Introduction
The wall of natural plant fibers, consisting essentially of α-cellulose, hemicelluloses and lignin, is smooth and hardly porous in the absence of water, thus displaying a low surface area (Topgaard and Söderman
2002). This can change drastically along the papermaking process, particularly during pulping and refining (Aguado et al.
2016; Przybysz et al.
2020). Indeed, the surface area of fibers, both inner and outer, is of utmost importance in papermaking, among other reasons because it strongly influences bulk density, air permeability, and water absorbency (Koponen et al.
2017; Mao et al.
2017; Azevedo et al.
2020). Furthermore, as paper strength is largely due to hydrogen bonding and other non-covalent interactions in pulps, the availability of a large area for bonding is generally translated into good mechanical properties (Motamedian et al.
2019). Not less importantly, this available area is reduced by hornification, a side effect in the structure of fibers as a consequence of the recycling process, mainly the subsequent drying and wetting stages (Delgado-Aguilar et al.
2015; Moser et al.
2018).
In the case of chemical pulping, pores are created by dissolving the hemicellulose and lignin fractions of fibers (Topgaard and Söderman
2002). And while these fractions remain after high-yield pulping, swelling still takes place through the hydration of fibers, especially in thermomechanical pulping (TMP) (Moral et al.
2017; Serra-Parareda et al.
2021). Nonetheless, the presence of lignin interferes with intra- and interfiber bonding, which makes a refining stage more necessary. During refining, fibers undergo shortening (or cutting), partial removal of the wall, redistribution of hemicelluloses, and fibrillation (Gharehkhani et al.
2015; Espinosa et al.
2018; Przybysz et al.
2020). External and internal fibrillation increase the outer and inner surface areas, respectively. The former is due to fibrils protruding from the surface of fibers, which becomes rougher (Moral et al.
2016), whereas the latter is evidenced by swelling (Lecourt et al.
2010). When it comes to TMP fibers, their negative charges (owing mostly to carboxyl groups) become more openly exposed (Zhao et al.
2016).
Despite the usefulness of measuring the specific surface area (SSA), this property is seldom reported in the context of papermaking, at least as of today. Earlier, the Pulmac permeability test was more commonly used for this purpose (Robertson and Mason
1949; Rouger and Mutjé
1984; Carrasco et al.
1996). It used to be a troublesome measurement, lacking reproducibility and reliability (Ramarao
1999). Another popular liquid–solid method implies the adsorption of dyes, such as methylene blue and Congo Red, onto the available surfaces of fibers, and then recording electronic absorption spectra (Kaewprasit et al.
1998).
It is in regards of cellulose-based sorbent materials and cellulose nanofibers that the SSA is often reported in recent works. This calculation of the SSA has preferentially used N
2 adsorption isotherms, and more specifically the BET model (Nemr et al.
2017; Hina et al.
2018; Darpentigny et al.
2020). While there is no doubt about the suitability of the BET method for this measurement, it requires specific equipment, often named “surface area analyzer”. Furthermore, a monolayer of adsorbed N
2 is formed all across the porous structure of the material, including those pores whose size is as small as 2 nm. Hence, the information gathered is extremely useful for nanocellulose-based gels (Tarrés et al.
2016; Darpentigny et al.
2020), but it would be not so relevant for the paper manufacturing process, which takes place in aqueous media and involving low residence times.
In this work, we evaluate a potentiometric titration with a cationic polyelectrolyte, poly(diallyldimethylammonium chloride) (PDADMAC), as a practical and reliable way to measure the SSA of TMP fibers. The surface area of fibers from their cationic demand has been previously estimated in previous works (Tarrés et al.
2018; Filipova et al.
2020), but no work justifying the method on the basis of an in-depth study and strong correlations has been published yet. It is worth highlighting a valuable contribution from Zhang et al. (
2016), who showed that the cationic demand measured by researchers actually depends on the molecular weight (MW) of PDADMAC. This inspires us to infer that PDADMAC of medium or high MW can be easily adsorbed along the outer surfaces of lignocellulosic fibers and can diffuse through relatively large pores, not reaching the small pores to which N
2 and low-MW dyes can adsorb. Instead of a drawback, the SSA value estimated from a titration with high-MW PDADMAC is hypothesized to correspond more faithfully to the actual bonding area of fibers in the fast processes of a paper mill. This estimation of the SSA could then be used to quantify the degree of refining, to monitor hornification as a result of recycling, and to predict paper wet strength.
Experimental
Materials
PDADMAC with an average MW of 107 kDa was kindly supplied by L.C. Paper (Besalú, Spain). The source of lignocellulosic fibers was a TMP from softwood, of industrial origin, that had undergone chlorine-free bleaching and whose freeness was 21.5°SR. Sodium polyethylene sulfonate (PES-Na) was provided by BTG Instruments. Congo red (CR) and all other reagents were purchased from Sigma-Aldrich.
Refining
30 g of pulp (on the basis of dry pulp weight) were dispersed in a pulp disintegrator at a consistency of 1.5%, for 10 min at 3,000 rpm. The suspension was filtered so as to adjust its consistency to 10% (wt), and then refined in a PFI mill from Metrotec, model NPFI 02, according to ISO 5264–2. TMP was refined to 5,000, 10,000, 20,000, and 30,000 PFI revolutions. The energy input was quantified by means of a device from Circutor, model CVM-C10.
Freeness of pulp was measured by means of a Schopper-Riegler tester, 95,587 PTI, in accordance to the ISO standard 5267–1. Canadian Standard Freeness (CSF) was interpolated from a freeness conversion table (González et al.
2012). Moreover, the water retention value (WRV) was measured gravimetrically, in accordance to the SCAN-C method 62:00. Fiber pads containing excess water were centrifuged at 3000 g in containers provided with a nitrocellulose membrane (0.22 µm of pore size), and by means of a Sigma Laborzentrifugen apparatus, model 6K15. After 15 min, the filter cake was collected, weighted (m
W), and oven-dried at 105 ºC until constant weight (m
D). Then, WRV equals the relative difference between those weights:
$$ WRV = \left( {m_{W} {-} \, m_{D} } \right)/m_{D} $$
(1)
Adsorption of Congo red
After verifying the compliance of CR to the Beer-Lambert’s law, by plotting the absorbance at a wavelength of 488 nm against the concentration, adsorption experiments were carried out as reported elsewhere (Inglesby and Zeronian
2002). Briefly, TMP samples were suspended in phosphate buffer (0.1 M) at pH 6 and mixed with varying amounts of CR, from 5 to 15% (wt.), then stored at 60 °C for 24 h. We added NaCl (0.004%, wt.) and centrifuged the samples for 20 min. Then, UV–visible absorption spectra were recorded from the free liquid and the concentration of CR in each case was computed. The maximum adsorption capacity (
qmax) was calculated by fitting to a linearized Langmuir isotherm:
$$ \frac{{c_{eq} }}{q} = \frac{1}{{K_{ad} q_{\max } }} + \frac{{c_{eq} }}{{q_{\max } }} $$
(2)
where
ceq is the concentration of CR at equilibrium (mg/L),
q is the adsorbed mass of CR (mg/g), and
Kad is the equilibrium constant. Then, the SSA can be estimated from:
$$ SSA_{CR} = \frac{{q_{\max } \times N_{A} \times SA_{CR} }}{{10^{21} Mw_{CR} }} $$
(3)
where
NA is the Avogadro number,
SACR is the surface area of a single molecule of adsorbate (1.73 nm
2), and
MwCR is the molecular weight of CR (696.7 g/mol).
Carboxyl content and cationic demand
Carboxyl groups were quantified by means of a conductimetric titration. A sample of TMP was suspended in 15 mL of HCl 0.01 M. This suspension was titrated with an aqueous NaOH solution, continuously recording the conductivity values. Then, we plotted a titration curve, which encompassed two inflection points, one being due to the strong acid (HCl) and the other one being due to weak acids (carboxyl groups). Then, the carboxyl content (
CC, in mmol per gram of dry sample) is calculated from:
$$ CC = \left( {V_{2} {-} \, V_{1} } \right) \times c/m $$
(4)
where
V1 and
V2 are the volumes of NaOH (mL) that corresponded to the first inflection point of the curve and the equivalence point, respectively;
c is the concentration of NaOH (10 mM), and
m is the mass of pulp (on a dry basis) in the suspension.
The cationic demand was determined potentiometrically, performing a back titration with a particle charge detector Mütek PCD-04 (BTG Instruments). 0.1 g of dry TMP were mixed with a known excess of PDADMAC, typically 10 mL (
VPDADMAC), and in deionized water medium. Other than the cationic polyelectrolyte and its counter-ion (Cl
–), the presence of ionic species can be neglected, given that the pulp had been thoroughly washed with deionized water. The suspension was centrifuged for 30 min at 10,000 rpm. Then, the supernatant was titrated with PES-Na until the isoelectric point (0 mV). The cationic demand (
CD) was then determined as:
$$ CD = \left( {c_{PDADMAC} \times \, V_{PDADMAC} } \right){-}\left( {c_{PES - Na} \times \, V_{PES - Na} } \right) $$
(5)
where
cPDADMAC \({c}_{PolyDADMAC}\) is the normal concentration of PDADMAC (typically 0.001 N),
cPES-Na \({c}_{PES-Na}\) is the normal concentration of titrating agent (typically 0.001 N), and
VPES-Na \({V}_{PES-Na}\) is the titration volume.
The hypothesis that these cationic demand measurements can be correlated to SSA, along with other properties that are also affected by refining, was evaluated by calculating the Pearson correlation coefficient.
Furthermore, the influence of the ionic strength on PDADMAC adsorption was assessed by dissolving different amounts of NaCl in the medium, before interacting with the pulp. Likewise, small additions of diluted NaOH or diluted HCl allowed us to study the effect of pH.
Separation of fines
In this work, fines are understood as those particles in a cellulosic or lignocellulosic pulp whose length lies below 75 µm. The consistency of a TMP suspension was adjusted to 1% and it was filtered through a 200-mesh screen. The fraction that passed through the screen was regarded as the fines content, which was determined gravimetrically, on the basis of dry pulp weight. Fibers, on the other hand, remained onto the mesh screen.
Then, the aforementioned procedures to estimate the surface area from PDADMAC adsorption, including measurements of the carboxyl content and the cationic demand, were performed separately for fibers and for fines. For comparison purposes, the Congo red sorption method was also carried out.
Conclusions
Results supported the main hypothesis of the work, i.e., that a potentiometric titration to measure the cationic demand allows for a useful estimation of the SSA of lignocellulosic fibers. Such titration can be combined with a conductimetric one, aiming to discriminate between the amount of cationic polyelectrolyte ionically exchanged with carboxyl groups and the adsorption driven by ion–dipole interactions. In any case, this estimation of the SSA cannot replace the BET method (N2) or dye sorption (in this work, Congo red), as it will yield lower values than these determinations, owing to the diffusion coefficient of PDADMAC being much lower. While this would imply an underestimation for other applications, in papermaking it is of great usefulness. Further, the proposed method was tested at increasing ionic strength and different pH values, revealing that both conditions had a notorious influence on the PDADMAC adsorption onto TMP fibers. In the case of ionic strength, it was attributed to the change on the persistence length of PDADMAC, which presumably modified the conformation of the polyelectrolyte during its adsorption. In terms of pH, the differences were attributed to fiber swelling and cleavage, for basic and acid pH. The SSA estimated from the methodology we present can be used as an indicator of refining, as shown by successfully correlating it to the Schopper-Riegler degree. Furthermore, it allows papermakers to differentiate between fibers, for which the values provided by our estimation are much lower those of dye sorption (e.g., 17.6 and 6.31 m2 g–1 in the case of 30,000 PFI revolutions), and fines, for which both methods attain similar results. More importantly, in light of the limitations to the diffusion of PDADMAC into fibers, the estimation of SSA from the cationic demand is thought to attain a valuable prediction of the behavior of the pulp with cationic polyacrylamides and in terms of inter-fiber interactions.
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