Chemistry Reference
In-Depth Information
2.7.2 Evaluation of Chemical Resistance for Cured Resins
There are two types of test method to evaluate the durability of coatings: chemical
resistance tests and mechanical tests. In the previous section, the results of mechanical
tests were evaluated for the cured epoxy based on AbA and DAK. In the present
section, the chemical resistance test will be evaluated. To this end, panels were coated
with cured epoxy systems (based on both AbA and DAK) and the resistance of the
coatings to alkali, acid, solvents, water and to salt spray was determied. The coating
films which fail the tests lose their adhesion to panels and show cracking and flaking of
the film. There are many reasons for coating failure but some are readily identifiable,
and attempts can be made to explain or illustrate them. Failure of coating systems will
be due to either one of adhesion loss and film durability or to a combination of them.
An extensive range of tests has been devised and the results obtained from these give
an indication of the probable performance of coating films based on their durability.
Solvent resistance may be tested for many different reasons. Tests for resistance
to petrol and diesel fuel are carried out on compositions that may be expected to
encounter contact or intermittent splashing with these liquids, e.g., motor vehicle
finishes and storage tank coatings. The measuring of solvent resistance is also
important because polar solvents such as methyl isobutyl ketone or acetone are
often used to assess the degree of cure of a crosslinkable composition. In addition to
immersion testing, solvent resistance may be assessed by a solvent rub test. Acetone
has been used to determine the degree of curing of all coating systems based on rosin
by both the immersion and rub methods. Failure of the tests was demonstrated either
by disruption or dissolution of the coating films from the panels. Generally, solvent
resistance depends primarily on the polarity of cured network resins. Nonpolar
polymers show solvent resistance to water, acetone and other polar solvents, whereas
polymers containing sites for hydrogen bonding are most effected by moisture,
humidity and polar solvents. Moreover, the molecular weight and crosslink density
of polymer networks are directly related to their resistance to solvent attack. This is
due to the thermodynamic relationship between the polymer network structures and
the solvent. Crosslinking is the ultimate structural factor in preventing a polymer
from dissolving in a solvent. Although this cannot completely eliminate the effects
of polarity and hydrogen bonding, it raises the molecular weight to the size of an
infinite network, preventing 'individual' polymer chains from dissolving in the
solvent. The higher the degree of crosslinking, the less free volume and segmental
mobility remain available in the polymer so solvent molecules can hardly penetrate
the crosslinked network at all. The crosslink density can be controlled by changing
the type of curing agents and the functionality of epoxy resins [70]
.
In this respect, it
was found that all the prepared epoxy resins based on both AbA and DAK had good
solvent resistance with all the types of prepared curing agents at epoxy:curing agent
ratios of 2:1. It was also noted that increasing of the epoxy functionality from 2 to 5
