Proceeding Articles

In the present study, the surface properties of cellulosic fibres have been systematically varied and the dispersive and acid-base properties have been determined by inverse gas chromatography (IGC) at infinite dilution.

Bleached kraft pulp fibres were carboxy methylated to different degrees. The results from the IGC measurements on the carboxymethylated fibres showed a linear relationship between the interaction energy with a basic probe (acidic properties) and the carboxylic acid group content. The extrapolation to zero degree of substitution ofcarboxylic acid groups indicates that,even in the absence of carboxylic groups, the fibres have acidic properties. The hydroxyl groups on the fibres obviously also contribute to the acidic properties. The interaction energy with an acidic probe (basic properties) was relatively constant with increasing carboxylic acid group content. An increase in the degree of carboxymethylation also seemed to slightly increase the dispersive part of the surface free energy. This could be a consequence of an increase in electron density, a more compact structure after drying the carboxymethylated fibres or removal of low molecular weight impurities. The dispersive as well as the acid-base properties are approximately the same for pulps in both their proton and sodium forms.

The carboxymethylated fibres were peeled after the modification. The carboxylic group contents of the fibres and of the removed outer layers were determined by conductometric titration. The results showed that the carboxymethylation procedure is somewhat more effective in the outer layers of the fibres.

IGC results for the peeled fibres pointed in the same direction. Diethylaminoethyl{DEAF}cellulose, which has a basic functional group was also characterised. The IGC results showed that the DEAE cellulose interacts more strongly with the acidic probe than the reference cellulose material

Adsorption of polyethylenimine (PEI) of different sizes on swollen delignified pulp fibers indicates that for PEI molecules of diameter smaller than 25 nm, the accessible internal surface area within the pores in the cell wall is independent of the size of the PEI molecule. This suggests that a minimum pore radius R. exists in the fiber wall (with the possible exception of very small pores of about 1 nm) through which all PEI molecules in the range 2-25 nn can pass freely. Since the molecules must be able to pass through pores with walls fully coated by PEI and since the thickness of an adsorbed PEI layer is comparable to the size of PEI in solution,the pore size must be at least 3 times the size of PEI, implying that R40 mn. A value of the pore radius in the range 45-50 nm is found from estimates of the area of pores accessible to PEI and the corresponding pore volume. No pores are found in the range 3-40 nm. These findings differ from the pore radii obtained by the solute exclusion technique which usually are around 10 nm. The difference might be due to the ease with which the pores contract and expand under different conditions. Non-adsorbing molecules could cause the pores to contract due to depletion effects, while adsorbing molecules might cause pores to expand.

Effects of fibre morphology on in-plane hygroexpansivity of paper have been studied using a general micromechanics formula and experimental results obtained for different fibre fractions. It was deduced from the micro-mechanics analysis that the major fibre geometry factors affecting paper hygroexpansivity are fibre width, wall thickness, and fibre length,all of which control the stress transfer ratio parameter defined in the formula. The microfibril angle was found to bean other major factor affecting the paper hygroexpansivity through the hygroexpansivity of a single fibre.

The results from the cut-fibre experiment showed that decreasing fibre length increased the hygroexpansivity, but the effect was small within the range of fibre length more than 1mm. The results from Bauer-McNett classification, on the other hand, showed considerable increases of hygroexpansivity as the fraction became shorter. This large variation was attributed to the cell wall structure, in particular microfibril angle and cell wall thickness, both of which directly affect the key parameters in the formula. Fibre fractionation was found to be a very effective tool to consistently change all the fibre morphology factors for controlling dimensional stability of paper.

Paper sheets containing polyacrylamide(PAM)as a paper chemical additive have been analysed by attenuated total reflectance Fourier transform infrared (ATR/FT-IRS spectroscopy. Absorbance ratios of the selected band of PAM to that of cellulose were used to determine PAM content. In order to determine the distribution of the additive in the radial direction of pulp fibre, ATR/FT-IR analysis was carried out after successive etching of the paper sheet. From the relationship between etching time and the thickness of removed surface layer it is possible to follow the partial concentration profiles of the additive as a function of distance from the original surface. The obtained profiles are found to be consistent with those of variable-angle ATR/FT-IR depth profiling method qualitatively. Being distributed from fibre surface toward the centre of fibre wall, PAM exists, on the whole, close to fibre surface. Addition level does not affect the distribution curve, i.e. the concentration at every depth level is roughly proportional to the addition level. In the case of a paper sheet from heavily beaten pulp, this tendency is not so clear i.e. PAM shows broad distribution at near surface layer. PAM whose molecular weight is low is distributed widely toward the inner side of the fibre wall.

A kinetic model for diffusion of ions within the fiber wall is introduced. A good fit for the model was obtained with data from experiments, where diffusion of electrolyte through the fiber wall was studied by means of simple conductivity measurements. This model together with the respective experimental data makes it possible to study the pore structure of fibers.

For the experiments done the pore structure of fibers was first controlled by precipitating calcium carbonate within the fiber walls. It was found that the diffusion coefficient of ions within unfilled and calcium carbonate filled fiber wall was about 1 .5 and 0.5 % of that in water, respectively, when experiments were done for fibers without pit apertures. Experiments were made for fibers with different degree of drying as well. The results showed that the diffusion of electrolyte reduced with replicate dryings as expected. After the dryings, the diffusion coefficient was less than half of that within newer dried fiber.

Metal cations modify the optical and mechanical properties of pulps and in pulp mills can have adverse effects on such parameters as corrosion rate and recovery furnace temperature. With today’s emphasis on recycling effluents and on tighter control of all operations, there is a need for quantitative theory predicting the uptake of cations by pulps and the buildup of cations in process liquors.

A variation of the Donnan Equilibrium Theory was recently developed to describe the partitioning of cations between the fibres and the liquor in a pulp suspension [1]. In this report we extend use of the theory to examine the ion-exchange of pulps from saturation with one cation to saturation with another. Theoretical predictions are closely duplicated by experiments using sodium, magnesium and lanthanum as model cations of different valencies. Pulps readily exchange cations even with very dilute solutions, preferentially taking up and most tenaciously holding the higher valency cations. Consequently, the replacement of high valency cations by monovalent cations requires higher concentrations of the replacement ion than for the reverse exchange. The differing tenacities by which cations are held are also shown during acid washing of pulps. Upon progressively lowering the pH of a suspension of pulp in mixed ionic form, monovalent, divalent and trivalent cations are successively released and replaced by hydrogen ions. The implication of these results for the “acid washing” of pulps is discussed.

The objectives of this research were (i) to obtain a comprehensive description of the surface chemistry of cellulosic fibres by thermodynamic and spectroscopic methods, (ii) to use this description to clarify the effect of kraft cooking as well as ECF and TCF bleaching on the surface chemical and adhesive properties of cellulosic fibres, (iii) to clarify and understand the connection between the fibre surface properties and some key sheet properties (e.g. tensile strength). The charge, surface energy and hydrophobic material in the surface of cellulosic fibres (mainly pine, Pinus sylvestr is and birch, Betula verrucosa) were studied by potentiometric titration, determination of polyelectrolyte adsorption isotherms, determination of contact angles of single fibers and ESCA. Lignin (remnants of middle lamellae or reprecipitated during cooking) is enriched in the surface of unbleached kraft pulp fibres. The fraction of surface lignin removed by oxygen or hydrogen peroxide is much smaller than the total decrease in lignin content. Ozone removes both bulk and surface lignin, while the effect of chlorine dioxide depends on the number of treatments. The low reactivity of surface lignin is due to condensed lignin structures and/orto lignin-carbohydrate complexes.

Theoretical considerations have led us to formulate a general equation for the collapse of wood pulp fibres in terms of trans verse dimensions, transverse elastic modulus of the fibre wall, and the collapse pressure. This equation is in agreement with experimental results on the effects of fibre transverse dimensions on collapse. We have also developed an equation for the transverse elastic modulus of the fibre wall in terms of fibril angle and the orthotropic elastic constants. We can therefore confirm the dependence of collapse on the transverse elastic modulus through the effect of fibril angle on the collapse of chemical pulp fibres.