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Colorcoat® Column: Colour and gloss retention

4 January 2010 No Comment

Colour and gloss retention of pre-finished steel are not only aesthetic requirements, they can often be used as indicators of the durability of a product as they are the first signs of coating failure. To understand this, it is important to detail the ageing process of coatings due to exposure to sunlight and how they can be assessed using both natural weathering exposure and accelerated testing.

Product durability can be defined by the ability of the pre-finished steel to withstand attack from a number of different sources including:

1. Water, oxygen, and salts
2. Chemicals
3. Abrasives
4. Sunlight

The ultra violet light portion of the sunlight spectrum is the most destructive to organic coatings as it is directly absorbed at the molecular level by the coating.

What are the effects of exposure to sunlight?

When exposed to sunlight, organic paint systems show a number of reactions indicating that the coating is being affected. These occur in the following order:

1. Loss of gloss or surface shine of the paint. This occurs as the surface breaks down and roughens slightly leading to surface scattering of the visible light.
2. Colour fade due to pigment degradation and surface chalking.
3. Micro-cracking and embrittlement of the paint surface.
4. Total coating failure in terms of localised cracking and peeling of the paint, and in worse cases delamination from the substrate leading to expensive repaint or re-sheet.

The time taken to progress through these to coating failure depends on the pre-finished steel coating, more durable paint systems, which have excellent colour and gloss retention, have better resistance to sunlight and so will last longer.

How is colour and gloss retention assessed?

Product performance can be assessed by a combination of accelerated laboratory testing, accelerated natural exposure  (EMMAQUA) and natural exposure at a range of weathering sites world-wide, which have different climatic conditions.

EN10169 defines the test methodology and classification standards. For UV resistance, products can be classified from RUV1 to RUV4 (most durable) – Colorcoat HPS200 Ultra® and Colorcoat Prisma® both exceed the requirements for RUV 4 classification.

Colour and gloss retention are only one factor to consider when looking at the overall durability of pre-finished steel. Corrosion resistance of the substrate, particularly at the cut edges and abrasion and scratch resistance of the paint coating are also key parameters. The best pointer for durability is the guarantee period and coverage, e.g. does it include cut edges.

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