Chemistry Reference
In-Depth Information
2.7 Coating Applications of Modified Rosin Epoxy
The major concern in protecting ferrous metals is the problem of preventing them
from returning to their naturally occurring state, that is to prevent corrosion. Epoxy
resins provide durable coatings of high mechanical strength with good adhesion to
many substrates. Solvent resistant and chemical resistant films are obtained by curing
at ambient and at elevated temperatures. Low molecular weight liquid or solid epoxy
resins are dissolved in solvents at room temperature to cure with amines. The curing
usually takes 7 days; the resin and hardener components are packed separately. Amine-
cured systems are suitable for marine and maintenance coatings where corrosion
resistance is required. The working pot life of amine/epoxy resin systems depends
on the curing agent, solvent, accelerators, and temperature. Normal stoichiometry
calculations for curing systems use the equivalent weight of amino hydrogen per
equivalent weight of epoxy resin but in practice, the best performance may be given
by other ratios. For good epoxy coatings, the pot life should be maximised and the
cure time at ambient temperature should be minimised.
Cured thermoset polymers are more difficult to analyse than cured thermoplastic
ones. They are insoluble and generally inert. However, the properties are influenced
by factors at the molecular level, such as the backbone structures of the epoxy resin
and curing agent, the nature of the covalent bond developed between the epoxy resin
and the curing agent during crosslinking and the density of cross linking (degree of
cure). Epoxy resins have a wide and increasing use in industry owing to their unique
combination of properties. The absence of volatile byproducts and low shrinkage
during cure results in good moulding characteristics, and the minimisation of internal
stresses gives the thermoset excellent mechanical properties. Furthermore, the cured
resins possess outstanding chemical resistance and they have high adhesive strengths,
due mainly to the generation of polar hydroxyl groups during the curing process.
Although epoxy resins are used as adhesives, they suffer from a lack of flexibility,
which reduces the shear strength and results in low peel strength when cured with
conventional 'small molecule' reagents. Inceasing the epoxide content of resins leads
to a densely crosslinked structure. Improvements in flexibility can be obtained by
incorporating long chain aliphatic compounds into the resin before cure. These may
either remain uncured during curing when they act as plasticisers, or they may be
linked chemically into the system by the cure process when they may be described as
flexibilisers. It is therefore desirable to design both epoxy binders and hardeners based
on cycloaliphatic systems in order to produce cured epoxy resins with good durability
and excellent mechanical properties. The mechanical properties of cured resins have
tested by determining the impact resistance, adhesion, bending and hardness.
