The measurement of fibre diameter-the potential of the OFDA measurement technique
Dr Ho Phan (DWI, Germany)
The production of fine fibre in Europe is of increasing importance. Among the criteria determining the market price of speciality fibres, the mean fibre diameter ist the most important one. For example, a small difference of 2 µm in cashmere fineness (Chinese vs. Iranian cashmere) can make the finer fibres 50% more valuable. However, there is a lack of an accurate, standardised method for the determination of the fineness of speciality fibres . For the same keratin fibre sample, a divergence of 3-4 µm in the fineness of the fibre can be measured, depending upon the laboratory and the technique being used.
In 1995, a round trial was organised by the EFFN (European Fine Fibre Network) and the DWI (German Wool Research Institute, Aachen). The main aims were to check the uniformity in measuring cashmere fineness at different stages of processing (dehaired and non-dehaired samples) and to identify the most accurate and reliable tool among the measurement systems:
It was shown that the results gained by measuring the mean fibre diameter of the samples varied considerably between the laboratories, even when the techniques being applied were the same. Within the four techniques mentioned above, the OFDA allows a better differentiation between very similar samples. Compared to the traditional projection microscope, the OFDA technique is much less labourious: a measurement cycle on a clean, conditioned sample takes only about 2 minutes (when using a projection microscope, it would take about 90 - 100 min for measuring 600 fibres.)
Principally the OFDA is an automatic image analysis system: moving fibres of a sample (in form of snippets) are magnified by a microscope being set above the sample holder. The images are captured by a video camera and then identified and measured by a computer system. After measurement, a histogram printout is produced, showing the fibre diameter distribution. The mean fibre diameter, standard deviation (SD) and the coefficient of variation (CV) are included in OFDA’s histogram. Within 2 minutes, more than four thousands fibre snippets can be identified and measured. The accuracy of the OFDA method is therefore higher compared to the traditional one.
The OFDA method has been accepted by the IWTO (International Wool Textile Organisation) as a standard test method for the measurement of the mean and distribution of fibre diameter of sheep's wool (IWTO-47-95). For calibration of this image analysis system, INTERWOOLLABS standard tops have been used, whereby the lowest threshold of the mean fibre diameter of the wool standard lies at 17 µm. The average mean fibre diameters of these standard tops had been determined in projection microscope round trials previously.
Dehaired, fine cashmere down has a mean fibre diameter range from approx. 13 - 18 µm. This is still a lack of fine fibre samples as standards when using the OFDA as a tool for measuring such small mean fibre diameters and this must be rectified. The same applied to mohair fibres. The round trial proposed will overcome this current deficiency.
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