When a kraft pulp is repeatedly wetted in water and dried for several times, the strength properties of handsheets arc considerably deteriorated. The repeated wetting and drying of a recycling process affects the micro-structure of the pulp fibers. The shrinkage of internal pore structure under recycling was not easily reversed. The surface behavior of a single pulp fiber during recycling was characterized by measuring directly changes in contact angle of water with an optical technique. A remarkable increase in water contact angle of softwood bleached kraft pulp (SBKP) fiber was observed by recycling. The recycled softwood kraft fiber is clearly much less hydrophilic than the original fiber. The contact angle of the original CTMP fiber was higher than that of the original SBKP fiber. Furthermore, increased number of recycling decreased the contact angle of the CTMP fiber. That is to say, the increase in hydrophobicity of softwood kraft pulp fiber reduced the inter-fiber bond potential, which causes a loss of tensile strength. A method using confocal laser scanning microscopy and fluorescence resonance energy transfer (FRET) was also applied to image the areas of a fiber-fiber interface which are within the realm of molecular interaction. The FRET range for the SBKP fibers which is within the bonding range of the fiber-fiber interactions decreased with increasing the number of the recycles. In particular, the more rapid decrease was shown in the FRET range after third recycle.
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