Chemistry Reference
In-Depth Information
enhanced the solvent resistance for all the mixing ratios of epoxy resins and curing
agents. This can be attributed to increasing the crosslink density by increasing the
epoxy functionalities, although the epoxy polarity is also increased by an increase of
epoxy functionalities. The high solvent resistance of the cured epoxy systems based
on AbA and DAK indicates that the solvent (xylene), used to solublise the polyamide
hardener, was not trapped in the cured system [71].
Whereas most conventional structural materials are suffer severely from attack by
many aqueous acids and bases, most organic polymers are relatively resistant to these
corrosive environments. Only specific functional groups in the polymer may cause
susceptibility to such reagents. In the present systems the structures of the networks
based on AbA and DAK as epoxy binders and curing agents, have acidic, hydroxyl,
ester and amide groups which are sensitive to both aqueous acidic and alkaline
solutions. The results of tests for acid and alkali chemical resistance for cured AbA/
FA3 epoxy resins indicate that these networks posses high resistance to alkaline and
acidic aqueous solutions, although they contain ester groups. The high alkaline and
acidic resistance can be attributed to the high crossink density of the networks which
results from high epoxy functionalities, and this high crosslink density of the networks
decreases their exposure to environment [72]. Reduced chemical resistance was
observed for cured epoxy based on AbA polyamide resins based on DAK at mixing
ratios of 1:1 (wt:wt). It was also observed that the cured epoxy/polyamide resins
based on AbA at mixing ratios of 4:1 showed defects when exposed to both alkaline
and acidic aqueous solutions. This can be attributed to the lower of hardener content
which decreases the crosslink density of the network and increases the susceptibility
of the amide and hydroxyl groups of the network to attack by acidic and alkaline
solutions. The systems based on AbA epoxy and polyamide curing agents based on
DAK have lower functionalities (1.8) and this leads to a cured network with a high
content of amide groups at mixing ratios of 1:1 (wt:wt), which were affected by
acidic and alkaline solutions.
2.7.3 Testing the Corrosion Resistance of Coatings
Salt spray tests are probably the most common tests for corrosion resistance and the
most controversial
.
It is well-established that salts such as sodium chloride can cause
rapid corrosion of ferrous substrates, and it is useful to have data on the ability of
a particular coating system to protecting such substrates from corrosion, both with
intact and damaged coating films.
.
However, salt spray tests are well-established and,
despite the problem of reproducibility, they are quite a useful guide to performance in
the absence of longer term corrosion data and so are unlikely to be discontinued. They
are considered to be unrealistic by some workers because of the degree of acceleration
of the corrosion process that they cause and the variability of the extent of damage
