Sustainable Hygiene Product Development from Recycled Pulp: Surfactant-Driven Enhancement of Tissue Handsheets Properties

Abstract

Deinked pulp (DIP) could be a sustainable fiber source for tissue and hygiene products, but it suffers from losses in the physical, mechanical, and softness properties required. To overcome this limitation, DIP was treated with cationic and nonionic surfactants under various refining conditions to enhance the bulk and softness of the DIP handsheets without compromising tensile properties. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) and energy-dispersive X-ray spectroscopy (EDS) analyses ensure the successful fiber-surfactant interactions. Additionally, the molecular dynamics simulation reveals interactions between cellulose and surfactants, leading to improved bulk and softness, and a higher tensile index (TI)-to-softness ratio, which aligns with the experimental findings. The bulk of the surfactant-treated fibers increased by 16%–57%, resulting in a 70%–76% improvement in softness, and the water absorption capacity also increased by 66.72%–93.31%. The hard-to-remove water content decreased by 14.10%–55.31%, and the tensile index (TI) to softness ratio was 2.71 times and 3.05 times higher than the control unrefined handsheets for cationic and non-ionic surfactants, respectively. Scanning electron microscopy (SEM) analysis revealed successive pore openings resulting from surfactant treatments, leading to greater spring-back than in the untreated fiber. The surfactant-treated fibers produced tissue handsheets with higher softness, bulk, and enhanced absorption capacity, achieving 93.29%–95.03% spring-back within the first 5 minutes without compromising mechanical properties. Therefore, this chemo-mechanical modification strategy demonstrates strong potential to enhance the properties of recycled fibers for tissue and hygiene product applications.

Graphical abstract

Keywords

Deinked pulp / Surfactant treatment / Refining / MD simulation / Fiber collapsing / Tissue paper application

Highlight

	● Recycled deinked pulp was modified with cationic and non-ionic surfactants.
	● Molecular dynamics simulation confirmed the fiber–surfactant interaction.
	● The bulk and softness of tissue handsheets increased by 57% and 76%, respectively.
	● The hard-to-remove water content was reduced by 14% to 55%.
	● Water absorption and elastic spring-back improved up to 93% and 97%, respectively.

1 Introduction

The tissue paper industry has been experiencing significant global growth, driven by its role in providing a hygienic, comfortable, economical, and convenient lifestyle. Tissue paper products, including toilet paper, towels, napkins, facial tissues, and wipes, are distinguished by their lightweight nature, superior flexibility, softness, high bulk, and availability in various forms, such as creped, non-creped, or embossed sheets, depending on their particular usage [1–3]. According to market reports from Statista, the global tissue market revenue is projected to reach $370.8 billion in the current year, with an expected annual growth rate of 5.10% over the next five years [4]. Softness, strength, and absorption are the three most important characteristics of tissue products, which vary according to their intended use and are significantly influenced by the source of raw materials, processing chemicals, and manufacturing methods [5,6]. Tissue paper production is highly dependent on wood fibers; however, recycled fibers recovered from various waste paper sources have emerged as an alternative to help conserve natural resources by reducing the use of virgin fiber [7–9]. Moreover, the use of recycled paper products significantly reduces carbon footprints and minimizes environmental pollution to a great extent by reducing landfilling and incineration practices [9–11].

Although recycled fibers are emerging as an alternative source, they remain limited in use due to their poor mechanical strength and inability to regain their original filament shape upon rehydration [11]. Virgin fibers undergo significant changes in their mechanical, chemical, and physical properties during refining, drying, and pressing. When virgin cellulosic fibers are dried, they undergo a structural transformation due to partially irreversible collapsing, known as hornification [12,13]. The effects of hornification are far-reaching, involving loss of fiber flexibility and swelling capacity as well as a significant decline in bulk, softness, and water absorption capacity [14–17]. Several efforts have been made to mitigate this problem by choosing alternative drying methods, enzymatic modifications, chemical modifications using cross-linkers or additives (e.g., softeners, strengthening agents), and mechanical refining [18–20]. For example, freeze-drying and glycerol-drying result in cellulose fibers with significantly lower fiber collapse and higher reswelling [21]. However, the former method is impractical on an industrial scale, and the latter method restricts the end use of cellulose due to residual glycerol [20,22]. On the contrary, the green treatment of cellulose fibers with enzymes (cellulase being the most used) has been an effective approach to improve their flexibility, smoothness, swelling capacity, and tensile strength [19,23]. However, its potential is restricted by the high cost, low efficiency, and sensitivity to process conditions [2,24,25]. Regarding chemical modifications, cross-linking cellulose with PVA has shown great promise in retaining both good tensile properties and water absorption, but only when PVA is fully or almost fully hydrolyzed [18]. Although softeners show great promise in improving the softness and wettability of fibers, they markedly decrease bonding between cellulose fibers and, consequently, the strength [7].

Recently, surfactant-driven fiber treatment with enhanced characteristics has gained popularity [6,25,26]. Surfactants, which contain both hydrophilic and hydrophobic groups, have been used in the pulping and papermaking industries [27–30]. Cellulose surfaces contain both hydrophilic and hydrophobic regions, and the cellulose fiber becomes negatively charged in a neutral aqueous medium due to its hydrophilic sites and therefore interacts with surfactants [30,31]. Surfactant treatment of cellulosic fibers can reduce the surface tension of water by breaking down water–water interactions through enhanced water–surfactant interactions, which in turn reduce the pulling force of the liquid bridge and prevent fiber collapse [32,33]. Many studies have reported significant electrostatic and ionic interactions between cationic surfactants and cellulosic surfaces, making them an excellent option for modifying cellulosic surfaces and enhancing their properties for various applications [34,35]. Cationic surfactants can neutralize the negative charge on the cellulose fiber surface through strong electrostatic and ionic interactions, leading to weaker fiber–fiber interactions and increased bulk without a significant reduction in tensile properties [35]. The hydrophilic part of the surfactant forms bonds with the hydrophilic surfaces of the fiber, creating a dynamic layer of hydrophobic carbon chains. Such interaction enhances lubricity and also delivers a softer feel [36–38]. Nonionic surfactants exhibit lower foaming actions and interact with cellulosic fibers through hydrophobic and van der Waals interactions, which enhance the wetting and swelling of fibers and thus increase the molecular contact surface between fibers [31,39]. Figure 1A illustrates the effect of surfactants on the surfaces of fibers and interactions between fibers in the fiber network. In systems without surfactant treatment, cellulose fibers are surrounded by large hydration layers that facilitate effective interaction between the fibers and water. Water molecules occupy a larger area of the fiber interface, thereby facilitating stronger fiber–water interactions. Consequently, upon drying, this hydration exerts a strong pull, increasing the rate of fiber collapse and reducing the bulk and stiffness of the end sheet. On the contrary, the presence of surfactants radically changes fiber–water interactions. Surfactant molecules adsorb onto the fiber surface, displacing water molecules and reducing the extent of hydrogen bonding between water and cellulose. The larger, longer amphiphilic structures of the surfactants fill inter-fiber spaces and create a protective, lubricating layer that reduces capillary pull during drying, resulting in less fiber collapse, greater bulk, and greater softness. Figure 1B represents the detailed structure and interaction chemistry of the cetyltrimethylammonium bromide (CTAB) and Triton X-100 (TX-100) surfactant. CTAB interacts with the surface of the cellulose mainly by electrostatic interactions, involving the positive head group of the quaternary ammonium [N⁺(CH₃)₃] and the negative carboxylate and hydroxyl groups on the cellulose surface. The TX-100 adsorbs onto the hydrophobic parts of the cellulose surface, such as the C–H faces of the pyranose rings, by forming hydrophobic and van der Waals interactions between the octylphenyl group and the hydrophobic portion of the cellulose surface, while the polyoxyethylene chains are oriented towards the aqueous phase.

Anionic surfactants have negatively charged head groups, and therefore, the repulsion or weak adsorption of anionic heads on the surface of the cellulose fibers is due to the net negative charge of the fiber: the anionic heads appear tailed by their hydrophobic heads [39]. Consequently, the positive charge of the fibers is not neutralized by anionic surfactants, and fiber surfaces are not substantially penetrated by them. Also, anionic varieties are more likely to form stable foams, which may disrupt sheet formation and dewatering in papermaking [35]. Such foam and charge-repulsion problems limit the use of anionic surfactants to improve tissue softness and bulk. Therefore, in this study, we chose Cetyltrimethylammonium bromide (cationic) and Triton X-100 (non-ionic) surfactants to pretreat DIP pulp and enhance the bulk, softness, and strength of tissue handsheets. Both surfactants resulted in increased bulk, enhanced softness, and increased water absorbency. It was found that tissue paper with higher bulk and a softer hand feel can be produced without significantly compromising strength by using a simple, expectedly environmentally friendly process involving appropriate surfactant treatment and mechanical modifications. This is a unique approach that combines a systematic chemo-mechanical approach that couples surfactant treatment with Papirindustriens Forskningsinstitutt (PFI) refining at different intensities for recycled deinked pulp, and molecular dynamics (MD) simulations that provide atomistic insight into cellulose–surfactant interactions [radial distribution function (RDF) analysis and solvation dynamics]. This is an important scientific development that brings together computational and experimental evidence for recycled DIP tissue handsheets. This work also investigated the empirical relationship between different types of surfactants and DIP pulp at varying refining levels, resulting in the removal of hard-to-remove water and improvements in tissue paper properties, including bulk, softness, and water absorption, in flexible handsheets prepared from surfactant-modified DIP pulp. Hence, this paper discusses the role of surfactants in a more positive light under improved conditions, providing guidelines for selecting surfactant strategies to produce soft, absorbent tissue from recycled fibers without a significant decline in tensile strength.

2 Materials and Methods

2.1 Materials

Deinked Pulp (DIP) was collected locally from Bashundhara Paper Mills Limited. The compositional analysis of DIP was cellulose 72.49%, hemicellulose 13.73%, lignin 3.65%, ash 6.29% and moisture 3.84%. CTAB, a cationic surfactant, and the non-ionic surfactant Triton X-100, both with 98% purity, were purchased from Fisher Scientific International, Inc.

2.2 Methodology

2.2.1 Compositional analysis

The cellulose, hemicellulose, and lignin content of DIP were analyzed using Technical Association of the Pulp and Paper Industry (TAPPI) test methods T203-99, T249-75, and T222-88, respectively.

2.2.2 Pulp preparation and refining

DIP was soaked in water, and a pulp slurry with a consistency of 10% was prepared. The unrefined and refined DIP samples were prepared by refining the slurry at 0, 650, 1250, and 2500 revolutions using a PFI mill following TAPPI T248 sp-00.

2.2.3 Pretreatment and post-treatment of DIP

The refined and unrefined DIP were diluted to a consistency of 3% and treated with CTAB and TX-100 using the following two processing routes:

(1) Pretreatment

The pulp slurry (3% consistency) was taken, and subsequently, 0.33% (w/w) of surfactants (Triton X-100 or CTAB) on the oven-dry basis of pulp were added to it. The surfactant percentage was chosen based on previous findings [6,26]. The slurry was then disintegrated at 10000 rpm for 5 min using a pulp disintegrator, UEC-2008, and further mixed at 1000 rpm for 1 h at 23 ± 2 °C using a mixer. The fiber thus modified using TX-100 and CTAB was termed pre-treated Triton X-100 (PrTX) and pre-treated cetyltrimethylammonium bromide (PrCT), respectively.

(2) Post-treatment

Pulp slurry (3% consistency) was disintegrated using a pulp disintegrator at 10,000 rpm for 5 min, and then the slurry was treated with 0.33% (w/w, on an oven-dry pulp basis) Triton X-100 or CTAB after disintegration. The slurry was mixed for 1 h at 1000 rpm at room temperature before handsheets were prepared using the stock. The fiber thus modified using TX-100 and CTAB was termed post-treated Triton X-100 (PoTX) and post-treated cetyltrimethylammonium bromide (PoCT), respectively.

The treatment conditions are summarized in Table 1.

2.2.4 Handsheets preparation

The pre- and post-treated pulps, after mixing with surfactants, were diluted to 0.3% consistency for handsheets preparation. The tissue handsheets were made at a target basis weight of 40 gram per square meter (GSM), following the TAPPI T205 method, with a lightweight foam roller (0.15 kg) instead of the standard heavy brass roller (13 kg). The handsheets were dried twice at 220 °F without pressing and stored at 23 °C and 50% RH before testing [3]. Tissue handsheets were prepared in a controlled laboratory model system to isolate and quantify the effects of interactions between surfactants and fibers, as well as between fibers and other components of the fiber materials, on tissue-relevant properties (bulk, softness, water absorption, spring-back). Mechanical and Chemical treatment with Triton X-100 (TX-100) and Cetyltrimethylammonium bromide (CTAB) surfactants and treatment methods are represented in Figure 2.

2.3 Characterization techniques

2.3.1 Fiber quality analysis

Fiber properties were measured using a high-resolution fiber quality analyzer (FQA), HiRes FQA, OpTest Equipment Inc., Hawkesbury, Ontario, Canada. Before analysis, FQA was calibrated, and the samples were disintegrated using a laboratory disintegrator. Particles with a size less than 0.20 mm were regarded as fines.

2.3.2 Time-of-flight secondary ion mass spectrometry (ToF-SIMS) analysis

A ToF-SIMS V (ION-TOF, Inc., Chestnut Ridge, NY), consisting of a Bi³⁺ liquid metal ion gun, Cs⁺ sputtering gun, and an electron flood gun to compensate for charging, was used to obtain ToF-SIMS spectra. The samples were sliced into 500 × 500 µm (raster size) pieces and placed on the ToF-SIMS sample holder under ultra-high vacuum to acquire high-lateral-resolution mass spectral images. To visualize the distribution of surfactant molecules on the handsheets, a burst alignment of a 25 keV Bi³⁺ ion beam was employed to create a 500 × 500 µm² area at 300 nm resolution using negative-ion beams.

2.3.3 Mechanical properties

The mechanical properties of the handsheets were evaluated by standard testing procedures. The bulk was measured using the TAPPI T580 method. The dry tensile strength of the prepared handsheets was measured using a universal tensile tester (model STM 566, SATRA, UK) following TAPPI T494 standard procedures, and the tensile index was calculated by dividing the sample weight basis, following test method T494 Om22. Each experiment was repeated at least five times, and the average value was calculated. The softness of tissue handsheets was examined using a tissue softness analyzer (TSA), manufactured by EMTEC Electronic GmbH, Leipzig, Germany. Water absorption was analyzed using a gravimetric immersion test in accordance with ISO 12625–8 (2016). Samples were dried at 105 ± 2 °C and immersed in 100 mL of distilled water for 120 s, and the water absorbed was calculated as the percentage change in weight using equation (1).

(1)\mathrm{W}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{r}\mathrm{a}\mathrm{b}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{p}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}(\mathrm{g}/\mathrm{g})=(W_{\mathrm{t}}{-W}_0)/W_0

Where, W₀ = oven dry weight of the handsheets and W_t = weight of the sample after immersion.

2.3.4 Hard to remove water

The thermogravimetric analysis (TGA) method is used to determine the hard-to-remove water content. The DIP handsheets were placed in a platinum sample pan to support the samples during testing. The sample was heated to 110 °C at 10 °C/min. When the desired temperature was achieved, an isothermal drying process was conducted at 110 °C for 15 minutes. The amount of hard-to-remove water (HR water) was determined by subtracting the total freezable water from the total moisture content, which was determined gravimetrically by drying a sample using a TGA microbalance under an air flow of 3 L·min⁻¹ at 110 °C until a constant weight was obtained [40].

2.3.5 Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS)

The morphological analysis of DIP handsheets was performed using a Hitachi S-3200N variable-pressure scanning electron microscope (Hitachi High-Technologies, Tokyo). A thin Au/Pd layer was sputtered onto the surface of DIP handsheets to enable current flow through the handsheets. The analyses were performed in low-vacuum mode at approximately 30 Pa, with an accelerating voltage of 10 kV and a secondary-electron detector [41,42]. Energy-dispersive X-ray spectroscopy images were obtained using a field-emission scanning electron microscope equipped with an Energy-Dispersive X-ray detector (Hitachi, Tokyo, Japan). The cross sections of the handsheets samples were set on SEM mounts, sputter-coated with a thin layer of gold (Au) to enhance conductivity and prevent surface charging. Imaging was performed at an accelerating voltage of 15–20 kV, with an emission current of 15 μA, a working distance of 15 mm, and magnification of up to 1000×. A total of 120 accumulation scans (each scan lasting 30 s) were conducted in the highest quantitative mode for EDS.

2.3.6 Spring-back measurement

The effectiveness of surfactant treatment on deinked pulp fibers was tested by assessing caliper recovery after applying compression loading to the deinked pulp fibers. DIP handsheets were compressed with a load of 50 kPa for 2 min, and caliper readings were taken before (C₀), immediately after (C₁), and after recovery (C₂) by using a low-load micrometer (2 kPa). [6] The Spring-back percentage (SB%) was calculated by using equation (2).

(2)\mathrm{S}\mathrm{B}\mathrm{\%}=[(C_2-C_1)/{(C}_0{-C}_1)]\times 100

The recovered caliper was monitored at 5 min, 1 h, 3 h, 6 h, and 24 h post-compression. This method allowed quantification of elastic recovery in the z-direction and provided insight into the bulk resilience of the recycled fiber network.

2.4 Computational analysis

2.4.1 System preparation and parameterization

A structural model comprising 100 cellulose decamers arranged in a 10 × 10 stacked configuration was constructed to mimic the assembly of crystalline cellulose. Each decamer was parameterized using the Chemistry at HARvard Macromolecular Mechanics (CHARMM)36-jul2022 carbohydrate force field [43,44], ensuring accurate representation of glycosidic linkages and hydroxyl groups. Two separate surfactant-containing systems were also prepared: one incorporating TX100 molecules and another with CTAB molecules. Surfactant structures were geometry-optimized [44] and then parameterized using the CHARMM General Force Field (CGenFF, version 4.6) [45,46] via the ParamChem web server [47], generating CHARMM-compatible topologies and partial atomic charges. The resulting CGenFF parameter and topology files were merged with the CHARMM36-jul2022 force field to ensure consistency in bonded and nonbonded terms. Charge-group definitions were disabled to maintain atom-based electrostatics, ensuring full compatibility with the Particle Mesh Ewald (PME) scheme in GROningen MAchine for Chemical Simulations (GROMACS) [48–50].

2.4.2 Solvation and ionic conditions

The cellulose assemblies (with and without surfactants) were placed in a triclinic simulation box with at least 1.0 nm buffer between solute atoms and the box edges. Each system was solvated using TIP3P water molecules [51]. For the CTAB-containing system, counterions (Br⁻) were explicitly included to maintain charge neutrality. Additional Na⁺/Cl⁻ ions were added to achieve an ionic strength of 0.1 M, replicating near-physiological ionic conditions and stabilizing the electrostatic environment around the surfactant–cellulose complexes.

2.4.3 Energy minimization

All systems underwent energy minimization using the steepest descent algorithm until the maximum force on any atom was reduced to below 1000 kJ·mol⁻¹·nm⁻¹, or a maximum of 1 × 10⁶ steps was reached. Non-bonded interactions were modeled with a 1.1 nm cutoff for both Coulombic and van der Waals terms. Electrostatics were initially handled using the reaction-field method (ε_r = 15) to alleviate steric clashes and stabilize the packing of cellulose and surfactant prior to equilibration.

2.4.4 Equilibration

Equilibration was performed in two stages with positional restraints applied to the cellulose heavy atoms to maintain structural integrity while allowing solvent and surfactants to relax:

(1) Number of particles, volume, temperature (NVT) equilibration (2000 ps)

The systems were equilibrated under constant number, volume, and temperature (300 K) using the V-rescale thermostat. Surfactant, cellulose, and solvent were defined as separate coupling groups. Long-range electrostatics were switched to Particle Mesh Ewald (PME) with a grid spacing of 0.16 nm.

(2) Number of particles, pressure, and temperature (NPT) equilibration (4000 ps)

Pressure equilibration was conducted at 1 bar using the Parrinello-Rahman barostat, retaining PME and thermostat conditions. For stability, the surfactant-cellulose aggregates were grouped as a single index, ensuring consistent equilibration under isotropic pressure.

2.4.5 Production of molecular dynamics

Unrestrained production simulations were performed using the leap-frog integrator with a 1 fs time step. All bonds involving hydrogen atoms were constrained using the LINCS algorithm [52]. Thermostat and barostat conditions from equilibration were maintained. Three independent production simulations were carried out:

(1) Cellulose + TX-100 system

40 ns simulation to characterize surfactant–cellulose interactions, interfacial disruption, and structural stability.

(2) Cellulose + CTAB system

40 ns simulation to probe cationic surfactant binding, aggregation, and impact on cellulose surface morphology.

This design enables direct comparison between the native cellulose assembly and surfactant-modified systems, revealing how nonionic and cationic surfactants differentially modulate cellulose structural organization and surface dynamics.

3 Results and Discussion

The changes in fiber properties, fiber length, curl, and kink index, for DIP tissue handsheets at various refining intensities are shown in Table 2. It was observed that the fiber length of the unrefined handsheets was higher than that of the handsheets with refining intensities of 650, 1250, and 2500 rev. This is because refining applies higher shear stress to the fibers, leading to fiber cutting and breaking, as well as internal and external fibrillation, which, in turn, makes the fibers more flexible and fibrillated, thereby improving the bonding potential in the handsheets [53]. The percentage of fine formation varied with the intensity of the mechanical refining. Refining generates secondary fines content resulting from both external fibrillation and fiber shortening, and therefore, enhances the number of inter-fiber bonds per volume but decreases the drainability of the pulp slurries, which is consistent with a higher proportional fines content [54].

The DIP pulp was treated with the non-ionic surfactant TX-100 and the cationic surfactant CTAB. ToF-SIMS and energy-dispersive X-ray spectroscopy (EDS) analyses (Fig. 3) were performed to investigate the interaction between the surfactants and the fiber, thereby confirming surface modification. Figure 3A and 3B depict that there was an increase in peak intensity of 43 mass units and 60 mass units, respectively, due to the presence of C₂H₅O⁺ and C₃H₈N⁺ ions, which confirmed the successful surface modification of TX-100 and CTAB surfactant with the fiber, respectively. This was due to the interaction of the surfactants with the fiber's carboxylate sites, which attached the long alkyl chains to the fiber, thereby increasing the peak intensity [55]. The maximum intensity was observed at a refining level of 650 rev for both surfactants, and it decreased as refining intensity increased. At low refining, pulp fiber becomes more fibrillated, increasing surface area and allowing both surfactants to interact with it. At higher refining levels, the fines content increases, and surfactants interact more with the fines, thereby reducing their availability to interact with the fiber surface. As a result, fewer surfactants were bonded to the surface of the fiber, hence lowering the peak intensity at high refining levels as shown in Figure 3A and 3B. Additionally, Figure 3C–F and 3G–J show ToF-SIMS images that confirm interactions between the fiber and the surfactant. The interaction of surfactant and fiber was further confirmed by EDS (Fig. 3K–M and 3N–P). Since both CTAB and TX-100 introduce long hydrocarbon chains (rich in C and O), the carbon and oxygen content increased (Table 3) due to surfactant treatment.

Surfactant-treated DIP tissue handsheets exhibited higher bulk values than untreated handsheets for both pretreatment and post-treatment methods (Fig. 4A–D). This was because CTAB and TX-100 surfactants could neutralize the negative charge on the fiber surface, thereby enhancing stronger fiber-surfactant interactions and reducing fiber-fiber interactions, thereby increasing the bulk. As shown in Figure 4B–D, the bulk value increased with increasing refining intensity for the surfactant-treated samples, reaching a maximum at 2500 rpm. The refining process induced external and internal fibrillation, thereby increasing the cellulosic surface available for interaction [56,57]. This increased surfactant-fiber interaction weakened the strong cohesive forces between the water molecules and fibers. This resulted in comparatively weaker surfactant-water interactions, which eventually decreased the surface tension of the water surrounding the fiber. The decrease in surface tension facilitated the formation of a weaker liquid bridge during drying, thereby promoting the removal of HR water from the surroundings and within the fiber [6,13]. Consequently, the fiber lumens experienced less pulling force as HR water is expelled, due to the presence of surfactants, leading to less fiber collapse and thus increased bulk. However, the post-treatment method showed a slightly higher bulk value than the pre-treatment method when surfactant was added to the pulp slurry after disintegration. This was due to the homogeneous dispersion of the cellulosic fiber after disintegration, resulting in a greater exposed surface area for interaction [58]. Therefore, a more effective interaction between the fiber and the surfactant led to the removal of water at a lower pulling force and, consequently, to a higher bulk.

In addition, the application of TX-100 to handsheets increased the bulk more than CTAB did. This was due to stronger interaction between cellulose and CTAB than between cellulose and TX-100, as suggested by Fig. 4I. The RDF profiles highlight a clear difference between how CTAB and TX-100 interact with the cellulose surface. The sharp initial rise in g(r) for CTAB within the 0.4 nm to 0.9 nm range indicates strong localization of the cationic headgroups near the cellulose surface, reflecting electrostatic attraction between CTAB’s quaternary ammonium group and the surface hydroxyl oxygens. This distinct ordering of the first shell suggests the formation of a compact, well-defined interfacial layer. On the other hand, TX-100 shows a broader, more gradual increase in g(r) and lacks a distinct first-shell peak. This behavior is characteristic of non-ionic adsorption driven primarily by hydrophobic association. The absence of a sharp peak suggests that TX-100 molecules are distributed more loosely within the hydration layer, forming a softer, less organized interface. Therefore, handsheets made from CTAB-treated fibers show less bulk than those made from TX-100-treated fibers [58].

The tensile index is considered an important property of tissue handsheets, essential for meeting market demand. Figure 4E shows that the addition of TX-100 and CTAB during pre- and post-treatment reduced the tensile index of tissue handsheets by 0.15%–16.27% and 2.66%–13.49%, respectively, compared to untreated samples. The decrease in tensile strength was due to the highest interaction of surfactants with the fiber surface, which was obtained in the DIP_650 rev samples, as shown in Figure 4A and B. Surfactants are adsorbed on the fiber surface and introduce hydrophobic groups between the fibers, which weaken inter-fiber hydrogen bonds and inter-fiber adhesion. As a result, surfactant-modified tissue handsheets had a slightly lower tensile index than the untreated samples and exhibited increased bulk (Fig. 4A), although the decrease was not significant [30]. As the refining intensity increased from 0 to 2500 revolutions, the tensile index of both control and surfactant-treated handsheets increased gradually. The refining process increased external fibrillation and fiber flexibility, resulting in a higher fines content, as shown in Table 2, and a larger area for fiber–fiber bonding via mechanisms such as mechanical interlocking, thereby enhancing the tensile index [53]. As the refining intensity increased from 0 rev to 650 rev, the tensile index of the control handsheets increased by 6.05%, primarily due to increased external fibrillation and fines generation, which enhance fiber–fiber bonding and improve the tensile strength of the handsheets. However, the claim that fines are generated solely by fiber shortening is somewhat misleading, as fines also result from external fibrillation and partial delamination of the fiber walls during refining. For surfactant-treated handsheets, the tensile index of PrTX, PrCT, PoTX, and PoCT was respectively increased by 1.15%, 0.67%, 2.20% and 1.11% compared to the DIP_unref surfactant-treated samples (Fig. 4F). A similar trend was observed for DIP_1250 rev and DIP_2500 rev samples. In DIP_2500 rev, the tensile index of surfactant-treated samples was nearly close to that of the DIP_unref control samples (for PrCT and PoCT, it was increased by 1.90% and 7.62%, respectively, and for PrTX and PoTX, it was reduced by 3.86% and 2.23%, respectively). CTAB-treated samples (PrCT and PoCT) showed slightly higher tensile index than TX-100-treated (PrTX and PoTX) samples (Fig. 4E–H). This is because CTAB forms a compact, electrostatically anchored layer that repels surrounding water, creating a relatively dehydrated, stable surface. While TX-100 forms a more diffuse, partially hydrated region, stabilized mainly by van der Waals and hydrophobic interactions. This difference in how each surfactant arranges itself helps explain the experimental trend: CTAB-treated samples show stronger surface modification and slightly higher fiber-to-fiber bonding, while TX-100 leaves a more flexible interface. This stronger interaction with CTAB resulted in a slightly higher pulling force than with TX-100 when water was removed during drying [59]. In Figure 4I, the sharp first shell RDF peak at 0.4–0.9 nm confirms a compact and well-ordered interfacial layer, which is strongly anchored to the surface of the negatively charged cellulose, and supported by CTAB. This thin layer is pretty good at displacing water and reducing capillary pull on the fiber surface. The rigidity of this layer also limits the fiber's flexibility and the lumen's diameter expansion. On the other hand, TX-100 creates a more dispersed and less defined hydrophobic interface by van der Waals interactions. This is a weaker force of attraction, but it enables greater swelling of the fibers, increased pore opening (lumen diameter), and greater elastic spring-back when the fibers are compressed. Figure 4J represents the surface charge characterization of the untreated and surfactant-treated recycled DIP pulp. Untreated DIP pulp exhibited the zeta potential values of -26.3 mV, whereas, after being treated with CTAB surfactants, the negative surface charge of the cellulose fiber gradually decreased to -7.1 mV for 0.33% CTAB concentration. This is because of the electrostatic interaction between the positively charged quaternary ammonium head group [–N⁺(CH₃)₃] of CTAB and the negatively charged carboxylate and hydroxyl groups on the cellulose fiber surface, resulting in the surface charge neutralization. However, the integration of TX-100 at the same dosages results in a modest reduction of zeta potential of -25.4 mV, which is mechanistically consistent with the non-ionic nature of the TX-100 surfactant, because the polyoxyethylene chain of the TX-100 cannot have a fixed charge to directly neutralize the surface charge of the fiber. Rather, TX-100 adsorbs by hydrogen bonding of its polyoxyethylene chain with hydroxyl groups of cellulose, and hydrophobic interaction of its octylphenyl with the non-polar CH faces of the pyranose rings of cellulose.

The softness of tissue handsheets, which is directly related to bulk, is a very important characteristic of bath and hygiene tissues. [3] The surfactants (CTAB and TX-100)-treated tissue handsheets were softer (i.e., showed a lower TS7 value) than control handsheets under both conditions. Figure 5A shows that under unrefined conditions, DIP handsheets had a TS7 of approximately 88 dB for untreated sheets, which decreased to 35 and 57 dB, respectively, for the pre- and post-treated sheets, indicating that both CTAB- and TX-100-treated sheets increased softness. Handsheets treated with TX-100 surfactant yielded the lowest TS7 values (softest surface). Both surfactants were strongly adsorbed to the anionic cellulose fibers, and their long hydrophobic tails lubricated the fiber surface, giving it a softer handfeel. However, TX-100-treated handsheets exhibited improved softness due to enhanced mixing and interaction after disintegration, resulting in higher bulk and, consequently, greater softness. The refining intensity also affected the softness. As the refining intensity increased from 0 to 2500 rev, in Figures. In Figure 5B–D, the softness of the DIP samples also increased under both untreated and surfactant-treated conditions (indicating a decreased TS7 value), thereby improving web uniformity, increasing bulk, and decreasing surface friction. At the DIP_650 rev, in Figure 5B, the softness of the untreated samples increased by 33.06%. In surfactant-treated handsheets, the increase was 12.41% and 39.21% for PrTX and PrCT, respectively, and 26.11% and 28.56% for PoTX and PoCT, respectively, compared to DIP_unref samples. The DIP_1250 and DIP_2500 samples also showed a similar trend. At DIP_1250 rev, PoTX samples showed a 72.67% increase in softness compared to untreated samples. The highest softness of the surfactant-treated handsheets was achieved in the DIP_2500 rev PoTX samples, which was 76.06% higher than the control unrefined sample. The tensile index/softness ratio (TI/TS7) indicates the critical mechanical integrity-to-unit-of-softness ratio for the issue products. As shown in Figure 5E–H, surfactant-treated samples exhibited a higher TI/TS7 ratio than the control handsheets. Refining intensity also enhanced the TI/TS7 ratio. At DIP_2500 rev, PoTX handsheets yielded a TI/TS7 value that is 84.78%, 32.81%, and 21.43% higher than unrefined, DIP_650, and DIP_1250 rev samples, respectively, indicating the presence of fibrillated fibers and an increased bond area, which raised the tensile index and also the softness due to attachment of the surfactant tails, which gave velvety texture [7]. The highest TI/TS7 value was observed in the TX-100-treated PoTX sample, which was 2.9 times higher than that of the control unrefined handsheets. In addition, the highest softness, with moderate strength, was observed in the TX-100-treated PoTX handsheets. As shown in Figure 5H, PoTX at DIP_2500 rev offered the best balance for tissue applications (TI/TS7 ~ 0.85, TS7 ~ 21.05 dB), enhancing hand feel without minimal tensile strength loss. Post-treatment methods retained strength and softness, especially with TX-100. Therefore, PoTX at a refining intensity of 2500 rev can be used in tissue paper applications to maximize softness while minimizing tensile loss.

Figure 6A shows that surfactant-treated handsheets exhibit higher water absorption than the control at each refining level. Figure 6A–D indicate that TX-100 (PrTX, PoTX)-treated handsheets had higher water absorption than CTAB (PrCT, PoCT)-treated sheets, consistent with the higher bulk value. The higher bulk value of the TX-100-treated handsheets consequently enhances water absorption, as there is more space to hold more water. The water absorption of tissue handsheets also increased with increasing refining level from 0 to 2500 rev. Refining causes external fibrillation and the breakdown of the internal bonding between the cellulose fibers. This enables fibers to exhibit greater swelling, allowing water to move more easily through the web, thereby resulting in a higher water absorption value (Fig. 6B–D) [18,60,61]. The highest water absorption value for the treated handsheets was observed in the PoTX samples at a PFI refining of 2500 rev, which was 93.31% higher than the control DIP_unref samples. This phenomenon indicates that the surfactant effectively reduced fiber collapse and helped fibers maintain their tubular shape, as shown in Figure 8, thereby retaining more water than the untreated, unrefined handsheets.

Figure 7A–D shows that the HR water of the control handsheets is higher than that of the surfactant-treated samples at each refining condition. HR water increased with refining intensity and reached a maximum at 2500 rev for the untreated handsheets, indicating that extensive refining generates more fines, which interact with more water, thereby increasing HR water. However, surfactant-treated handsheets showed significantly lower HR water, as they can enter fiber walls and displace bound water by reducing water-fiber interactions through a decrease in surface tension, facilitating the easier removal of water molecules during drying. Also, they bind at different fiber sites, which reduces the access of water to the fiber hydroxyl groups [13]. In all cases, TX-100 has a slightly lower HR water value in both pre- and post-treatment conditions than CTAB, although the difference is not significant. This validates the higher bulk and softness values of the TX-100-treated handsheets.

Figure 8 shows SEM images of the untreated and surfactant-treated handsheets, along with the fiber pore sizes. The cross-sectional images showed that surfactant treatment increased the thickness of the DIP handsheets (Fig. 8A–C). This resulted from successive pore openings, increasing the lumen diameter and ultimately improving bulk, water absorption, and softness. Figure 8A showed that the untreated and unrefined fibers formed a relatively flat, closed network with minimal inter-fiber voids, as they collapsed during drying. As the fibers were refined to different intensities, the internal layer opened (Fig. 8D, G and J), resulting in a higher bulk that enhanced softness (Fig. 5B–D). After introducing the surfactant, fiber swelling increased due to its interaction with the fiber. Surfactants reduce the interaction between fibers and water, enabling water to pass through by reducing surface tension. The TX-100 showed a greater lumen diameter/pore opening than CTAB (Fig. 8B, E, H and K). This was due to stronger interaction between Fiber-CTAB, as shown in Figure 4F, which consequently induced higher pulling force during drying, resulting in more swollen fibers and ultimately demonstrating higher bulk, softness, and water absorption.

Figure 9 represents the elastic recovery of unrefined and refined fibers at 650, 1250, and 2500 rev under pre- and post-treatment methods. It can be observed that surfactant-treated handsheets exhibited higher spring-back than untreated handsheets at each refining condition (Fig. 9A–D). In Figure 9A, for unrefined fibers, the initial (after 5 min) and final elastic recovery (after 24 h) of untreated fibers were 84.16% and 85.45% respectively, whereas those of surfactant-treated fibers were 90.23%–95.03% and 91.21%–95.03% respectively. This was due to the lubricating effect of the surfactants, imparted by their long non-polar chain, which were adsorbed onto the fiber surface. Upon application of forces, the fibers glided past one another, and when the forces were withdrawn, the fibers regained their original shape much faster than the untreated fibers [6]. Refining conditions also influenced the elastic recovery. At PFI refining 650 rev, both TX-100 and CTAB surfactants treated handsheets showed higher initial elastic recovery by 8.92%–9.07% and 7.02%–9.20%, respectively, compared to untreated DIP handsheets refined at 650 rev. The final recovery also increased by 11.40%–11.98% for TX-100 and 10.40%–10.57% for CTAB-treated DIP handsheets compared to the untreated ones. Similar trends were observed for the samples refined at 1250 rev and 2500 rev; the initial elastic recovery increased by 11.42%–14.49% and 17.96%–18.05% for TX-100, respectively, and 9.35%–12.48% and 11.72%–13.62% for CTAB, respectively. The final recovery was also enhanced by 14.06%–15.43% and 16.48%–16.54% for TX-100, respectively, and 9.21%–13.45% and 12.03%–12.73% for CTAB, respectively (Fig. 9C–D). Among the surfactants, TX-100-treated handsheets exhibited slightly higher elastic recovery than CTAB-treated handsheets because TX-100 interacts with the cellulosic surface via van der Waals forces, thereby increasing bulk. Additionally, its hydrophobic tails created a lubricating surface that allowed fibers to slide past one another under applied force, thereby enhancing their elastic behavior.

4 Conclusions

We have successfully prepared surfactant-treated tissue handsheets from recycled DIP pulps and evaluated the effectiveness of different treatment conditions (pre- and post-treatment) at varying refining intensities (0, 650, 1250, and 2500 rev). The successful addition of cationic (CTAB) and non-ionic (TX-100) surfactants was confirmed by ToF-SIMS and EDS analysis, which improved several essential characteristics of the tissue handsheets. Additionally, the radial distribution function from the molecular dynamics simulation confirms cellulose-surfactant interactions. All CTAB and TX-100-treated samples showed higher pore opening (higher in TX-100 treatment), i.e., bigger diameter, which led to a higher bulk value than the control, and the bulk of surfactant–fiber networks increased by the order of 16.71%–57% (post-treatment > pretreatment and TX-100 > CTAB). On the other hand, the tensile index was slightly reduced due to less fiber-fiber interaction. At 1250 and 2500 refining intensities, the tensile index was near that of the control unrefined handsheets, and for the DIP_2500 rev PoTX sample, it was 8% higher than the control samples. The softness of Tissue Handsheets had significantly increased to 71.16%, 72.66%, and 76.06% for DIP_650 rev, DIP_1250 rev, and DIP_2500 rev samples, respectively, with a TX-100 post-treatment, whereas DIP_2500 rev PoTX was found to experience the maximum softness value of the tissue handsheets sample. The (TI/TS7) ratio also increased (e.g., PoTX at 2500 rev approached 0.85, which was 3.05 times higher than that of the control), indicating improved strength-per-softness. The hard-to-remove (HR) water content of the handsheets was lower across all refining conditions when treated with both surfactants. However, TX-100 reduced HR water content across all conditions by 20.06%–55.31%, and for CTAB, it was 14.10%–49.85% compared to the control treatment. Water absorption of treated handsheets increased by 66.72% to 93.31%. The benefits that were achieved by 93.29%–95.03% and 90.16%–97.37%, for initial (after 5 min) and final (after 24 h) elastic recovery, respectively. In the treatment methods, TX-100 made the softest sheets (lowest TS7) and the strongest fiber bonding (highest TI/TS7). Pre- and post-treatment had a slight effect on tissue handsheets (post-treatment slightly increased bulk). The optimum was the PoTX at 2500 rev, which raised the bulk by approximately 57%, increased tensile index by about 8%, raised softness by approximately 76%, roughly 3.05 times higher the TI/TS7 ratio, and increased water uptake by about 93% and lowered HR water content by about 55%. The findings validated that surfactant-treated DIP tissue handsheets can produce ultra-soft, high-bulk tissue with minimal strength trade-off, suggesting a sustainable path to greener tissue development.

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