COLOUR FASTNESS LEATHER TO THE LIGHT BY EGEMEN

There are a quantity of test methods systematically used for colour fastness and for dyes. The more appropriate are:

- Grey Scale for Assessing Change in Colour: This Grey Scale is for assessing changes in colour of leather in colour fastness tests, for example,  wash fastness, perspiration fastness, etc. The scale consists of nine pairs of grey colour chips all representing a visual difference and contrast.

- Grey Scale for Assessing Staining: This Grey Scale is for assessing the degree of staining caused by a dyed leather in colour fastness tests. For  example, the staining of wool and cotton fabrics in the wash fastness,  perspiration fastness, etc. The scale consists of nine pairs of grey colour chips each representing a visual difference and contrast.

- Colour Fastness of Leather to Light: This method is intended for determining the resistance of the colour of leather to the action of a standard artificial light source. The Xenon lamp has an emission wavelength profile close to daylight. The side to be tested of the leather sample is exposed to light from a Xenon Lamp, under controlled conditions, along with eight blue dyed wool standards (blue scale). The light fastness is assessed by comparing the fading of the leather with the fading of the blue standards. The fading is typically made in 2 exposure times to better assist the evaluation.

- Colour Fastness of Leather to Mild Washing: Fastness of the colour of leather to hand washing is the resistance to washing under mild domestic laundering in water. In washing leather, not only changes in colour can
Occur in the leather, but coloured substances may bleed from it and may stain adjacent textile materials.

- Colour Fastness of Leather to Machine Washing: Fastness of the colour of leather to machine washing is the resistance to washing under domestic machine laundering in water. In washing leather, not only changes in colour can occur in the leather, but coloured substances may bleed from it and may stain adjacent textile materials.

- Colour Fastness of Small Samples to Dry Cleaning Solutions: This method is intended only for determining the resistance of the colour and the finish of leather to dry cleaning solutions. It does not cover the suitability of composites or complete leather garments to dry cleaning processes.

- Colour Fastness of Leather to Migration into Plasticized PVC: The colour fastness in respect of migration into plasticised poly(vinyl chloride) - PVC - is the transfer of colour from leather to white plasticised PVC at 50 C. The side of the leather sample to be tested is placed on a white pigmented sheet of
Plasticised PVC and the composite specimen is exposed to heat under pressure in an appropriate apparatus for16 h at 50 C.

- Colour Fastness of Leather to Perspiration: By fastness of colour of leather to perspiration is meant its resistance to the prolonged action of an artifical perspiration solution.

Another way of test is Chrome-free leather. This leather has gradually gained commercial importance, particularly for automobile upholstery applications. In many respects, however, chrome-free leather is inferior to chrome-tanned leather. UV and heat are known to be more detrimental to chrome-free leather than to chrome-tanned leather, especially in regard to the colorfastness of dyestuff and mechanical properties. Temperature, UV radiation, and humidity are key environmental factors that affect leather properties. The role of humidity and its interaction with UV radiation and temperature on leather properties, however, are not clear to the leather industry , and this information is needed for formulation of antioxidants that will protect chrome-free leather from UV and heat damage.

Light of my life!

The problem of poor light fastness is discussed by BLC Leather Technology Centre, Northampton, UK

Published:  08 July, 2003

The problem

Problems due to changes in the colour of leather when exposed to light present themselves on a fairly regular basis and occur across a wide spectrum of leather articles. Most commonly poor lightfastness results in a fading of the colour intensity from dark to pale.

However, sometimes it can also result in distinct changes from one colour to another. This usually occurs when just one dye in the dye mixture used fades, eg black leather often fades to green as the blue and red dyes in the mixture fade. In addition, colour changes in other process chemicals, eg vegetable tanning agents, can cause colour shifts.

The cause

For the colour of an object to be perceived by the eye, it has to be illuminated by visible light (400 to 700nm wavelength). Assuming the object is illuminated with white light (an homogeneous mixture of all wavelengths between 400 and 700nm (Figure 2)), if it reflects all of the light then the object will appear white. If it absorbs all of the light and reflects none then it will appear black. If the object absorbs some wavelengths of light and reflects others it will appear coloured depending on which wavelengths are reflected. For example, if an object reflects only light with a wavelength of 650nm it will appear red. The use of dyestuffs enables the tanner to determine which wavelengths of light will be absorbed and which will be reflected, thus producing the required colour.

When a dyestuff absorbs light energy it raises the dye molecule to an electronically excited state. This excited state is very short lived and the dye molecule rapidly returns to its original state. The excess absorbed energy can be lost in several ways:

* The evolution of heat

* The emission of radiation (fluorescence or phosphorescence)

* Photochemical reactions which can cause degradation of the dye molecule resulting in fading of the colour

It is thought that there are several different photochemical reactions that can occur to produce fading of the colour which can be influenced by:

1. Dye structure; the rate of fading of azo dyes tends to follow the order chemical reactivity: blues>reds>yellows. Also particle size can be influential.

2. Dye concentration; it is thought that high concentrations of dye or pigments has the effect of protecting them from oxidation.

3. Substrate composition; direct contact of the dye molecule with the functional groups of the substrate can degrade the dye by photo-redox reactions, ie oxidation or reduction. Photo-reduction is more common on protein substrates such as leather¹.

4. Atmospheric conditions; moisture allows the diffusion of reactants such as oxygen. Atmospheric pollutants such as sulfur dioxide and oxides of nitrogen can also be influential.

5. Other chemicals within the substrate; some metal ions can affect the length of time that the dye molecule stays in its excited state. Photo-degradation of tannages and retannages can also affect the light fading of leather.

6. The wavelength of light; light at the ultra-violet end of the spectrum is of higher intensity and likely to accelerate fading.

Prevention

It can be seen that the problem of poor light fastness is a highly complex issue and is not solely due to dyestuff selection, although dye selection obviously plays a vital role; premetallised and mordant dyes generally having good lightfastness and basic dyes having poorer lightfastness on leather. The photo-stability of other components in the leather needs to be taken into consideration, eg retannages and fatliquors etc. The presence of metal ions such as copper or nickel should be avoided with iron complex dyes. However, copper can improve the lightfastness of some anionic dyes. The use of water softening agents and sequestering agents such as EDTA should be avoided with all metal complex dyes.

Preventing light reaching the dyestuff in the leather is a useful approach; the use of pigments in the finish or 'pigment dyeing' techniques² provides a significant improvement in lightfastness and UV absorbers can also be used to filter out the more damaging UV light.