Banana leaf residue as raw material for the production of high lignin content micro/nano fibers -

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Banana leaf residue as raw material for the production of high lignin content micro/nano fibers

The present work aims to produce lignocellulosic micro/nanofibers (LCMNF) from pulp made of banana leaf residue and to assess their reinforcement potential and production costs for the paper making sector. For the first time, high-lignin content LCMNF will be produced with the same reinforcing capacity than CMNF prepared from woody and bleached pulp, using methodologies under the umbrella of bioeconomy, waste valorization and sustainable growth.

Raw material characterization

The banana leaves were the raw material; Banana is a herbaceous plant (2–16 m high), composed of long fibers strongly overlapped forming a pseudo-stem. Banana plants generally produce large leaves (almost 2 m long and 30–60 cm wide) that were used in this work. The content of -cellulose, holocellulose, lignin, ashes, ethanol extractable, hot water soluble, 1% NaOH soluble, Kappa number and viscosity were determined according to TAPPI standards T–9m54, T–222, T–203os61, T–211, T–204, T–257, T–212, T–236 cm and T230–om–94, respectively.

Pulping

The raw material was pulped in a 15 L batch reactor under Specel® process conditions, (100 ◦C ± 1 ◦C, 150 min, 7% (o.d.m) NaOH as reagent and a liquid/solid ratio 10:1). After the pulping process, cooked material was washed in order to remove residual cooking liquor and then the cooked material was dispersed in a pulp disintegrator at 1200 rpm during 30 min. After that, the pulp was passed through Sprout-Bauer refiner, and using a Sommerville equipment with a netting of 0.14 mm mesh size, the uncooked material was separated.

Banana leaf pulp characterization

The resulting pulp was chemically characterized in the same way as the raw material. In addition, the morphology of fibers was determined using a MorFi Compact equipment (TechPap, Grenoble).

LCMNF production

In the mechanical treatment, the banana leaves residue sodaSpecel® pulp was refined in a PFI mill (NPFI 02 Metrotec SA) according to ISO 5264-2 until achieving a drainage rate of 90o SR. Then, 1% aqueous suspension was prepared and passed through a high pressure homogenizer (Panda Plus 2000) following the next sequence: 4 times at 300 bar, 3 times at 600 bar and 3 times at 900 bar. The energy consumption of the homogenizer during the production of LCMNF was carried out with an energy measuring equipment (Circutor CVM-C10), which gives values of the energy consumption of the equipment or, what is the same, the energy required from the electrical grid.

LCMNF characterization

Conduct metric titration was used to determine the carboxyl content (CC) of LCMNF. In this procedure, first, a dried sample (50–100 mg) was suspended and stirred in 15 mL of 0.01 M HCl solution, which exchanges Na cations bound to the COOH group by H ions. Then, the suspensions were titrated with 0.01 M NaOH, adding 0.1 mL of NaOH to the suspensions and recording the conductivity in mS/cm until observing a reduction, stabilization and increase in the conductivity.

The Nano fibrillation yield was determined by centrifugation at 0.2 wt% suspension and 4500 rpm for 20 min in order to isolate the Nano fibrillated fraction. After that the supernatant was recovered, weighted and oven-dried at 90 ◦C until constant weight.

Transmittance measurements were performed on LCMNF suspensions with 0.1% of solid content. The sample was introduced in quartz cuvettes and the transmittance measured with a UV–VIS Shimadzu spectrophotometer UV-160A set in the range between 400 and 800 nm. Distilled water was used as the reference. The cationic demand and the carboxyl content also serve as an indirect indicator of the specific surface area of LCMNF through the diameter of LCMNF that can be calculated. These samples were tested according to the cited methodology because of the impossibility of measuring values lower than 75 eq-g/g through conduct metric titration

Incorporation of LCMNF into the pulp suspension

LCMNF were added and dispersed into a bleached kraft hardwood pulp suspension by means of a pulp disintegrator operating at 3000 rpm during 60 min and 1.5% consistency using tap water (289 S/cm) as the background. The dosage was established at 3% (o.d.m.). After this step, cationic starch and colloidal silica were added at the corresponding doses of 0.5 and 0.8%, respectively, expressed on dry pulp, with the purpose of retaining LCMNF on fibers surface. This combination between cationic starch and colloidal silica was proved to retain all the added LCMNF by means of forming paper over a nitrocellulose membrane (0.22 m of pore size) with 3 wt% LCMNF without any retention agent. The obtained tensile strength of the resulting paper was the same than the obtained using such dual retention system. The application of these retention agents was done at a gentle agitation of the suspension at 1% consistency for 20 min. Paper sheets presented an average basis weight of 75 g/m2 (using a sheet former SP mod. 786FH) according to ISO standard 5269-2 and were conditioned in a weather chamber at 23 ◦C and 50% humidity for 48 h before testing them in tensile mode. 2.7. Paper sheet characterization Paper sheets were tested in tensile mode according to ISO 1924- 2 testing conditions with an Instron Universal Testing machine equipped with 2.5 kN load cell. Papers prepared from neat pulp presented a breaking length of 2054m.

Results

Raw material characterization

The holocellulose content of the banana leaf residue (55.48%) is lower than the content found for other agricultural residues (64.05, 61.22, 60.82, 63.01 and 60.6%, for wheat straw, olive tree prunings, vine shoots, sunflower stalks and cotton stalks, respectively). However, the lignin content (25.25%) is higher than those found in the agricultural residues mentioned above (14.46, 18.85, 21.62, 14.08 and 18.29%). The extractive content (7.59%) is lower in comparison to the values of extractive of other raw materials considered (13.84–28.74%). The ash content (15.35%) is the greatest one compared (from 3.72% for the sunflower stalks up to 9.51% for vine shoots). Finally, fixed carbon value (14.51%) is less than other raw materials compared (15.86–20.33%), with the exception of olive tree pruning (10.73%).

Production costs of LCMNF

The benefits of using LCMNF prepared by mechanical methods instead of using TEMPO-oxidized CMNF. For the obtaining of LCMNF, mechanical refining at 22,000 revolutions is required and, then, some passes through the homogenizer at different pressures. LCMNF through mechanical methods limits the yield of fibrillation and, thus, the resulting LCMNF present diameters higher than 20 nm, their performance as paper reinforcement is supposed to be almost the same than those obtained by oxidative methods. On the other hand, enzymatically obtained CNF presented the same properties enhancement than those LCMNF from banana leaf residue obtained by fully mechanical methods with higher production costs, mainly due to the use of enzymes during the obtaining procedure. However, it must be pointed that the obtained LCMNF in the present study are not suitable to be used in white papers were brightness is a market requirement, since the presence of lignin affects drastically this property. In this sense, enzymatically obtained CNF would represent the most appropriate properties/cost ratio.

Conclusions

The present work has brought to light the feasibility of providing added value to banana leaf waste, derived from banana exploitation in agri-food industry. This value has been provided by its conversion into lignocellulosic micro/nanofibers (LCMNF) and their use as paper additive with the purpose of enhancing mechanical properties. The main conclusions of the present work can be listed as follows:

1.      The produced pulp presented a pulping yield higher than 80%, which lead to a high conversion rate of the banana leaf waste to value-added product.

2.      The high lignin and hemicellulose content promoted the fibrillation of the obtained fibers, driving to high performance LCMNF in terms of reinforcing potential and production costs.

3.      The obtained LCMNF presented different properties than those obtained by oxidative and enzymatic methods although they provided almost the same property increase to paper.

4.      For the first time, high-lignin content LCMNF with high reinforcing capacity have been produced, increasing the global yield of the production.

5.      The lower fibrillation degree of the obtained LCMNF should lead to obtain pulps with good run ability in the paper machine due to their lower water retention capacity. Finally, the obtained LCMNF from banana leaf residue presented almost the same reinforcing potential than those obtained from bleached Kraft hardwood pulp, which is a pulp prepared from fresh wood resources with treatment yields of 50%, both those prepared by oxidative methods and mechanical methods. Moreover, the production costs of the LCMNF from banana leaf residue is significantly lower than for TEMPO-oxidized CNF.

In this sense, it is conceivable that the environmental impact should be lower, but further research must be performed in this sense

Reference:

Tarrés, Q., Espinosa, E., Domínguez-Robles, J., Rodríguez, A., Mutjé, P. and Delgado-Aguilar, M., 2017. The suitability of banana leaf residue as raw material for the production of high lignin content micro/nano fibers: From residue to value-added products. Industrial Crops and Products, 99, pp.27-33.