3.1. Comparison of fiber morphology
Diluted and fully disintegrated pulp samples of bleached fique residue, NBSK, and BEK were analyzed in terms of their fiber length, width, fines content, and fiber coarseness. Fiber population in terms of millions of fibers per gram of pulp was calculated from the total number of fibers and the total mass of pulp samples used in the FQA analysis. Table 2 shows fiber morphology data as obtained from FQA for all the pulp samples. Fique residue bleached pulps and NBSK market pulps had remarkably similar fiber morphology. The average fiber length of fique residue bleached pulp (~ 2.3 mm) was comparable to the NBSK market pulp (~2.4 mm). Fiber length is one of the most critical parameters for reinforcing pulps as longer fibers have the ability to form inter-fiber bonds with multiple fibers and provide superior strength to the tissue paper (De Assis et al. 2018). Moreover, fique residue bleached pulp had lower fiber width, fiber coarseness, and lesser fines content than the NBSK market pulp. These properties of fique residue bleached pulp are more conducive to developing favorable tissue paper’s properties than the NBSK market pulp. Due to the combination of comparable fiber length and narrower fiber width, fibers from fique residue bleached pulps had a higher aspect ratio (length/width). Slightly smaller fiber length and lower fiber coarseness resulted in fique residue bleached having a higher fiber population (~10% higher) than NBSK market pulp.
In addition to the average values, the distribution of fiber length and width are also shown in Figure 2. Compared to NBSK market pulp which is likely manufactured from tress with different ages and species, fique residue pulp was prepared from a single plant species of similar age. As a result, fique residue bleached pulp had a narrower distribution of fiber length and width. A narrower distribution of fiber morphology improves product uniformity and gives better control on achieving final properties (de Assis et al. 2019).
3.2. Effect of refining on fiber properties
A lab-scale PFI refiner was used to refine the pulps in this study. The compressive and shear forces applied during the PFI refining process produces several changes in the fiber structure, such as external fibrillation on the fiber surface, internal fibrillation (delamination of cell wall layers and changes in the internal structure of cell wall), fiber shortening, and fines generation (Page 1989). The relative predominance of these refining effects depends on the physical and chemical properties of fibers. Thus different fibers respond differently to refining (Page 1989; Gharehkhani et al. 2015). The cumulative effect of these changes in the fibers’ structure can be observed by measuring changes in pulp freeness, apparent density, light scattering coefficients, and strength properties of the final paper web (Wang et al. 2005, 2007; Kang and Paulapuro 2006). In this study, the effect of refining on fiber properties was evaluated by measuring the freeness of the pulp and bulk (inverse of the apparent density) of the final tissue handsheet. In addition, changes in the fiber’s internal structure were investigated using SEM (scanning electron microscopy) micrographs.
Figure 3 shows SEM micrographs of the surface and cross-section of tissue handsheets made with unrefined and refined fibers of bleached fique residue and NBSK pulp. Lower resolution images of the surface of the tissue samples present in the first column (Figures 3a, 3d, 3g, 3j) are used to evaluate the network structure of fibers. Higher-resolution surface micrographs present in the second column (Figures 3b, 3e, 3h, 3k) are used to compare any changes in the internal structure of fibers before and after refining. Micrographs present in the third column (Figures 3c, 3f, 3i, 3l) represent the cross-section of the tissue samples and are used to evaluate the collapsibility of the fibers lumen and relative consolidation (loss of bulkiness) of the fibrous assembly with the refining process. Overall, refining consolidated the paper web structure and reduced the pore size and pore volume of both pulps. However, it can be observed that fique residue bleached pulp formed a more open, porous, and bulkier paper structure than NBSK pulp in both unrefined and refined fiber states. The cell wall of NBSK fiber completely collapsed into a flat ribbon structure with refining (Figures 3k and 3l). In contrast, some fibers in fique residue pulp can be observed to retain their original cylindrical shape and intact fiber lumen (Figure 3i). Moreover, some sporadic external fiber fibrillation was observed on the outer surface of refined fique residue pulp, while the outer surface of NBSK pulp fibers was relatively cleaner and smoother. Overall, the cell walls of NBSK pulp fibers completely collapsed and seemed to favor more internal fibrillation and negligible external fibrillation with refining. Cell walls of fique residue pulp did collapse with refining; however, they were more resistant to the internal fibrillation than NBSK pulp and favored a certain degree of sporadic external fibrillation. The milder conditions of refining conducted in this study did not generate secondary fines or reduce the fiber length significantly.
To further investigate the effect of refining on the properties of the fibers and corroborate the findings from SEM micrographs, changes in freeness and bulk of the tissue paper were measured for fique residue and NBSK pulp. Figure 4a indicates the change in pulp freeness with the number of PFI revolutions, and Figure 4b compares the change in bulk of the tissue paper at different pulp freeness values. Freeness of pulp determines its drainage behavior when used on paper machines to produce paper. As the number of PFI revolutions increased, the freeness of both pulps decreased proportionally (Figure 4a). Fique residue and NBSK almost followed a similar drainage profile with the refining; however, fique residue seems to have a faster drop in pulp freeness at higher PFI revolutions. Compared to pulp freeness, fique residue pulp displayed higher bulk than NBSK pulp for a given value of pulp freeness (Figure 4b). However, fique residue pulp had a higher reduction in bulk than NBSK pulp with increasing refining. Thus, the bulk of both pulps seems to be converging at higher refining energy (lower pulp freeness). It appears that the thin cell wall of NBSK pulp easily develops internal fibrillation, thus making densified paper sheets than fique residue pulp. On the other hand, fique residue pulp also favors a certain degree of external fibrillations apart from internal fibrillation, particularly in the later stage of refining. These observations agree with previous studies aimed to separate the effects of external and internal fibrillation on final paper properties. Wang et al. (2007) and Kang and Paulapuro (2006) reported that internal fibrillation increases the collapsibility and flexibility of fibers and is the primary factor responsible for the densification of a paper sheet, while external fibrillation leads to an increased specific surface area of fibers and mainly correlates with the decreased freeness of pulp.
3.3. Comparison of strength properties
Tissue paper products must have sufficient strength and durability to resist tear and rupture during the manufacturing and consumer use stages (De Assis et al. 2018). In this context, tensile strength and tear strength are considered two essential indicators of tissue paper’s strength properties (Shannon 2016). Page (1969) and Van den Akker (1958) showed that the strength of a lightly bonded paper structure such as tissue papers is mainly limited by the inter-fiber bonding strength between individual fibers rather than the strength of individual fibers (Van Den Akker et al. 1958; Page 1969). Mechanical refining and wet pressing can be used to improve the inter-fiber bonding in a tissue paper sheet; however, there are undesirable tradeoffs with other important tissue properties such as softness and water absorbency. NBSK fibers with long fiber length, low coarseness, and easily collapsible thin cell walls are highly desirable in the tissue industry to provide the necessary strength to the paper while balancing softness and other important tissue properties (Shannon 2016; De Assis et al. 2018). Thus, the potential of fique residue bleached pulp to act as a reinforcing fiber in tissue paper products was investigated by comparing its strength properties against the NBSK market pulp. Figure 5ashows the tensile strength of fique residue bleached pulp and NBSK pulp at different pulp freeness, and Figure 5b compares the tear strength of both pulps for a given tensile strength value. Both fibers were subjected to the same amount of refining energy (measured by the number of PFI revolutions), and their response to this refining energy was observed.
As expected, the tensile strength of both pulps increased, and freeness decreased as refining forces increased the flexibility, conformability, and total surface area of the fibers. At higher pulp freeness, when the bonding between fibers is low, the tensile strength of fique residue is higher than the NBSK market pulp. When refining is increased, NBSK pulp develops tensile strength faster than the fique residue pulp. At pulp freeness below 600 ml when interfiber bonding is relatively developed, NBSK market pulp displays superior tensile strength than the fique residue pulp. The difference in the tensile strength curve between both fibers can be understood by observing the morphology of fibers in the unrefined state and changes occurring in the structure of fibers throughout the refining process. Higher fiber population, higher aspect ratio, and lower fiber coarseness of fique residue pulp (Table 2) translate into better fiber coverage and a higher number of fiber contacts than the NBSK market pulp and contribute to its higher tensile strength in the unrefined state (Seth 1990a, b). As refining proceeds, NBSK fiber with its wider lumen and thinner cell walls collapses easily into a flat ribbon-like structure (Figure 3 and Figure 4b), thus significantly increasing the relative bonded area between the fibers. Also, at higher refining when bonding between fibers is well developed, a higher percentage of fibers are broken during the tensile failure of paper, and individual fibers’ tensile strength becomes more important for the total strength of paper (Van Den Akker et al. 1958; Page 1969). Zero span tensile strength of both pulps was measured as an indicator of tensile strength of individual fibers in that pulp (Van Den Akker et al. 1958; Seth and Chan 1999). It was observed (Table 2 and Figure A1 in the supplementary information) that NBSK pulp has approximately 20% - 35% higher zero span tensile strength than fique residue bleached pulp indicating superior tensile strength of individual fibers in the NBSK pulp. Moreover, SEM micrographs (Figure 3) show that the refining actions expose cellulosic fibrils and microfibrils on the outer cell wall layers of fique bleached residue pulp and make its fiber surfaces rougher than the refined NBSK pulp. A rougher surface hinders the contact between adjacent fibers and reduces the relative bonded area (Hubbe 2006), thus resulting in weaker inter-fiber bonding for fique residue bleached pulps than NBSK market pulp at higher refining levels.
After a direct fiber-to-fiber comparison between the fique residue pulp and the NBSK market pulp, both reinforcement pulps were separately blended with BEK market pulp at different weight proportions, and tissue handsheets were made using the resulting pulp furnish. Since fique residue bleached pulp provided better bulk properties even at higher refining, it was refined at two different pulp freeness values and separately blended with BEK pulp. Figure 6 shows the tensile strength of the resulting tissue handsheets as a function of the weight percentage of the reinforcing pulps in the pulp furnish. As the amount of reinforcement pulp increased in the furnish, the tensile strength of the final paper increased proportionally. As expected, the addition of fique residue bleached pulp refined at higher refining energy (or to a lower freeness) provided the highest improvement in the tensile strength at any given weight percentage. Despite NBSK pulp showing faster tensile strength development with refining, the addition of fique residue bleached pulp refined at similar refining energy as NBSK pulp (1000 PFI revolutions) provided either similar or better tensile strength than the NBSK market pulp. The reinforcement mechanism is based on longer fibers forming bonds with multiple fibers leading to a more efficient stress transfer between fibers when a load is applied to the fibrous assembly (Seth 1990b). Moreover, blending two different fibers in a pulp furnish can lead to either positive or negative synergy between the fibers depending on the packing structure of fibers in the fibrous network. Considering that NBSK fibers had a compact structure even at 1000 PFI refining revolutions, BEK pulp’s addition might disrupt its existing compact network structure. In comparison, fique residue bleached pulp had an open network structure with lots of voids and gaps between the fibers. The addition of shorter fibers from BEK pulp might fill the open space and bridge the gap between fibers leading to better stress distribution when a load is applied.
3.4. Water absorption capacity
Hygiene tissue products are designed to absorb the maximum amount of water in the lowest possible time (Beuther et al. 2010). Hence, water absorbency properties are one of the most crucial properties for tissue paper products. Water absorption capacity determines the total amount of water that a tissue paper can retain under saturated conditions (Ko et al. 2016). From a water absorption capacity point of view, tissue paper can be considered as a two-phase system of cellulosic fibers and interfiber pores where both phases can potentially absorb water (Bristow 1986). The function of the cellulosic fibers forming the underlying tissue structure is to maximize the volume of interfiber pores and provide sufficient hydrophilicity for additional sorption of water into fibers (Ko et al. 2016; de Assis et al. 2019; Zambrano et al. 2021a).
The primary objective of adding a reinforcement pulp is to provide strength and durability to the tissue structure. However, the addition of a reinforcing fiber might negatively impact the water absorbency properties of the tissue paper (De Assis 2018; Stankovská et al. 2020). Hence, the best performing reinforcement pulp should provide the maximum water absorbency for a given tensile strength of tissue paper. Figure 7 compares the water absorption capacity of fique residue pulp with NBSK pulp at different refining energies. Refining has been used to change the tensile index and the apparent bulk of paper made with both pulps. Figure 7a and Figure 7b show that fique residue bleached pulp provided better water absorption capacity than NBSK pulp for any given value of the tensile index. As refining progressed, the water absorption capacity of both pulps decreased proportionally; however, the same trend of fique residue pulp having higher water absorption capacity than NBSK pulp was observed at all refining levels used in this study. Previous studies have shown that the majority of water absorbed in a tissue paper sheet is located inside the pores between fibers and fiber lumens (Bristow 1986; de Assis et al. 2019; Zambrano et al. 2021b). De Assis et al. (2019) and Zambrano et al. (2021) reported a high correlation between water absorption capacity and the bulk of the tissue paper. For tissue papers made with highly bleached cellulosic fibers, when there is not much difference between the chemical composition of fibers, the difference in water absorption capacity can be explained based on the total apparent pore volume available in the paper (Zambrano et al. 2021b). Thus, fique residue bleached pulp’s superior water absorption capacity can be attributed to the higher bulk of its paper structure (Figure 7b). Overall, water absorption capacity showed a 92% linear correlation with the bulk of all tissue handsheets evaluated in this study. This effect can also be visualized through the SEM micrographs in Figure 3, wherein higher fiber curl and tubular fiber structure of the fique residue pulp created a relatively porous and bulkier web structure than the compact and consolidated paper web structure generated by NBSK pulp.
Additionally, it is important to note that all tissue papers displayed higher water absorption capacity than the original pore volume present in the paper, which can be calculated using the apparent bulk (cm3/g) of the paper in the dry state. The extra water being absorbed is accommodated between the plies, the pores within the fiber cell wall, and changes in the original pore volume of paper caused by the swelling of the cellulosic fibers(Bristow 1986; de Assis et al. 2019). An equal number of handsheets was used to measure the water absorption capacity for both pulps; hence, it can be assumed that a similar amount of water was present between the plies in all observations. However, it is essential to acknowledge that NBSK pulp is a once-dried pulp, and fique residue is a never-dried pulp. Drying brings some irreversible changes in the morphology of fibers (i.e., hornification) and negatively impacts their swellability with water upon rewetting (Gurnagul et al. 2001; Hubbe et al. 2007).
Figure 8 shows the water absorption capacity as a function of the tensile index of tissue handsheets prepared from pulp blends of BEK and the individual reinforcement pulps. Tissue handsheets made with 100% unrefined BEK pulp have a water absorption capacity of 7.1 grams water per gram of fiber which was in a similar range as observed for fique residue bleached pulp refined at 1000 PFI (635 CSF mL). Hence, the addition of fique bleached pulp refined at 1000 PFI did not have a negative impact on the water absorption capacity of the tissue handsheets. Compared to that, adding NBSK pulp or fique residue pulp refined at higher refining energy (2000 PFI) reduced the water absorption capacity of the resulting tissue handsheets (Figure 8). However, in both cases (similar refining energy or higher refining energy), the fique residue pulp provided better water absorption capacity than NBSK pulp for a given value of the tensile index, which can be attributed to the bulkier fibrous structures imparted by the introduction of fique residue pulp
The perceived softness of a hygiene tissue product is a subjective impression that the human mind generates when tactile sensors present in human hands interact with the sample material (Wang et al. 2019). Previous studies have shown a high correlation between perceived softness and measurable physical properties of tissue papers such as surface roughness, stiffness, compressibility, flexibility, free fiber ends, fiber flexibility, etc. (Hollmark 1983; Hollmark and Ampulski 2004; Wang et al. 2019). Accordingly, two different aspects of softness have been proposed in terms of surface softness and bulk softness. The surface softness relates to the perception of softness when fingertips move over the surface of a tissue paper, while bulk softness indicates the softness perception when a human hand folds and crumples the tissue paper (Hollmark 1983). In this work, the softness of the tissue handsheets was measured using a tissue softness analyzer (TSA, Emtec Electronic GmbH, Germany), which measures the surface and bulk softness independently using three basic parameters: TS7 (surface softness), TS750 (surface smoothness/roughness), and in-plane flexibility (bulk softness) (Wang et al. 2019; Prinz et al. 2021). As with the water absorption capacity, the objective is to maximize the softness of tissue handsheets for a given value of the tensile index. Figure 9a and 9b compare the TS7 (surface softness) and the in-plane flexibility (bulk softness) of tissue handsheets made with 100% fique residue bleached pulps and 100% NBSK pulp without any fiber blending with BEK pulp.
The comparison among BEK, NBSK, and fique residue bleached pulp in their unrefined state showed that BEK pulp had the best softness properties in terms of surface softness (TS7), bulk softness (in-plane flexibility), and TS750 (surface smoothness/roughness). TS750 values for tissue handsheets are presented in Figures A3 and A4 and provided in the supplementary information. BEK pulp produced the lowest TS7, highest in-plane flexibility, and highest surface smoothness (lowest TS750). Fique residue bleached pulp provided better surface softness (lower TS7) and higher surface smoothness (lower TS750) than the NBSK pulp with a similar bulk softness (in-plane flexibility). As refining was used to develop the tensile strength of NBSK and fique residue pulp, it negatively impacted all aspects of softness. NBSK pulp with lower coarseness and flexible thin cell walls are desirable in hygiene tissue products over other long-length fibers such as SBSK (southern bleached softwood kraft) given their ability to impart superior surface softness property (Shannon 2016; De Assis 2018). However, fique residue bleached pulp consistently produced lower TS7 values than NBSK pulp at all refining levels which indicates its ability to provide superior surface softness when used in hygiene tissue products. It appears that the combination of shorter fiber length, lower coarseness, low fines content, higher fiber curl, and higher fiber population contributes to the fique residue pulp having lower TS7 values than NBSK pulp (Wang et al. 2019; Prinz et al. 2021). However, it is also important to acknowledge the limitations of TSA equipment regarding its ability to accurately compare the softness of samples made from different fiber sources. Apart from flexible and free fiber ends, a more porous sample can also dampen the amplitude of vibrations more efficiently, resulting in lower TS7 values recorded in comparison with a less porous surface (Prinz et al. 2021). Fique residue pulp had a lower apparent density (higher porosity) than the NBSK pulp at all refining levels (Figure 4b), which could have also contributed to it producing lower TS7 values than NBSK pulp. The open and porous nature of tissue samples made with fique residue bleached pulp can also be visualized through the SEM micrographs presented in Figure 3.
Compared to the surface softness, it was surprising to observe both pulps showing similar in-plane flexibility or bulk softness at all refining levels despite fique residue pulp having superior bulk. To further evaluate this observation, tensile stiffness of handsheets was recorded from the tensile testing of handsheets. Figure A2 (provided in the supplementary information) indicates that both pulps have similar tensile stiffness in their unrefined state, but the NBSK pulp has significantly higher tensile stiffness (~35% higher) than the fique residue pulp at higher refining levels. The in-plane flexibility as measured by the TSA equipment is closely related to the flexural rigidity or bending stiffness of the sample (Zambrano et al. 2021a). The flexural rigidity, in turn, is a function of the tensile stiffness and the third power of the sheet thickness (Hollmark 1983). Hence, it appears that the sheet thickness is also having significant impacts on the measured in-plane flexibility of tissue samples (Hollmark 1983). This might explain why NBSK pulp with higher tensile stiffness but lower caliper has similar in-plane flexibility as fique residue pulp with lower tensile stiffness but higher caliper.
After comparing the softness properties of individual pulps, the softness of tissue handsheets prepared using pulp blends of BEK and reinforcement fibers was studied. Figure 9c and Figure 9d present the TS7 (surface softness) and in-plane flexibility (bulk softness) of tissue handsheets made with different pulp blends. Since reinforcement pulps were successively added in smaller weight percentages, it is expected that tissue properties should be mainly dominated by the BEK fibers present in the majority. It is argued that this effect should also negate the impact of porosity on softness measurement, thus providing a more accurate comparison of softness than that obtained by comparing pulps individually. Shorter fibers have a higher tendency to protrude out from the paper surface, which is augmented by the remarkably higher number of individual fibers in the pulp. On the other hand, adding refined and long-length reinforcing fibers reduces the number of free fiber ends and increases the stiffness of the paper structure by increasing the degree of bonding between the fibers. As a result, both surface softness (TS7) and in-plane flexibility got worsened as the amount of reinforcing pulp was increased in the tissue handsheet. Individually, tissue handsheets prepared with the pulp blend of BEK and fique residue refined at lower refining energy provided superior surface softness (lower TS7) than the NBSK pulp. However, when fique residue pulp was refined at higher refining energy (580 CSF), its surface softness became comparable to the NBSK pulp (Figure 9c). Similar to the trend observed when comparing individual pulps, in-plane flexibility or the bulk softness of all pulp blends was comparable (Figure 9d), which was attributed to the combined effects of tensile stiffness and the thickness of tissue handsheets.