranges, then the solvent
balance should be modified to provide for better coating application and solvent release. In colder weather, faster evaporating solvents should be used;
and conversely, in hot weather, slower evaporating solvents are required.
Classes and characteristics of some common solvents are given in Table 7.4.
In some cases, organic paint can be mixed and applied without the presence
of solvents. These paint systems are referred to as “solvent free.” Examples
of these are low-viscosity, two-component epoxies and powder coatings. The
application and curing of powder coatings were discussed previously. The
epoxy coatings can be mixed and applied without the use of a solvent, as
the two components typically have low viscosity. Mixing and application of
these coatings are often done at elevated temperatures to reduce the viscosity as much as possible.
7.2.4 additives
Most additives are formulated into paint often in trace amounts to provide a
specific function. For example, cobalt and manganese naphthanates are used
as dryers for alkyds and oil-based coatings to facilitate surface and thorough
drying. These drying additives are added to the paint in amounts usually
less than 0.01%.
Other additives are incorporated into the formula for different purposes.
For example, zinc oxide can be added to retard deterioration of the resin by
Fundamentals of Corrosion
between the components is necessary for the formation of a continuous film.
The interaction with the solvent should decrease to enable the solvent to
evaporate from the curing film. To achieve optimum storage and application
properties, a correct choice of additives is vital. Correct material selection for
coating formulation is often a complicated operation where elaborate practical experience is a requirement.
Organic solvents (water is considered either a solvent or an emulsifier)
usually are required only to apply the coating and, after the application,
are designed to evaporate from the wet paint film. The rate at which the
solvents evaporate strongly influences the application characteristics of the
coating; and if the solvents are partially retained and do not completely
evaporate, quite often the coating will prematurely fail due to blistering
and pinholing. As a general rule, the synthetic resins (vinyls, epoxies, chlorinated rubbers, etc.) are more polar and therefore more readily dissolve
in polar solvents. However, polar solvents are more apt to be retained by
a polar resin system and therefore, when using such resins, particularly in
immersion service, it is imperative that sufficient time is allowed for the
coating to dry. Because these resins depend more on solvents for penetration and flow, they require a greater degree of surface preparation than do
oleoresinous or oil-modified coatings.
Coatings usually are formulated to be applied at ambient conditions of
approximately 75°F (24°C) and 50% relative humidity. If ambient conditions
are considerably higher or lower than these optimum ranges, then the solvent
balance should be modified to provide for better coating application and solvent release. In colder weather, faster evaporating solvents should be used;
and conversely, in hot weather, slower evaporating solvents are required.
Classes and characteristics of some common solvents are given in Table 7.4.
In some cases, organic paint can be mixed and applied without the presence
of solvents. These paint systems are referred to as “solvent free.” Examples
of these are low-viscosity, two-component epoxies and powder coatings. The
application and curing of powder coatings were discussed previously. The
epoxy coatings can be mixed and applied without the use of a solvent, as
the two components typically have low viscosity. Mixing and application of
these coatings are often done at elevated temperatures to reduce the viscosity as much as possible.
7.2.4 additives
Most additives are formulated into paint often in trace amounts to provide a
specific function. For example, cobalt and manganese naphthanates are used
as dryers for alkyds and oil-based coatings to facilitate surface and thorough
drying. These drying additives are added to the paint in amounts usually
less than 0.01%.
Other additives are incorporated into the formula for different purposes.
For example, zinc oxide can be added to retard deterioration of the resin by