Architects memo no.
33: October 1983
etch primers
Etch (or wash) primers were first developed in America during
the Second World War. The need to be able to quickly refurbish ships
with limited dry docking facilities spawned the need for a fast drying
primer which was tolerant of application over damp surfaces.
Etch primers, a mixture of zinc chromate, phosphoric acid and polyvinyl
butyral, was the answer to the need. They dried very quickly; could
be overcoated with standard paints, and developed a remarkable adhesion
by a series of complex interactions which are not fully understood even
today. They were two-pack materials in which the acid component was
added just before mixing, applied in very thin films, and had a characteristic
translucent appearance.
It can be seen that one of the major uses of etch primers today (the
pretreatment of 'difficult' non-ferrous metals) was not even considered
in the development of the product.
The major drawback of this type of product is water sensitivity (even
though it can be applied to damp surfaces), and premature failure can
occur through exposure to rain, dew, or even high humidity in the early
life of the film. It is also sensitive to UV light. The product, as
mentioned, is a two-pack system with a limited pot life at the expiry
of which it is unable to achieve adhesion. Unfortunately, this change
occurs without any other physical signs and it is possible to carry
on applying the material unknowingly past its useful pot-life.
Clearly the usefulness of this type of product could be increased
if these shortcomings were overcome. The water and UV resistance were
upgraded by modifications with phenolic resins and iron oxides and this
type of formulation has found a lot of use. The appearance is very different
from the original 'wash' primer and its resistance allowed it to be
used as a 'shop' or 'holding' primer where it may have to take some
weathering before overcoating. This type have found use as primers for
galvanised iron roofs because they can withstand the vagaries of the
weather that can often delay the application of the topcoats.
The next step, formulating this type of product into a stable one-pack
coating, required some sophisticated chemistry. It involved temporarily
complexing the acid component while it is in the can, and releasing
it when solvent evaporation occurs during drying.
In 1983 Resene released Resene
Vinyl Etch that represents these developments and Data
Sheet RA31 describes some of the remarkable properties of the product.
To summarise, the improvements this type of product has over the original
etch primers are:
- Improved water and weather resistance allowing delayed topcoating
schedules.
- No onsite metering of individual components thus removing a major
possibility of error.
- No finite pot-life removing the danger of applying 'spent' material.
- Higher film builds with consequent improvements in corrosion resistance.
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