The Fogging Test for Automotive Upholstery Leather

The Fogging Test for Automotive Upholstery Leathers

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?

Of all the possible factors that influence the fogging result, natural fats and fatliquors play the greatest role. Natural fats must be removed as completely as possible by the use of suitable surfactants and degreasers. Only fatliquors with very low fogging values should be used for automotive upholstery processing. Fortunately there is a wide range available on the market for the tanner to choose from.
Good clean fleshing is important as this removes the maximum amount of flesh and grease.
Lime splitting makes it easier for natural greases to be removed from the skin, and also makes it easier for the surfactants to penetrate deeper into the skin to assist with grease removal.
A thorough washing after deliming is important to remove any residual ammonium salts as these can combine with formic acid to form ammonium formate which can have a negative impact on the fogging test result. Ammonia-free deliming produces the lowest fogging results of the deliming agents.
Fatliquor stabilizing agents and dyeing auxiliaries can have a negative impact if they contain free or low-ethoxylated fatty alcohols or fatty amines.
Fixation of chemicals in the dyehouse is critical to prevent evaporation of products when exposed to heat. Many tanners fix for too short periods of time and do not allow sufficient time for the chemicals to fix properly to the collagen protein.
Washes should be done thoroughly and preferably by using hot water. This will have the effect of removing water soluble chemicals, salts, unbound auxiliaries and natural greases. The higher the washing temperature, and the more often the process is carried out, the better the removal of these chemicals.
Drying will evaporate volatile chemicals from the leather. The more intensive the drying process, the greater their removal from leather.
Finishing auxiliaries can have a major impact on the fogging test result and therefore tanners should check the entire finishing formulation for its effect on the fogging test.