A SIMPLE EXPERIMENTAL-METHOD FOR THE MEASUREMENT OF THE SURFACE-TENSION OF CELLULOSIC FIBERS AND ITS RELATION WITH CHEMICAL-COMPOSITION

A method for the determination of the surface tension gamma(S) of cellulosic fibres has been developed. A number of fibres are placed on a liquid surface with a surface tension gamma(L). The liquid surface tension at which 50% of the fibres float on the surface, gamma(F), is determined. It can be shown that gamma(F) is the liquid surface tension at which theta = 0-degrees. gamma(F) is measured in both a polar and an apolar liquid system from which gamma(S)d, gamma(S)P and gamma(S) can then be calculated. Advantages of this method are that it avoids contact angle measurements and that it is operational in spite of the fact that the surface is rough and porous. The coefficient of variation of the gamma(F) measurements appears to be 1-3%. The values found for flax fibres are in good agreement with literature data for surface tensions of various cell wall components such as cellulose, hemicellulose, lignin and waxes. Untreated flax fibres proved to be very hydrophobic (gamma(S) = 28.5-34.2 mN m-1), comparable to waxes. Extraction of fatty substances significantly increases the fibre surface tension (gamma(S) = 40.3-43.1 mN m-1). Additional extractions of pectins and hemicelluloses further raise the fibre surface tension. The remaining fibre mainly consists of cellulose and has a surface tension comparable to that of highly crystalline cellulose (gamma(S) = 60.5-66.1 mN m-1). For the application of cellulosic fibres in composite materials this method can be very useful in predicting the wettability of the fibres by the surrounding polymer matrix.