ACS Sustainable Chemistry & Engineering
pubs.acs.org/journal/ascecg
Research Article
(Figure 2C). At 30 wt % (hairy) cellulose fiber loading, a full coverage of the EFB fibers can be achieved, creating a continuous layer of (hairy) cellulose fibers over the entire nonwoven EFB fiber surface (Figure 2D). The hygroscopic nature of EFB fibers 34 facilitated a flow-and-attach process, where water absorption brought (hairy) cellulose pulp fibers into contact with the EFB fibers during the mixing stage. Once adhered, dewatering, wet pressing, and restrained drying formed a rigid nonwoven EFB fiber network, held together by the bonding of (hairy) cellulose pulp fibers through van der Waals’ interactions, hydrogen bonding, and fiber/molecular entanglement, binding the EFB fibers. The porosity of EFB/(hairy) cellulose fiber nonwovens is summarized in Figure 3A. Without any cellulose binders, the neat EFB nonwoven possessed a porosity of ∼ 80%. This can be attributed to poor adhesion between adjacent EFB fibers, leading to a loose fibrous packing. The addition of unrefined cellulose fibers reduced the porosity of the resulting nonwoven material to ∼ 71%. At low hairy cellulose fiber loading, the porosity of the resulting EFB/hairy cellulose fiber nonwovens remained constant at ∼ 70%, independent of the refining time of the cellulose fibers. This finding corroborates with the morphology of the nonwovens shown in Figure 2B, whereby the coverage of EFB fibers by the (hairy) cellulose fibers is low. At a high hairy cellulose fiber loading of 30 wt %, the porosity of the EFB/hairy cellulose fiber nonwovens decreased to ∼ 60%. This reduction in porosity can be attributed to the higher coverage of the hairy cellulose fibers, and the fibrillated surface fills void spaces within the nonwoven structure. Consequently, this improves fiber packing efficiency and results in a denser material. 35 2.3. Permeability of EFB/(Hairy) Cellulose Fiber Nonwovens. Air permeability, herein defined as the time taken for a given volume of air flowing through a constant area (in our work, a flow area of 0.1 in 2 was used) at a differential pressure of 12.2 inH 2 O was measured by using the Gurley method (Figure 3B). It follows that longer time taken for air to flow through corresponds to lower air permeability. For the air permeability of nonwovens without cellulose fiber binder and those containing 10 wt % (hairy) cellulose fibers, an air volume of 30 cm 3 was used in the Gurley setup. Even at such high air volume, it only took ∼ 1 − 3 s under 12.2 in H 2 O of pressure to pass through these nonwovens. Due to the higher air resistance, the air permeability of the nonwovens containing 20 and 30 wt % (hairy) cellulosic fibers were quantified using an air volume of 10 cm 3 . Even when a lower air volume was used, a higher air resistance (i.e., higher Gurley seconds) was obtained. At 20 wt % cellulose fiber loading, the air resistance increased from 15 s for unrefined cellulose fibers as the binder to 123 s for hairy cellulose fibers refined for 30 min as the binder. A similar trend was observed at 30 wt % hairy cellulose fiber loading, whereby air resistance increased from 110 s for unrefined cellulose fibers as the binder to ∼ 1200 s for hairy cellulose fibers refined for 30 min as the binder for the resulting nonwovens. The permeability of a porous structure is dependent on its porosity and equivalent pore diameter (see Carman − Kozeny 36 or Ergun 37 equations), as well as the tortuosity of the interconnected porous structure. 38 Helium pycnometry con- firms that our nonwovens possessed an interconnected pore network. Therefore, the increase in air permeability of our nonwovens when the loading of (hairy) cellulose fibers was increased at a constant refining time can be attributed to an
increase in EFB fiber coverage. At the same (hairy) cellulose fiber loading, particularly 20 and 30 wt %, it is postulated that the increased degree of fibrillation of the cellulose fibers created smaller and more tortuous channels for air to flow through. Similar effects had been observed in hand sheets made from beaten pulp, whereby those produced from pulp with a higher degree of beating (fibrillation) exhibited greater air resistance. 39 For particulate filtration applications, pressure drop is an important parameter. Higher pressure drop is undesirable, as it is associated with higher energy consumption. 40 The pressure drop across our EFB/(hairy) cellulose fiber nonwovens was quantified by measuring the pressure downstream and upstream of our nonwovens mounted in our in-house built open-circuit wind tunnel. 41 The face velocity in our wind tunnel in this experiment was 0.15 ± 0.05m s − 1 . As shown in Figure 3C, the pressure drop across our nonwovens is consistent with their respective air permeability. Higher loading of (hairy) cellulose fibers as binder and longer refining time of the (hairy) cellulose fibers lead to higher pressure drop across the resulting EFB/(hairy) cellulose fiber nonwovens. The EFB nonwoven without any cellulose fiber binder was found to possess a pressure drop of 0.02 kPa, and doubled to 0.04 kPa when 10 wt % unrefined cellulose fibers were used as the binder. The pressure drop increased up to 0.13 kPa when 30 min refined hairy cellulose fibers were used at the same loading. As expected, the increase in the loading of (hairy) cellulose fiber binder used further increased the pressure drop of the nonwovens. The use of 30 wt % 30 min refined hairy cellulose fibers as the binder leads to a nonwoven with a pressure drop of 0.42 kPa. As a comparison, the pressure drop across a three-ply FFP2 face mask was measured to be ∼ 0.1 kPa in our wind tunnel setup under the same condition. It is also worth mentioning that the pressure drop of our nonwovens is comparable to other natural fiber-based nonwovens, including the combination of kapok and hardwood fibers ( ∼ 0.15 kPa), 42 fibrillated lyocell nanofiber, softwood, and PET blends (0.04 − 0.4 kPa), 43 and needle-punched flax mats ( ∼ 0.1 kPa) 17 when tested at the similar face velocity of ∼ 0.15 m s − 1 . We also evaluated the water vapor transmission rate (WVTR, see Figure 3D) of the fabricated EFB/(hairy) cellulose fiber nonwovens using the Turl dish method. The results indicate that WVTR is independent of both the amount and refining time of the (hairy) cellulose pulp fibers used as a binder. This suggests that the hygroscopic nature of the EFB fibers (measured contact angle in the form of fiber sheet = ∼ 40 ° ) 44 and the cellulose pulp fibers (measured contact angle of cellulose network in a hand sheet = ∼ 25 ° ), 45 driven by the hydroxyl groups on cellulose and hemicellulose molecules, 46 dominates water vapor transport than the pore structure of the EFB/(hairy) cellulose fiber nonwovens. A similar trend was also observed in paperboards with the addition of nano- cellulose, where the increased air resistance from nanocellulose incorporation was not accompanied by a reduction in the WVTR. 47 2.4. Aerosolized Particulate Filtration Efficiency of EFB/(Hairy) Cellulose Fiber Nonwovens. Figure 3E presents the aerosolized particulate filtration efficiency of the fabricated EFB/(hairy) cellulose fiber nonwoven. This measurement was also conducted in our in-house built open circuit wind tunnel. 41 An air flow containing aerosolized NaCl particles (number-averaged diameter = 0.27 ± 0.01 μ m) was
6213
https://doi.org/10.1021/acssuschemeng.5c00041 ACS Sustainable Chem. Eng. 2025, 13, 6209 − 6221
Made with FlippingBook. PDF to flipbook with ease