ISO Blue Wool No. 6 for Hot Light Fastness

Solar Radiation Through Car Windows

In their end-use environment, automotive interior materials are expose to very specific and extreme conditions. An important factor is the solar radiation filtered through the cabin glass, which triggers photochemical degradation reactions, e.g. degradation of
• pigments,
• dyes,
• polymers or the
• paint matrix.

Solar radiation can also lead to strong heating due to the closed volume (greenhouse effect).

These are extreme conditions for most materials in automobile interiors (plastics, textiles, paints, leather, electronic components), requiring specific tests. The special case of lightfastness testing under high temperatures is commonly referred to as Hot Light Fastness testing.

Atlas blog blue wool 6 cover

ISO 105-B06 on Hot Light Fastness

The most important test methods for automotive interior materials are summarized in ISO 105-B06

(2020): Textiles, tests for colour fastness: Colour fastness and ageing to artificial light at high temperatures.

This standard uses ISO Blue Wool No. 6 to determine the duration of one exposure period. One exposure period is finished, if Blue Wool No. 6 reaches a grey scale rating of 3 or a color change of ΔE* = 4.3 ± 0.3 (CIELAB). Today, mostly colorimetric measurement is done. Automotive manufacturers refer to this method, requiring total test durations up to five periods in their individual material specifications.

Fading Behavior of Blue Wool No. 6 Changed With Batch Change

During recent years, we have received feedback from customers about inconsistent fading behavior of ISO Blue Wool No. 6. The current batches KSL III and KSL V are fading faster than the previous batch DL 2230. The DL 2230 batch has been on the market for the past decade and users see this as de-facto standard. Test durations according to the requirements of ISO 105-B06 are much shorter with the new Blue Wool No. 6 batches, which results in a confusing situation, not acceptable to industry.

ISO 105-B08 specifies a strict validation procedure for Blue Wools. However, validation is only required to the test conditions described ISO 105-B02 (textile lightfastness). Blue Wool No. 6 has been used for decades also for hot light fastness testing but was never really validated for that purpose.

DEK Determined Batch to Batch Correction Factor

Now the German Fastness Committee (DEK), who is the major producer of ISO Blue Wools, published a correction factor for the latest batches of ISO Blue Wool No. 6 for hot light fastness testing. The correction factor was established by the Saxon Textile Research Institute (STFI) by directly testing next to each other
• new batches 2230 KSL III and 2230 KSL V and
• previous batch 2230 DL

in a Xenotest Beta with Xenochrome 320 filter and ISO 105-B06 conditions 3), until a color change of ΔE* = 4.3 ± 0.3 was achieve on the 2230 DL control specimen.

Atlas blog blue wool 6 image

The test condition 3) of ISO 105 B06 are:

• Irradiance E(300 nm – 400 nm) = 60 W/m²
• Black Standard Temperature: (100 ± 3)°C
• Radiant Exposure H(300 nm – 400 nm) = 14.47 MJ/m²
• Exposure time: 67 hours (average 3 test runs)

For the new batches the following correction factors have been determined:

• 1.24 for 2230 KSL III
• 1.27 for 2230 KSL V

This means that the target values for one exposure period of ISO 105-B06 Condition 3) for the new batches are: • 2230 KSL III: ΔE* = 5.3 ± 0.3

• 2230 KSL V: ΔE* = 5.4 ± 0.3

With these target values the same radiant exposure, and the same exposure duration will be achieved as for the older DL batches of Blue Wool No. 6

DEK Publishes Explanatory Statement

Just recently, DEK published a communication in English and German (soon published on about the above correction factors and stated that this correction cannot be applied to other test conditions or devices without further consideration.

However, a previous ring study resulted in similar results and the purpose of the Blue Wool is to improve test-to-test and instrument-to-instrument comparability. Therefore, the new target values should generally help to determine the end of an exposure period for testing according to ISO 105-B06 conditions 3 as well as for related OEM specifications.