Architects memo no. 4: October 1980
acrylic roof paints for galvanised steel
Architects memo No. 2 dealt with galvanised steel as a substrate
for paint and the measures needed to protect it from corrosion and thus
extend its useful life. In this memo we shall deal with the decorative
The two subjects cannot be completely divorced because they have each
affected the other.
Although it is possible to formulate acrylic roof paint for direct
application to galvanised steel, the modifications required to improve
its anti corrosive performance, such as an addition of anti corrosive
pigments, oils, etc., often leads to the detriment of its durability.
Resene maintains their policy of using a primer over all galvanised
The emphasis of corrosion resistance is taken off the topcoat, allowing
the topcoats to be formulated with durability as the single most important
Work in this area of improved durability lead to the present formulation
(these improvements were subsequently incorporated into the house paint
formulation that lead to the combining of the two products into Resene
The requirement for durability becomes obvious when it is realised
that a surface exposed at 45° gets about twice as much radiant energy
as one exposed in the vertical. (This does not take into account further
reductions in radiation to the vertical surface afforded by soffits,
awnings, etc.). Of the paint vehicles commonly available for on-site
application, the acrylic family has the best potential for UV resistance;
and within this class, vehicles of superior durability can be formulated.
Not only have the acrylics UV resistance, they also have the ability
to resist thermal hardening. This is especially important in the roof
paint area where strong colours are the norm. Heat-reflectively values
of the average roof paint colour would lie between 2-10%. That is, they absorb
from 90-92% of the heat striking them. This can result in surface temperatures
up to 80°C.
In discussing durability, special reference must be made to the role
of micaceous iron oxide (MIOX). Paints incorporating this pigment are
well known for their characteristic metallic glint, and have often been
used for this aesthetically pleasing effect. What may not be so well
known is the very high durability this pigment affords paints formulated
with it. MIOX (or flaky haemetite) has a lamella structure that overlaps
within the paint film providing excellent water resistance. It is also
an extremely efficient filter for UV light, protecting the paint vehicle
from its damaging influence and thus greatly extending the useful life
of the coating.
Pigmentation plays a significant role in the durability of roof paints
and would be classified roughly as follows (in order of durability):
- Micaceous iron oxide
- Chromium oxide
- Iron oxide
- Carbon black/white greys
- Phthalo blue/white blues
- Organic pigments in bright red and orange
Acrylics have been discussed as the best materials for durability
and maintenance of aesthetic appeal.
Where aggressive environments demand more corrosion protection than
can be offered by acrylics, chlorinated rubber should be considered
for their better anti-corrosive properties although lacking the UV
durability of the acrylics.
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