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Soiling and staining tests for upholstery
Resistance to soiling of textile and leather upholstery can influence the consumers’ perception of performance.
Soil-resistant materials will require less frequent and/or less severe cleaning, making them far easier to maintain – particularly in uses such as general household (domestic) and automotive upholstery where the covering may not be removable and will therefore be more difficult to clean.
Leather and textiles produced for upholstery are usually tested to see if they can resist everyday stains such as coffee or red wine. However, despite its importance, there is comparatively little effort placed on assessing materials for their resistance to general soiling.
Requirements of a soiling test
Any soiling test requires a soiling medium and a means of application, such that the resultant soiling is similar to that seen in everyday use.
The soiling medium has to both adhere to the material under test and be easily visible. The simplest way of making the soiling medium adhere, is to base it on an oily or greasy mixture similar to the oils found on human skin and hair which will be absorbed onto the surface of the material under test.
It is made easily visible by incorporating an insoluble marker such as carbon black, so that the more that is absorbed the darker the stain appears.
The method of application is also critical. Although a vigorous application method offers the potential advantage of much shorter test duration, too severe an application could cause abrasive damage to the test piece – which is not the purpose of the test.
The method must therefore be reasonably gentle, making a longer test time inevitable.
Although there is a British Standard method for assessment of the visible soiling by body contact of upholstery fabrics (BS 4948:1994), the soiling medium requires 1,1,trichloroethane, the use of which is subject to certain restrictions.
To overcome this problem and at the same time improve the test, SATRA has developed an alternative method (SATRA TM296:2011 – 'Visible soiling of upholstery materials'). This improved method now offers faster testing and more reliable results.
The soiling medium in the SATRA method is a mixture of oils, mainly of natural origin, for example triglycerides and fatty acids, together with carbon in the form of colloidal graphite as a marker.
This soiling medium is impregnated into wool felt cubes that are then tumbled against the material to be assessed. This produces a gentle soiling action and the lightweight cubes produce little abrasive damage. The duration of the test is controlled by testing a standard white cotton fabric along with each test piece and terminating the test when the cotton shows a predetermined amount of soiling when assessed with a reflectometer.
While BS 4948 is specifically intended for use on upholstery fabrics, with its primary aim to assess the degree of soiling that would occur from the transfer of oils from the body, the SATRA method is applicable to all types of leather, textile and coated fabrics.
Soiling of leather
When assessing upholstery leathers, there are two other advantages of the SATRA method over the BS procedure.
Firstly, it includes a means of artificial ageing or pretreating the leather prior to soiling – this preconditioning has been proven to give results that are much more representative of the soiling that occurs in real use. The ageing process involves firstly a moderate abrasion with grit paper over the grain surface (if appropriate) and then flexing using a modified form of the vamp flex test. These processes replicate the wear and tear of normal use and make the final results more realistic. We are able to vary the severity of these pretreatments depending on likely end-use conditions or omit one or other treatment if we feel it is irrelevant to the leather being tested.
Secondly, the measuring and interpretation of the results for leather include an assessment of shade change, colour change and also descriptive comments. Although primarily intended for upholstery, this test has other general uses such as for clothing and handbags.
Dye transfer or staining
As well as general soiling, light coloured upholstery is also susceptible to colour transfer or staining from dark coloured clothing – particularly denim. Although many clothing manufacturers attach a disclaimer to their products, it is wise to ensure that upholstery fabrics are resistant to this transfer. SATRA has developed two tests to evaluate this problem – a contact storage test and also a modified crockmeter test where a standard dark denim fabric is rubbed against the upholstery fabric or leather.
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