by
Dr Clive Jackson-Moss
Most tanners involved in the manufacture of automotive upholstery leather will be very familiar with the term “fogging” and have to design their processes around producing leathers that have a low fogging test result. What exactly is this test measuring, and what steps can the tanner take to reduce the potential of having leathers fail the fogging test?
Fogging occurs when volatile compounds contained in the leather evaporate and then condense as a film on the coldest surface in the car, which is usually the windscreen. This thin film of condensate on the inside of the window reduces the driver’s visibility, and thus reduces the safety of the car’s occupants. The problem is often made worse when the driver tries to wipe the inside of the windscreen with his or her hand, and merely smears the condensate, making the visibility even worse.
There are two different ways in which the leather can be tested (gravimetric or reflectometric). A piece of leather is placed in a glass beaker, which is placed in a hot bath. The sample is heated, and any volatile leather ingredients are condensed on a cooled glass plate or aluminium foil.
Diagram of the fogging test apparatus (Source: World Leather) on the left, and photograph of the fogging test apparatus (Source: www. dstester.co.kr) on the right.
With the gravimetric test, the condensate is collected on aluminium foil and measured by weight. With the reflection test the condensate is collected on glass and the reflectance expressed as a percentage compared with that of a clean glass plate. Every car manufacturer has their own set of standards that have to be met by the tanneries.
Since the motor manufacturers are very strict regarding this test, what can the tanner do to ensure that the leather that is produced meets the strict requirements?