A method for the determination of the surface tension of cellulosic fibres in their natural state and its relation with chemical composition

The surface tensions of several natural cellulosic fibres like flax, hemp, kenaf and cotton and a synthetic cellulosic fibre have been determined using the so-called floating test. This method determines the liquid surface tension δF at which fibres placed on a liquid surface remain just floating. It can be shown that γF is the liquid surface tension at which the contact angle θ≈0°. By measuring γF in both a polar and an apolar liquid system, the fibre surface tension γS and its dispersive and polar parts, γSd and γSp, respectively, can be calculated using the harmonic mean approximation. The fibre surface tensions found for untreated and extracted natural fibres are in good agreement with literature data for surface tensions of various fibre components such as cellulose, hemicellulose, lignin and waxes. Untreated natural cellulosic fibres proved to be very hydrophobic due to a waxy layer on their surface. Extraction of fatty substances significantly increases the fibre surface tension. This method can be very useful in predicting the wettability of fibres by the surrounding polymer matrix in fibre-reinforced composite materials