acrylic roof paints for galvanised steel

Architect's memo 4: October 1980

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 finishing coats.

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 steel.

The emphasis of corrosion resistance is taken off the topcoat, allowing the topcoats to be formulated with durability as the single most important factor.

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 Hi-Glo).

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):

1. Micaceous iron oxide
2. Chromium oxide
3. Iron oxide
4. Carbon black/white greys
5. Phthalo blue/white blues
6. 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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