By producing additional hydrogen bonds, due to the formation of carboxyl groups (COOH), we studied the effect of the N₂0₄ oxidation to the strength properties of the bleached kraft pulps. The main objective of this study was to observe the influence of the formation of carboxyl groups due to the nitrogen tetroxide (N₂0₄) oxidation to the bonding strength of the fibres, through the increasing the hydrophilic character and surface adhesion of the fibres. We intended to find out under what conditions of treatment, such as gas flow-rate, oxidation time, temperature and pulp consistency or by using the chemical substances, the improvement of the above properties may be obtained. The other objective was to determine whether the final achievements would be resulted to the higher paper strength of the sheet. An application of the page equation of I/T=I/F +I/B.

The experimental work first involved a three-design protocol applied to a standard bleached kraft softwood pulp as a refèrence. The designs used were the one factor at a time, the half-factorial and a 3-factor 5 level central composite design (CCD). It was found that carboxyl formation increased to a certain limit as the gas flow-rate, oxidation time increased. However, there was a simultaneous decrease in viscosity, which is a sign of fibre degradation. Regardless of the operating conditions, the decrease in viscosity was large enough to overcompensate the carboxyl formation effect, such that the overall effect was a decrease in tensile and z strength of the handsheets. The effect of temperature and consistency was to accelerate the viscosity loss.

The carboxyl formation was verified by inspection of FTIR spectra, which displayed a new peak at 1734-1738 cm^-1. The OH peak was perturbed due to the transformation of différent carbons in the glucosidic rings to aldehyde and ketone groups. The ESCA showed that the O/C ratio increased during the oxidation, which is a sign of greater hydrophilic character of the oxidised fibres. It was also found that the CI component decreased and the C2 increased for all three fibres. The C3 representative the carbonyl groups formation increased for the softwood as well as for the bagasse oxidised fibres. However, it was not possible to detect carboxyl groups (at binding energy of 288.4 eV) on the surface by ESCA. At the same time, the specific acid-base interaction, I_SP of the softwood and bagasse fibres, as determined by IGC, increased due to the higher H-bonding formation. In the case of aspen fibres, there was neither an increase in O/C ratio nor in the specific acid-base interaction. This can be explained by the acidic character of the untreated aspen fibres (K_A= 2.9, K_B= 1.9) as compared to the basic character of the bagasse (K_A=1.7, K_B= 2.3) and to the amphoteric character of softwood bleached kraft fibres (K_A= 0.49, K_B= 0.55).

In order to determine the possibility of limiting the degradation of fibres by this oxidation, the oxidation was repeated in the presence of 1%-MgC0₃, a salt which has been reported to provide some protection to the cellulose fibres in other oxidative environments. It was found out that this salt limited the reduction of the viscosity of pulp. Even though the formation of carboxyls was also lower than that during oxidation without MgC0₃, the net effect was to improve the strength properties of the paper over and beyond those of papers made from the original untreated fibres. This occurred at oxidation rates of 0.8% and 2.5%-N₂0₄/fiber, for softwood and bagasse, respectively. The improvements in tensile and burst strength were 25, 5%, and 12% for the softwood and bagasse, respectively. Under the saine conditions, it was also found that the treatment increased the opacity and the scattering coefficient of oxidised fibres.

The mathernatical evaluation based on the Carrasco method, led to the same conclusions as the graphical interpretations of experiments. Essentially, the multiple regression analysis showed that a paper strength is more strongly influenced by the morphological and physical properties of fibres, (CSF, fibre length, viscosity), than by chernical changes induced by the oxidation process (carboxyl formation, O/C ratio, 1_sp). However, the importance of the hydrophilic effect and surface adhesion of the fibres affecting the bonding strength of the oxidised fibres, B, on the tensile strength, T, in the Page equation is clear when the viscosity is kept within a certain range by MgC0₃ Interpretation of the three-factor model showed that even though there is an obvious reduction in fibre length and in viscosity due to the N₂0₄ oxidation, it is still possible in the case of using a protective agent, to improve the paper strength due to the increase in the hydrophilic character and surface adhesion of the fibres.

27 juillet 2000