1997 Volume 2

The swelling of fibres in water has a large impact on the consolidation of the web in papermaking and thus on the properties of the final paper. The water taken up by the fibre is held by many different mechanisms and it is not always clear from the techniques used which quantity of water is being measured. In this report, an attempt is made to obtain more knowledge regarding the water- holding mechanisms of fibres, by studying the amounts of bound water, of pore water and of the total water in the fibres. Effects of delignification, recycling, beating and ion exchange are examined. It is concluded that the amount of bound water is a reflection only of the wood polymer composition of the fibre, whereas the pore water is affected by physical changes of the fibre wall. The ionic charges mainly affect the surface water of the fibre.

The uptake of moisture by paper sheets was analyzed as an adsorption process occurring in a porous medium. Water vapor was assumed to diffuse into the pore space and was subsequently adsorbed onto the surfaces of the fibers constituting the paper sheet. The response of the sheets to variations in relative humidity was investigated.

Since the transient moisture profiles inside paper sheets depend upon the moisture sorption equilibria for paper, the equilibrium behavior was investigated with special emphasis on a description of sorption hysteresis. It is necessary to follow equilibrium trajectories inside the hysteresis loop for paper sheets. Sorption equilibria inside the hysteresis loop for paper sheets (bleached kraft linerboard, 290 gsm) were evaluated experimentally. Complete sets of desorption and adsorption scanning trajectories were obtained. Further higher order loops were obtained experimentally. The theory of independent domain complexions was applied to the hysteresis loop. By constructing a so called moisture distribution function for the hysteresis, arbitrary trajectories representing equilibrium sorption behavior under cyclic humidity changes could be predicted.

An investigation of transient sorption was also undertaken. A model for moisture uptake based upon diffusion inside the pore space and the fibers in the sheet was set up. Experimental data on transient moisture uptake was obtained under ramped changes in humidity. It was found that the model for moisture uptake incorporating a linearized isotherm could describe the sorption response of paper sheets to ramped changes in humidity adequately. From the experimental data, a value for the intro-fiber mass transfer coefficient representing moisture diffusion through fibers was determined.

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.

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.

It has been known for a long time that carboxylic acid groups in cellulosic fibres increase swelling in fibres- and hence this impacts flexibility, bonded area, and strength. In a few recent publications, attention has been drawn to the possibility that carboxyl groups may serve another function. Those located at the fibre surface have an effect on specific fibre-fibre bond strength . The evidence for this
possibility is at present indirect . Using a technique developed for this work, pulps were prepared with uniform distribution of carboxyl groups across the fibre cell wall, and with carboxyl groups located primarily at the fibre surface. Using values of light scattering coefficient, tensile strength, and z-bond strength, it was possible to determine the relative importance of increased bonded area caused by swelling, and increased bond strength caused by localized surface effects. It was found that specific bond strength could be enhanced roughly 50% by surface enrichment, and this could exceed the strength increase attributable to enhanced bonded area. Previous work on the effect of carboxyl groups on strength needs reevaluation to determine the extent of the two effects . The mechanism of strength increase by surface carboxylic acid groups is not understood . It may be an ionic effect, but it is
possible that the increased localized surface swelling allows more molecular flexibility and more intimate molecular contact and interdiffusion as found in other polymers . In practice, there are opportunities, especially in bleaching or other pulp treatments, for enhancing fibre surface carboxyl group concentration, and these may have practical value. The field looks ripe for exciting developments.

Inverse gas chromatography (IGC) and sessile drop contact angle measurement were used to study the surface energetics of calcium carbonates from different sources and the effects of surface conditioning . A compilation of the results using these two methods to determine the apolar and polar nature of the surface is given . The results of the two analytical methods are summarized individually . IGC analysis provided a sensitive tool for detecting differences in surface energy attributed to variation in surface water, or by contamination with other chemisorbed species . Basic probes were used to characterize surface acidic sites. Acidic probes, however, were found too reactive with the calcium carbonate to en-
able a quantification of the surface basicity . Strong adsorption of acidic probes indicates the presence of a strong basic component at the surface . The susceptibility of the IGC method to misinterpretation retention data obtained from chemisorbed probes was also identified . Contact angle experiments were conducted on cleaved calcite samples using multiple wetting liquids and two techniques to separate the polar and apolar contributions of surface energy. The results demonstrate a strong interaction of water with the calcite surface and the reduction of total surface energy when the surface is treated with al-
kylketene dimer. A comparison of results from both methods is provided . Differences in the application of the results of the two methods to access the surface energetics are addressed . The importance of distinguishing between the thermodynamics of gas/condensed and condensed/condensed phase interactions when comparing the results from IGC and wetting experiments is also discussed . For calcium carbonate, the trends observed by the two approaches are consistent, although a defining link between the two was not established.