A new approach for estimating purity of processed flax fibre by NIR spectroscopy. An Abstract from Journal of Near Infrared Spectroscopy

Flax must be retted, in which bast fibres are separated from non-fibre components, and then mechanically processed to clean the fibres before industrial application. In the USDA Flax Fiber Pilot Plant, flax is first cleaned through four separate modules and then passed through a Shirley Analyzer to further clean fibres for high-value applications such as textiles. Often, multiple passages through the Shirley Analyzer are employed to obtain higher quality fibres, but it is difficult to determine when the limit for cleanliness is reached by this method. Further, it is clear that materials other than the woody shive components are being removed by Shirley-cleaning, and a method is needed to assess cleanliness beyond the measure for shives. In this study, we attempted to establish an index to determine the degree of purity of flax fibre during the secondary cleaning stage for high quality fibre. Dew-retted (DR) flax and enzyme-retted (ER) flax, which had been first processed through the USDA Flax Fiber Pilot Plant and assessed for shive content, were processed with 10 repetitions of cleaning through the Shirley Analyzer. For both flax samples, absorbances at 1730, 1766, 2312 and 2350 nm decreased with successive Shirley-cleaning steps. These wavelengths appeared to originate from the epidermal layer (EL) that was associated with the flax fibre, an index was calculated using 11 training samples and validated using 10 independent test samples from the same flax samples. Index values gradually decreased with successive Shirley-cleaning steps for both retted flax samples; a lower index value indicated cleaner fibre. Different curves were apparent for the two flax samples, suggesting variations in the cleanliness of the starting material or perhaps differencess in fibre composition. The results suggest it is possible to determine the extent of cleaning of flax fibre using NIR spectroscopy beyond that for shive content based on the epidermal layer of the plant.