BLC News

Smelly leathers cause concern for retailers

24th May, 2011

An odour is caused by one or more volatilized chemical compounds, generally at a very low concentration, that humans or other animals perceive by the sense of olfaction. Leathers and finished leather products often have characteristic odours associated with them. Such odours are cause for concern if derived from harmful/toxic chemicals. Odours can also be chemically 'safe', but negatively affect consumer buying.

Persistence of odour of an object depends upon the relationship of the object to the source of odour. A woman wearing perfume will continue to radiate odour as the perfume evaporates, whereas objects in the immediate vicinity subsequently moving away will only smell similarly if there is a transfer of the perfume. Similarly leather in transit may take up an unusual odour associated with shipping conditions, but this should dissipate readily unless the leather has been in direct contact with the source of the odour.

Sense of smell gives rise to the perception of odours. Smell is mediated by the olfactory nerve. The olfactory receptor cells are neurons present in the olfactory epithelium - a small patch of tissue in back of nasal cavity. There are millions of olfactory receptor neurons that act as sensory signalling cells. Each neuron has cilia in direct contact with air. The olfactory nerve is considered the smell mediator; the axon connects the brain to the external air. Odorous molecules act as a chemical stimulus. Molecules bind to receptor proteins extended from cilia initiating an electric signal. When the signal reaches a threshold it fires, travelling along the axon to the olfactory bulb, part or the limbic system right outside of brain. Interpretation of the smell begins, relating the smell to past experiences and in relation to the substance(s) emitted.

Odour sensation depends on the concentration (number of molecules) available to the olfactory receptors. A single odorant is typically recognized by multiple receptors, and different odorants are recognized by combinations of receptors, the neuron fire helps to identify the smell. The olfactory system does not interpret a single compound but instead the whole odorous mix not corresponding to concentration or intensity.

An object will retain odour for as long as it is contaminated (or treated) with a material that can volatilise and which gives rise to the sensation of smell. For example, if leather in shipment comes into physical contact with a contaminant it may take on a smell that persists until all of the relevant volatile components have volatilised, which could be a considerable period of time. If the leather was simply in the presence of such a contaminant it may take on the smell as a result of the volatiles infusing the leather and subsequently re-volatilising. The persistence of odour in the second scenario should be limited, as the volatiles concerned would be the most readily volatile and therefore will quickly dissipate.

VOCs are emitted by a wide array of products; examples include paints and lacquers, paint strippers, cleaning supplies, pesticides, building materials and furnishings, glues and adhesives etc. The term 'VOC' is commonly associated with solvents and sometimes the terms are used interchangeably. Although most organic solvents can be classified as VOC, there are other chemicals not considered to be solvents that also fall into the classification. Furthermore, the definition of 'volatile organic compound' varies. Within the European Union, it is covered by Council Directive 1999/13/EC, which defines a VOC as 'any organic compound having at 293,15 K a vapour pressure of 0,01 kPa or more, or having a corresponding volatility under the particular conditions of use' (the directive also defines an organic compound as being 'any compound containing at least the element carbon and one or more of hydrogen, halogens, oxygen, sulphur, phosphorus, silicon or nitrogen, with the exception of carbon oxides and inorganic carbonates and bicarbonates'). In the United States, however, the Code of Federal Regulations defines VOCs as 'any compound of carbon, excluding carbon monoxide, carbon dioxide, carbonic acid, metallic carbides or carbonates, and ammonium carbonate, which participates in atmospheric photochemical reactions'. A tannery (or any other organisation for that matter) may or may not be using VOCs depending upon where they operate and which definition they use. In many parts of the world VOC emissions are subject to controls due to the adverse health and/or environmental effects associated with them.

Health effects

The concentration levels can build up in poorly ventilated spaces exaggerating the adverse effects that can be experienced. 'Sick Building Syndrome', for example, has been associated with the build up of VOCs in poorly ventilated offices. The ability of organic chemicals to cause health effects varies greatly from those that are highly toxic, to those with no known health effect. As with other pollutants, the extent and nature of the health effect will depend on many factors including level of exposure and length of time exposed. Key signs or symptoms associated with exposure to VOCs include conjunctival irritation, nose and throat discomfort, headache, allergic skin reaction, nausea, fatigue, dizziness etc .Some organics can cause cancer in animals; some are suspected or known to cause cancer in humans. Benzene, toluene and xylene for example are known carcinogens and, even in countries that have no specific VOC emissions limits, emissions levels of these are controlled.

Environmental effects

The most common VOC is methane, which is a green house gas. Major worldwide sources of atmospheric methane include wetlands, ruminants (such as cows), energy use, rice agriculture, landfills, and burning biomass such as wood.

Other volatile organic compounds (VOCs) are associated with both the creation and the destruction of ozone. At lower levels of the atmosphere (the troposphere - up to ~15 km) they are associated with the formation of ozone, a major component of smog. Reactions of VOC and oxides of nitrogen in the presence of sunlight form smog in the troposphere. The formation of ground-level ozone is a complex process that is affected by many variables. Conversely, when VOCs, especially chlorofluorocarbons, migrate to the stratosphere (15 - 50 km) they break down to form free radicals which react with and break down stratospheric ozone (thus forming the famous hole in the ozone layer). This results in an increase of ultraviolet radiation reaching ground levels with many potential harmful effects such as skin cancers and damage to plant and marine life.

VOC sources in the tannery

Within the tannery most tanners will be familiar with the VOCs that are encountered in the finishing operations associated with leather manufacture. The move from solvent based finishing systems to aqueous based finishing system was largely driven by increasingly stringent legislation designed to limit the emissions of VOCs. Other chemicals (apart from those used for finishing) can also contribute to VOC emissions either in the tannery or from the finished product. The manufacturers of automotive leathers, for example, need to control the emissions of VOCs from the finished leather in order to comply with the fogging specifications set for automotive leathers. A source of volatiles (from pre-finishing operations) that has a major impact arises from the fatliquors that are used, although these are not the only chemicals that can affect fogging performance.

The definitions of both the EU and the US EPA encompass a number of chemicals that many people do not automatically associate with the term VOC. Amongst the several chemicals that are not solvents but which are classed as VOCs, the example that will be known to all tanners is formic acid. However, formic acid is destroyed in the leather manufacturing operations and therefore does not contribute to emissions to the environment. Another non-solvent chemical classed as VOC that most people will be aware of is methane, which contributes to global warming climate change and has a global warming potential 25 times that of carbon dioxide.

Control of VOC emissions

The most common emissions route is to the wider environment and then into the atmosphere. Other pathways can include to water courses and soil systems, where certain VOCs could ultimately find their way into drinking water supplies. Emissions can be controlled by restricting the use of VOCs in manufacturing operations or by some form of abatement technique. The EU Directive indicates that VOCs that are recovered for reuse or destroyed are not included in the calculations for those that are emitted into air or other environmental receptors. The VOCs (or solvents if strictly speaking a solvent management plan is being followed) that would not be counted within the sum of environmental releases includes:

• Organic solvents and/or organic compounds lost due to chemical or physical reactions (including for example those which are destroyed, eg by incineration or other waste gas or waste water treatments, or captured, eg by adsorption.

The use of head-space analysis at BLC can be used to assess whether leather which has an undesirable odour exhibits this odour as a result of some inherent aspect of the leather or whether it is due to contamination. With regard to any undesirable odours that are inherently associated with the leather BLC can assist with overcoming the situation ie identification of odour, risk assessment of chemicals, advice on origin of chemical and replacements.

For further information on odours or VOCs, please contact info@blcleathertech.com .

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