Typical Result of Polymer Surface After Weathering
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Figure 1.1: Weathered and unweathered samples of painted steel. Note the colour change.
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Figure 1.2: Diagram showing paint pigments in a resin matrix. It can be seen that weathering only removes the smaller pigments, resulting in the colour change
Figure 1.1 shows typical weathering results of a sample of painted steel; the paint on the steel is an example of a common polymer system. The sample on the left had been placed in an outdoor exposure rack, and weathered for a total of 6 years. It can be seen that the sample has a chalky appearance and has undergone a colour change in comparison to the unweathered sample on the right.
Colour is determined by light-reflecting chemical particles, pigments, in the paint. These particles can have very different physical sizes, as shown in the diagram in figure 1.2. In this example, the black pigments are the small black dots; red pigments are larger spheres, while the yellow pigments are acicular. This combination of pigments produces the original brown colour. The upper diagram has had no weathering, and the surface is still smooth and undamaged. The lower diagram shows the painted surface after weathering has occurred. The surface has eroded with significant loss of the black and red pigments from the surface layer. The pitted surface scatters light, therefore reducing the gloss and creating the chalky affect. The larger acicular yellow pigments are more difficult to remove, resulting in a colour change towards a more yellowy appearance. Weather testing was paramount in discovering this mechanism. Pigment composition has recently been modified to help minimise this effect.
Read more about this topic: Weather Testing Of Polymers
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