Dirt collection index testing of architectural coatings

Dirt pickup resistance (DPUR)

Dirt pickup resistance (DPUR) represents the ability of an architectural coating (e.g. paint) to resist dirt in exposure to natural environments.

Though it is named “dirt pickup”, it is not defined in terms of the amount of dirt accumulated on a surface, but in terms of the color change of a surface before and after a period of exposure. Due to this definition, surface color changes due to other factors, such as exposure to UV radiation, also impact DPUR.

A detailed description of the dirt pickup mechanism is available in this article: Towards a comprehensive understanding of dirt pickup resistance.

In simple words, DPUR is the color change of an architectural coating due to exposure to the natural environment.

Dirt collection index?

Dirt collection index, Dc, is a property defined in ASTM D3719 for DPUR characterization:

Dc = L*exposed / L*unexposed

where:

  • Dc: dirt collection index
  • L*exposed: L* value of the exposed surface
  • L*unexposed: L* value of the unexposed surface

L* is the lightness of a color in the CIELAB color space. L* = 0 for a perfect black surface and L* = 100 for a perfect white surface.

There are other similar properties characterizing DPUR. To the author’s knowledge, dirt collection index is the only one defined in an international standard. Unfortunately, ASTM D3719 was withdrawn in 2019 and there is no replacement standard at the moment of writing (please leave your comment at the bottom of this page, if you are aware of some alternative international standards on DPUR characterization).

Laboratory testing of dirt collection index

For laboratory testing of dirt collection index, a test sample needs to be measured two times, in the unexposed state and in the exposed state, with the following 3 steps:

  1. Unexposed sample measurement: a new sample is measured before weathering
  2. Weathering: the sample is weathered outdoors for a certain period
  3. Exposed sample measurement: the exposed sample is measured after weathering

For the weathering part, the customers may perform the weathering by themselves according to their preferred conditions.

The colors can be measured with either a handheld spectrophotometer or our UV/VIS/NIR spectrophotometer.

Most customers perform the dirt collection index together with the solar reflectance index (SRI) testing (also in the unexposed and exposed states). In that case, the spectral reflectance data collected in the SRI testing can be re-used to calculate the dirt collection index.

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