A comparative study of air-dry and water swollen flax and cotton fibres

Abstract

Thermal stability and structural characteristics of air-dry and swollen crude flax, bleached flax and cotton fibres and the behaviour of bound water were analysed using thermogravimetry, microscopy, differential scanning calorimetry, low-temperature ¹H NMR spectroscopy and cryoporometry methods. Both air-dry and swollen fibres contain strongly (SBW) and weakly (WBW) bound water which differ in their behaviour at temperatures below 273 K. All samples have a higher content of SBW than WBW because of the structural features of natural fibres. The air-dry fibres studied have low porosity and similar inner mesoporous structure. The air-dry fibre samples of both cotton and bleached flax contain mainly SBW located in nanopores with radius R < 1–2 nm. In addition to SBW, the swollen fibres contain a significant fraction of WBW, which is located in mesopores with 1 < R < 10 nm, and practically unbound bulk water in large mesopores at R > 10 nm. According to cryoporometry, swelling substantially increases the pore volume (by a factor of 20–30) and specific surface area S_meso (two–three times) of fibres in the mesoporous region. The largest changes were observed in cotton fibres, owing to their chemical structure and textural characteristics affected by swelling. In the nanopore range, swelling reduced the specific surface area of nanopores (S_nano) in cotton fibres and increased S_nano in flax fibres, so that for air-dry samples of all fibres S_nano > S_meso but for swollen samples S_nano < S_meso.