In the laundry and garments treatment field, and especially talking about the use of bleaching, there are some questions to be answered, which we could briefly summarised as it follows: for what is the bleaching used for? How many types of bleaching there are? Which is the purpose of bleaching? Which damage can be caused by using the bleach and the optical bleaching? For a better understanding of the subject, it is necessary to illustrate some elements of optics, including the nature of the light, the reflection of the light on the objects and also on the colour perceived by the eye.
The colour comes from the light. The light (of the sun or from a lamp) is electromagnetic energy; the electromagnetic waves are travelling through the space transporting energy; these waves are characterised by the frequency, or better said, by the number of the waves per time unit (the higher the frequency, the greater the energy), and by the wavelength, as the distance between the waves. The eye responds to the radiations in a wavelength range starting from 380 nanometres nm (violet) and up to 740 nm (deep red).
This range is referring to the visible light. The eye is not able to perceive the:
• infrared radiations, with a longer wavelength
• ultraviolet radiations, with a shorter wavelength
The objects appear coloured to us because they reflect back to our eye only a certain wavelength of light. An apple appears red to our eye, because all the wavelengths excepting the longest ones (those that appear red) are absorbed by the surface of the apple. If the surface of a body absorbs all the white light radiations, then that body appears black; if it reflects all of it, it appears white.
The coloured objects contain pigments called chromophores that absorb some wavelengths, while the rest are reflected and they can reach our eyes. The ratio between the radiation reflected from the body and that reflected from a perfectly white reference surface with 100% reflection – evaluated at each wavelength – called reflectance.
Bleaching or whitening means all those treatments whose purpose is to remove from the fabric those particles called chromophores, that are absorbing the light, but that are also capable to reflect it (stains, dirt, ageing, etc.). In some fibres such as cotton, linen, wool and silk there are impurities that cannot be removed by bleaching (60% wool impurities – 25% straw yellow coloured cotton impurity) not found in the case of the synthetic fibres.
As it can be seen in the drawing below, with one hundred percent optimal white reflectance, the cotton has a reflectance of 70-75% due to the impurities present, so that after bleaching, the fabric still has a yellow shade. The same applies to the wool, which after washing is even more yellow than the cotton due to the higher amount of impurities. In the case of the wool, linen or cotton, to improve the whiteness it is necessary to use the optical bleacher with 100% reflectance and with 140% peaks tending towards the blue area (from the drawing).
Optical bleaching For fully understanding the difference between chemical bleaching and optical bleaching it is necessary to analyse the chart (below)
The chart is listing:
• the reflectance values of the standard white (R% 100)
• chemically bleached cotton where the curve is lower than that of the white standard (it is not reflecting the same radiations amount): the chemically bleached cotton sample, lacking the purple component, is more yellow than the standard white.
For improving the whiteness, it is necessary to compensate the yellow tone by applying a special substance called optical brightener. These are organic fluorescent substances capable of absorbing radiation in the “neighbouring ultraviolet” range and emitting optical radiation in the violet-blue range.
These radiations are added to the yellow-red radiations of the material and makes it appear whiter and brighter; the curve of the optically bleached cotton has a reflectance in the violet-blue range, much higher than that of the white standard. In the red-yellow range, while remaining slightly below the white standard, the observer gives more weight to the chromatic effect rather than to that of clarity, and the material becomes less bright (less clear) even considered whiter. The physical phenomenon produced in the optical brightener molecules is called fluorescence.
Fabric seen under the wood lamp with and without optical
Fluorescence is referring to the ability of certain materials of emitting light when they are struck by ultraviolet or visible light (when they emit light of a different colour). The name comes from the fluorite (a mineral composed of calcium and fluorine) that is fluorescent. The fluorescent minerals cease to be bright when the stimulus ends. For being able to observe the fluorescence, it is necessary to use a lamp emitting ultraviolet rays, known as “Wood’s lamp” (used to distinguish the genuine banknotes from the fake ones). The lamp is emitting a dim purple light and a certain amount of ultraviolet rays that are invisible to the human eye.
Defects that bleaching can cause to the fabric
There are two main types of bleaching agents: with hypochlorite and with oxygen.
a) bleach, whitener or sodium hypochlorite. Among the various chemical bleaches, this is the one with the strongest oxidising action and therefore, if it is not used in the right concentrations and at controlled temperatures, it can cause serious damage to the fabric (discoloration, loss of endurance, damages, etc.). The presence of metals can accelerate the bleach action with serious damages (catalytic action).
It has a strong antibacterial and germicidal action. It is unstable to the light and to the heat, so its concentration and effectiveness can vary over the time depending on the goods storage conditions. As already mentioned, there must be taken special precautions when introducing the hypochlorite into the washing bath. Normally, the hypochlorite is commercially available in diluted solutions, its concentration is measured in percentages of active chlorine; commercially available hypochlorite can be found in solutions of 1 to 4% active chlorine in the so-called bleaches, but also in more concentrated solutions of 4 to 8% in the industrial bleaches.
Sodium hypochlorite fault
b) Sodium perborate. It has a weaker action than the hypochlorite; it is a white powder, not very soluble but easily decomposable into the water, releasing hydrogen peroxide; it is generally used in the composition of the washing product or it comes added to another product of the range (enhancer). It is commercialised with active oxygen concentrations of about 10%. The presence of metals can accelerate the perborate action, creating a catalytic damage (transformation of cellulose into water-soluble oxycellulose). It is not efficient as a germicide.
c) Oxygenated water or hydrogen peroxide. It oxidises the fibre impurities, by bleaching it; it is used in concentrations of around 20-30% active oxygen for the bleaching processes. With a concentration of 3%, it is used as a disinfectant. The concentration of hydrogen peroxide in the water solutions is measured in volumes of oxygen, equal to the litres of fresh oxygen developed by each litre produced. 3% of hydrogen peroxide produces 12 litres of oxygen and therefore has 12 volumes. It has a less powerful action than the sodium perborate.
Fabrics colour resistance to the current water
Lart checked in various conditions the strong colour degradation of some materials, by simply soaking them in running water. A piece of fabric, placed on top of a glass, dripped with tap water for 4-5 minutes and then, when the sample is dry, the degradation of the dripping area is being assessed. The colour is obtained by mixing three basic colours (green-blue-red) in appropriate combinations.
One of the three colours (probably blue) tends to fade, causing colour changes and mottling, very similar to the defects caused by the washing with sodium hypochlorite (bleach). The defect affects some specific colours like the light blues, blues, pastels and colonial colours. There are no testing standards and regulations. The degradation is expressed on a grey scale with values from 1 (very bad) to 5 (very good). For selling and attracting the customers, the designers and the companies are creating bright, vivid colours by using the optical bleaching before the dyeing process.
But this practice is often: harmful to the environment and dangerous for the health. Harmful because it reduces the colours lasting – to the various factors of exposure of the garment during its life cycle – but it is also considerably reducing the fabrics maintenance. The bleaching agents may be absorbed by the body in contact with the skin. Various studies have associated the eczema and dermatosis outbreak with the use of these substances.
It is said that the skin exposure to the sunlight, after being in contact with these substances, can increase the probability of dermatosis appearance. These substances tend to accumulate in the animal organs (liver and kidneys) and in the plant roots. They are decomposing very slowly. In order to reduce the risks to health and the environment, it is therefore recommended to avoid the use of these substances. •
Ing. Vittorio Cianci
Direttore LART – Laboratorio Analisi e Ricerca Tessile
Detergo Magazine – Number 4, April 2022