Causes and prevention
The most common causes of yellowing are briefly discussed here:
Oxidation of fats and oils
Oxidised fats and oils have a characteristic yellow/brown colour. Fatliquors that contain large amounts of unsaturated oils are more likely to cause a problem. The amount of unsaturation can be determined by testing the iodine value; values over 100 suggest a highly unsaturated oil.
The risk of the problem occurring is reduced by avoiding unsaturated fats and oils. The use of antioxidants may also be of benefit. A simple test to determine if a fatliquor is likely to oxidise and yellow is to apply a 10% solution of the fatliquor to a piece of white filter paper and place in an oven at 100°C overnight. The degree of yellowing can than be assessed and measured using a grey scale if necessary.
Another feature of yellowing caused by oxidation of oils is that the yellow colour will bleach quite rapidly if exposed to strong light.
Optical brighteners
If an excessive amount of optical brightener is used a yellow colour can become apparent. Also, if optical brighteners become degraded the colour of the leather becomes less blue and, therefore, appears more yellow. Examination of affected and unaffected samples under ultra-violet light should reveal if this is the cause of the problem; the yellow areas will fluoresce less.
Poor light fastness of dyes or pigments
Most leathers will be coloured with a mixture of dyes and/or pigments, some of which may be more light fast than others. For example, yellow azo dyes tend to have better light fastness than red or blue. Therefore, if one or more of the dyes used fades in light, the yellow component may become more apparent.
Light fastness is a highly complex issue and is not solely due to dyestuff selection, eg there may be interactions with fatliquors and retanning agents; premetallised and mordant dyes generally having good light fastness and basic dyes having poorer light fastness on leather.
Oils/fats, dyes or plasticisers have all been known to migrate to the surface of the leather and cause yellow stains. Unsaturated oils are more fluid and, therefore, more likely to migrate. Degreasing a sample of affected leather should indicate if this is the cause.
Dye migration from the leather into the finish, or between layers of the finish can be controlled by careful dye selection, eg cationic dyes tend to be more prone to migration than some others. Dye migration can be identified by pvc migration testing.
Plasticisers can migrate if the leather is exposed to excessively high temperature, eg during drying or ironing.
At BLC we see many cases of discolouration caused by the migration of dyes etc from other components used in the construction of an article. For example, the yellowing of a white jacket due to migration of dyes from the lining as explained in Figures 1 and 2 show that it is essential that manufacturers ensure that all components are compatible with one another to avoid such problems.
Nitrous oxides
Phenolic substances such as fungicides and some types of antioxidants, eg BHT, can react with oxides of nitrogen in the atmosphere to produce a yellow colour. This colour change is reversible; it is yellow in alkali and in acid it becomes colourless (Figure 3). BHT is often found in film packaging, adhesives, polyurethane foams etc from where it can migrate to the leather surface.
Some reactive dyes used in leather can also react with oxides of nitrogen producing an orange/yellow colour, especially if the leather is wet. Oxides of nitrogen are found in low levels in the atmosphere, but can be found in higher concentrations where there is pollution from the burning of fossil fuels.
Acid hydrolysis
If pickled stock is stored for a long time it can acquire a yellow discolouration. While some of the discolouration may be attributed to oxidation of fats, especially on pickled sheep skins which can be very fatty, hydrolysis of the collagen is also likely, especially if it is associated with a loss of strength.
The problem is caused by an excess of free acid and will occur more quickly at elevated temperatures. It can be determined if yellowing of pickled pelts is due to acid hydrolysis by measuring the amount of soluble nitrogen in the collagen, more than 5% indicates a problem. Ensuring that pelts are pickled to equilibrium with the correct amount of acid and storing them as cool as possible should minimise the problem.
Nitrocellulose finishes
Nitrocellulose finishes will yellow with age, especially with prolonged exposure to light. Also, amines released from freshly moulded polyurethrane shoe soling materials can react with nitrocellulose to produce a yellow colouration.
The reaction that produces the yellow colour is also believed to break some of the ester linkages in the nitrocellulose thus increasing its water solubility resulting in a loss in performance with regards to wet rub fastness. Replacing nitrocellulose top coats with polyurethane or cellulose acetate butyrate (CAB) finishes should overcome this problem.