Principal component analysis (PCA) was used to evaluate the results of standard fiber analyses, determinations of charge, electron spectroscopy for chemical analysis (ESCA) measurements, and selective staining of kraft fibers prebleached with oxygen, followed by hydrogen peroxide or ozone. The majority of data variance is explained by the lignin content in fibers and by polarity (hydrophilicity vs hydrophobicity) of functional groups. The lignin determination methods (kappa number, C1 (ESCA), selective staining) gave similar but not equal results, because they measure different parts of lignin. The determination methods of the charged groups (total charge, surface charge, C4 (ESCA), and hexenuronic acids) also gave similar but not equal results. The results of staining by using cationic dyes do not correlate with the quantity of anionic (mainly carboxylic) groups in fibers, regardless of whether the dyes are selective for lignin or hemicellulose. Hydrogen bonding and hydrophobic interactions seem to overrule ionic interactions between dyes and fibers. Therefore, the majority of bonds formed between fibers themselves, as well as between fibers and paper additives, can to a great extent be expected to have the character of hydrogen bonds.
An investigation of the fiber samples with various amounts of lignin, hemicelluloses and cellulose was conducted by means of dye adsorption, spectrometric and chemical analyses. Five fiber samples were prepared by the oxidation of unbleached Norway spruce thermo-mechanical pulp with acidic potassium permanganate solution. Each sample was oxidized with defined amount of KMnO 4 and the degree of oxidation was determined as kappa number. Fibers were additionally characterized by conductometric titration, the determination of hemicelluloses and infrared spectrometry. Sorption of various types of dye, namely Methylene Blue (MB), Crystal Violet (CV) and Astra Blue (AB) on the fiber surface was studied and compared to the kappa number and other fiber characteristics. The adsorption of MB and CV is found to follow the amount of anionic (carboxylic) groups, which at first increase and later decrease toward the final stages of oxidation. The behavior of AB is nearly the opposite, its adsorption increases with the depletion of lignin and carboxylate groups, pointing to a different mechanism of AB binding on fibers.
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