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Indarti, E., Abdul Rahman, K. H., Ibrahim, M., and Wan Daud, W. R. (2023). "Enhancing strength properties of recycled paper with TEMPO-oxidized nanocellulose," BioResources 18(1), 1508-1524.


Recycled fibers used in the manufacturing of paper and board are associated with strength deficiencies. This study investigated the use of TEMPO-oxidized nanocellulose from oil palm empty fruit bunch (OPEFB-TEMPO) for reinforcing papers made from such fibers. Strength properties of tensile and tear were enhanced with the addition of OPEFB-TEMPO, with strong correlations, as indicated by the R2 values. The reinforcement capability was supported by the scattering coefficient-percent relationship. The only drawback of the nanocellulose addition is that it reduces pulp drainability, which can be minimized by adding drainage aids. Because only a relatively small amount is required, OPEFB-TEMPO has the potential to be used as paper strengthening agent, particularly in the production of low grammage papers.

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Enhancing Strength Properties of Recycled Paper with TEMPO-oxidized Nanocellulose

Eti Indarti,a,* Khairul Hafizuddin Abdul Rahman,b Mazlan Ibrahim,b and Wan Rosli Wan Daud b

Recycled fibers used in the manufacturing of paper and board are associated with strength deficiencies. This study investigated the use of TEMPO-oxidized nanocellulose from oil palm empty fruit bunch (OPEFB-TEMPO) for reinforcing papers made from such fibers. Strength properties of tensile and tear were enhanced with the addition of OPEFB-TEMPO, with strong correlations, as indicated by the R2 values. The reinforcement capability was supported by the scattering coefficient-percent relationship. The only drawback of the nanocellulose addition is that it reduces pulp drainability, which can be minimized by adding drainage aids. Because only a relatively small amount is required, OPEFB-TEMPO has the potential to be used as paper strengthening agent, particularly in the production of low grammage papers.

DOI: 10.15376/biores.18.1.1508-1524

Keywords: TEMPO-oxidized nanocellulose; Oil palm empty fruit bunch; Strengthening agents; Recycled paper; Carboxylic acid moieties; Paper strength properties

Contact information: a: Agriculture Product Technology Department, Faculty of Agriculture, Universitas Syiah Kuala. Banda Aceh Indonesia 23111; b: Bioresources Technology Division, School of Industrial Technology, Universiti Sains Malaysia, 11800 Penang Malaysia;

*Corresponding author:



Despite the advent of digital information and online publications, the world’s paper industry continues to expand from an annual production of 323 million tonnes in 2000 (FAO 2001) to 409 million tonnes in 2016 (FAO 2017), with a projected forecast of 482 million tonnes in 2030 (Papernews 2015). This increase is attributed to the growing global demand for packaging and tissue papers (FAO 2017). Although virgin fiber is the predominant raw material for papermaking, recycled fiber (also known as secondary or recovered fiber) has also increased its share as sources for papermaking. It is estimated that in 2017, 56% of recycled fiber is used in the manufacture of paper globally (FAO 2017).

One of the challenges in using recycled fibers is that the strength of paper made from these fibers decreases with the extent of recycling, which is due to the degradation of their fiber properties. This has been associated with the deterioration in the strength of bonds between fibers (Laivins and Scallan 1993). During recycling, fibers undergo various stages of drying and rewetting, and in doing so, they lose some of their conformability and swelling capabilities. Hornification, or the irreversible hardening of fibers (Jayme 1944), was suggested as being accountable for these fibers’ impairment, whereby the inter-fiber hydrogen bonds that were fully formed during the initial papermaking are resistant to being broken during the subsequent rewetting process of recycling. Because some of these bonds remain bonded, the fibers only experience partial swelling, resulting in paper with poorer strength qualities.

To overcome these setbacks, different strategies have been adopted, i.e., mechanical treatment (refining), using of chemical additives, and addition of long fibers so as to increase the strength of paper made from recycled fibers, with the latter method relying on the creation of new fiber-to-fiber interactions (hydrogen bonds). This forms the basis for the use of nanocelluloses as paper strength reinforcement agents. This material is biodegradable (and renewable), with high specific strength and stiffness, high reinforcing potential, and high specific surface area (Guimond et al. 2010; Osong et al. 2014; Kumar et al. 2016; Balea et al. 2018; Viana et al. 2018).

Nanocellulose (NC) is a general term that has been used to describe nanosized elements obtained from cellulose, which includes cellulosic nanofibrillar and nanocrystalline materials (Hamad 2006), cellulose nanocrystals (Habibi et al. 2010; Lin et al. 2011), cellulose nanowhiskers (Chen et al. 2009), cellulose crystallites (Fleming et al. 2001), and nanocrystalline cellulose (Li et al. 2012). The morphology and dimensions are related to the processes used to isolate them, amongst which are acid hydrolysis at elevated temperatures (Hamad 2006; Habibi et al. 2010; Qin et al. 2011; Fan et al. 2020), mechanical treatments such as using a disk grinder (Hu et al. 2015) and high intensity ultrasonication (Wang et al. 2015) of fibers, all of which demands a high energy usage. To mitigate such issues, the fibers are usually chemically pretreated, with 2,2,6,6-tetramethylpiperidine-1-oxy (TEMPO) being one of those successful pretreatments. It is believed that during this TEMPO-mediated oxidation, the hydroxyl primary groups on the cellulose chains (C6) undergo a selective oxidation, during which negatively charged carboxylic moieties are introduced, thus allowing the pretreated fibers to be easily broken down into nanocelluloses that are dispersible in water (Tahiri and Vignon 2000; Saito et al. 2007; Johnson et al. 2009; Isogai et al. 2011). In an attempt to increase the NC yield, Rohaizu and Wanrosli (2017) used two treatment sequences of sono-assisted TEMPO oxidation of the cellulosic material, followed by a highly powered ultrasonication mechanical treatment of the oxidized product, with yields exceeding 90% reported. The resultant NC was shown to be stable over a long period of time; even after 24 hours only partial sedimentation was observed. These observations were most likely due to the increase in the negative charge carboxylate groups generated during the TEMPO oxidation process.

Traditionally, NC can be obtained from wood; nevertheless they can also be isolated from non-woody materials such as sisal fibers (Moran et al. 2008), kenaf (Kargarzadeh et al. 2012), rice straw (Jiang and Hsieh 2013), sugarcane bagasse (Mandal and Chakrabarty 2011), bamboo (Wang et al. 2015), and oil palm lignocellulosic (OPL) biomass fibers (Fahma et al. 2010; Haafiz et al. 2014; Al-Dulaimi and Wanrosli 2016; Rohaizu and Wanrosli 2017). The latter, which is a by-product of the palm oil industry, is of particular interest in this research due to its abundance, low cost, and the ready availability of renewable cellulosic biomass. OPL consists of various types of residues, viz. trunks, fronds, and empty fruit bunches (OPEFB), which are rich in cellulose. However, because of its distinct advantage of being produced at the mill site, from which it can be easily obtained, OPEFB has been chosen as the material for investigation for the potential as paper strength reinforcement agents.

This study deals with the effects of adding nanocellulose produced via the TEMPO reaction (OPEFB-TEMPO) with recycled fibers (RF) obtained from old corrugated containerboard (OCC) on pulp and paper properties made thereof. It has great potential as strengthening agent; the presence of reactive carboxylic groups in OPEFB-TEMPO will confer a greater degree of inter-fiber (hydrogen) bonding, resulting in better paper qualities. For comparison purposes, microcrystalline cellulose (OPEFB-MCC) that was prepared by acid hydrolysis and beaten bleached OPEFB pulp (OPEFB-B) obtained via totally free chlorine (TCF) processes was used. Paper handsheets were prepared and evaluated according to the appropriate TAPPI Standards (TAPPI 2004).



OPEFB in the form of fibrous strands were obtained from a local Malaysian company. Before use, they were washed with water, air dried, and cut into lengths of 5 cm to ensure better penetration of the pulping chemicals. 4-Acetamido-TEMPO (2,2,6,6-tetramethylpiperidin-1-oxyl) 98% was procured from Sigma-Aldrich (St. Louis, MO, USA). Sodium bromide (NaBr) and other chemicals were obtained from Bendosen and used as received.

Preparation of Bleached Pulp

Bleached OPEFB pulp (OPEFB-B) was prepared using an environmentally benign process as described by WanRosli et al. (2003) and Leh et al. (2008). The two phases were preparation of the unbleached OPEFB pulp (OPEFB-UB), followed by bleaching. In the former, OPEFB fiber strands were pulped via water prehydrolysis followed by soda-anthraquinone pulping, while the latter involves the bleaching of OPEFB-UB pulp using a totally chlorine-free (TCF) bleaching sequence of oxygen (O), ozone (Z), and peroxide (P). The resultant OPEFB-B pulp has a Kappa number of 1.2.

Preparation of Microcrystalline Cellulose

Microcrystalline cellulose (OPEFB-MCC) was prepared by hydrolyzing the above OPEFB-B pulp with 2.5 M HCl while maintaining a solid to liquid ratio of 1:20 at a temperature of 105 ± 2 °C for 15 min. After hydrolysis, the material was thoroughly washed with distilled water before being air-dried and subsequently kept in a desiccator over phosphorous pentoxide until further use.

Preparation of TEMPO Oxidized Nanocellulose

TEMPO oxidized nanocellulose (OPEFB-TEMPO) were prepared based on the procedure described by Rohaizu and Wanrosli (2017), which involves sono-assisted TEMPO-oxidation of the cellulosic material with slight modifications. First, 3 g of OPEFB-UB pulp was suspended in a 1 L glass reactor containing an aqueous mixture of 500 mL water, 0.048 g of 4-acetamido-TEMPO, and 0.48 g sodium bromide, followed by drop-wise addition of a 30 mL solution of sodium hypochlorite with continuous stirring. The oxidation reaction was carried out in an ultrasonic water bath (model Branson 8510) at a frequency of 40 kHz and 320 W output power at a constant temperature of 30 °C, whilst maintaining the pH of 10.0 ± 0.2 by using 0.5 M NaOH for 2 h. The reaction was quenched by adding 30 mL of ethyl alcohol, with the final pH regulated to 5.0 by adding 1.0 M HCl, after which it was centrifuged using a Kubota model 5100 at 3500 rpm for 20 min. To ensure the TEMPO oxidized OPEFB is maximum free from any unreacted TEMPO, superfluous acids and inorganic salts, the water-insoluble product was re-dispersed and re-centrifuged three times using distilled water, followed by sonication in an ice-bath using an ultrasonic probe (Branson sonifier 450) with a 7 mm tip at 20 kHz and 400W output power for 30 min after which a colloidal suspension was obtained.

OPEFB-TEMPO nanocellulose was isolated by re-centrifuging the colloidal suspension at 3500 rpm for 1 h and the ensuing admixture filtered using a 15-µm filter for removal of any unwanted materials comprising of micro-contaminants and fibrous aggregates, and then stored in a refrigerator before further use.

Preparation of Recycled Fiber Pulp

Recycled fiber (RF) pulp was prepared from old corrugated containerboard (OCC) by tearing them into pieces of dimensions 1 cm x 1 cm, followed by soaking in water for 24 hours, after which it is disintegrated for 3000 revolutions in a standard laboratory British disintegrator. After concentrating the pulp to about 20% consistency, they were kept in plastic bags and stored in the refrigerator until further use.

Morphological Analysis

The surface morphology of the samples was investigated using SEM and TEM. For SEM, a SEM Oxford INCA 400 model was used. Prior to scanning, samples were coated with gold sputter in order to avoid charging effect. TEM micrographs were obtained using the transmission electron microscope Phillips CM12 with Docu Version 3.2 image analysis. Drops of the suspension (0.01% of w/v) were deposited on carbon coated electron microscope grids and allowed to dry and stained with phosphotungstic acid (around 2% of wt) for 30 seconds.

The lengths of OPEFB-B fibers were analyzed by the Fiber Quality Analyzer (FQA, Op Test Equipment, Canada), while for OPEFB-MCC and the OPEFB-TEMPO, the sample dimensions were measured directly from the TEM micrographs with the results based on a mean of 50 measurements.

Preparation of Paper Handsheets

One of the challenges regarding the use of specialized cellulosic materials as strengthening agents for papermaking is that it is almost impossible to determine the amount retained during sheet making, unless it is prepared by evaporation, which does not really represent the papermaking process. This is because they all belong to the same group of materials which have practically the same basic fiber properties. Papermaking is a filtration process wherein fibers are collected on a wire mesh. Thus, the objective is to retain as much as possible the fibers and other materials that are added. In designing the experimental procedure, the greatest constraint is to minimize the errors that ensued from the sheet making process and, in this regard, three approaches were considered. It should be emphasized that some amounts of the cellulosic materials, especially the micro- and nano- fractions will definitely be lost during the sheet forming process; however, since all experiments were carried under the same conditions, it can be assumed that the lost proportion will be consistent throughout and the results will be thus comparable.

In the first approach, the normal handsheet making technique, as outlined in the Second Report (Anon., 1936) using a Standard British Laboratory handsheet equipment, was carried out. In the second method, the cellulosic materials were blended for two minutes to ensure proper mixing, followed by handsheet making. In the third procedure, RF fibers were added during the initial stage of the handsheet equipment being filled with water, with the OPEFB cellulosic materials only added after the water had finally reached the maximum level.

Both of the first two methodologies gave unsatisfactory results because of significant variations in handsheet grammage. It is anticipated that the micro- and nanometric small particles of OPEFB-MCC and OPEFB-TEMPO are not trapped in the sheet structure, causing it to pass through the wire mesh as the water is drained. This is partly confirmed by the turbidity of the collected backwater, which can be inferred as resulting from the accumulation of the small particles that easily passed through the wire mesh.

The third procedure gave a more consistent grammage with much lesser turbidity in the backwater; hence it was adopted in this study. It is believed that in this method, the small particles have a better chance of being trapped because by the time they reach the wire mesh, the paper structure has already been formed, hence entrapping them. Nevertheless, it should be emphasized that there is no known technique to determine the exact amount of these cellulosic materials that are retained in the paper structure.

The amount of cellulosic material added is dependent on its type; ranging from 0 to 100% for OPEFB-B, and 0 to 20% for OPEFB-TEMPO and OPEFB-MCC.

Handsheet Making and Testing

Handsheets of 65 ± 2 g/m2 were prepared using the Standard British Laboratory handsheet equipment, as outlined in the Second Report (Anonymous 1936) with slight modifications whereby they were prepared according to the third procedure as discussed above. For OPEFB-B, prior to addition to the recycled fiber, the pulp samples were beaten in a PFI mill for 20,000 revolutions.

Pulp freeness was determined according to TAPPI Standard T227 om-99. Drainage times were expressed as the time taken for the water to drain completely from the maximum level in the handsheet equipment.

The completed handsheet sets were conditioned at 23 °C and 50 % RH for at least 24 hours before testing. Their properties were evaluated following the appropriate TAPPI standard methods (TAPPI 2004), such as: Tensile index (T 494 om-01), Tear index (T 414 om-98), and Opacity, from which scattering coefficients are derived (T425 om-91).


OPEFB cellulosic materials comprising of OPEFB bleached pulp (OPEFB-B), OPEFB microcrystalline cellulose (OPEFB-MCC), and OPEFB nanocellulose produced via the TEMPO oxidation method (OPEFB-TEMPO) were utilized as reinforcing materials for paper using recycled fibers (RF). The effects were compared and discussed in terms of fiber morphology, pulp freeness and drainability, sheet density, tensile, and tear indices.

Morphological Analysis

Figure 1 shows representative SEM and TEM images of various OPEFB cellulosic materials: OPEFB-B pulp, OPEFB-MCC, and OPEFB-TEMPO. The OPEFB-B pulp (Fig. 1a), as expected, demonstrated a fibrous structure. The dimensions were reduced greatly upon conversion into OPEFB-MCC, with the morphology apparently in the form of bundles of rod-like structure, as shown in Fig. 1(b).


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Fig. 1. (a) SEM of OPEFB-B pulp (x200 magnifications); TEM of (b) OPEFB-MCC (x31000 magnifications), (c) OPEFB-TEMPO (x88000 magnifications)

During the acid hydrolysis process, the amorphous regions were removed, leaving the MCC rod-like structures, and since all contaminants were removed during this treatment, the product was a highly purified cellulose. However, unlike the OPEFB-MCC, OPEFB-TEMPO (Fig. 1c) had distinct rod-like structures resembling the nanocrystalline cellulose crystallites.

The length of OPEFB-B as analyzed by the Fiber Quality Analyzer (FQA, Op Test Equipment, Canada) (expressed as length weighted average) was ca. 1.01 mm, while for the OPEFB-MCC and OPEFB-TEMPO, the lengths were ca. 1875 nm and 76 nm, respectively. The differences in size of these cellulosic materials will have an impact on the pulp and paper properties as will be discussed later.

Freeness and Drainability

Freeness of pulps as measured by Canadian Standard Freeness (CSF) decreased with the addition of OPEFB-MCC and OPEFB-B pulp (Fig. 2a).

Fig. 2. Effect of (a) freeness and (b) drainability on OPEFB cellulosic addition

However, reliable data for OPEFB-TEMPO addition could not be determined due to the very slow drainage rate, which affects the freeness readings. For the OPEFB-TEMPO, the water drained slowly (dripping) through the side walls of the apparatus sample chamber, making it impossible to have accurate CSF readings.

A possible explanation for the changes observed with both OPEFB-MCC and OPEFB-B is that the intrinsic properties of these materials differ greatly from the recycled fibers. The RF, by virtue of being hornified, are stiff fibers, which have markedly reduced swelling and absorption capabilities. This was reflected in the high freeness at zero addition of OPEFB cellulosics. As the virgin OPEFB cellulosics were added, some of the base RF were replaced with the active fibers, and because of their intact internal structure, they can swell and absorb water unhindered to their maximum potential, leading to a CSF reduction. It could also be seen that the freeness with OPEFB-MCC addition decreased at a faster rate than OPEFB-B pulp, which could be related to the size of the cellulosic material, whereby the fiber length of the OPEFB-MCC was much shorter than the OPEFB-B fibers and their fines as discussed above (Morphological analysis). Furthermore, chemical OPEFB-MCC is considered pure cellulose, which has large amounts of hydroxyl groups, which can participate in water absorption. Thus, both the high surface area and greater number of hydroxyl groups in the OPEFB-MCC will absorb more water, resulting in lower freeness as compared to OPEFB-B pulp.

The effect of OPEFB cellulosic additions was investigated in terms of drainage times, which in this study was measured by the time it takes for the pulp to drain from the handsheet machine. As shown in Fig. 2b, OPEFB-TEMPO showed the slowest rate (longer drainage times), followed by OPEFB-B and OPEFB-MCC. It is possible that this is due to the nanometric scale of the OPEFB-TEMPO nanocelluloses, which increases the water retention capacity of the pulp, resulting in slow drainage. It is also probable that these retained nano-sized elements in the fiber matrix could decrease the available pore area through plugging of the inter-fiber pores, hence reducing the drainage rate (Lin et al. 2007; Cole et al. 2008; Rantanen and Maloney 2013). They can penetrate the fiber network, leading to a partial closing of the pores, thus increasing the drainage resistance (Cole et al. 2008). These small particles can migrate in the fiber network, and as they become trapped, they tend to block the flow of water. In addition, since swelling is driven by osmosis, it is therefore dependent on the number of ions trapped in the fibers, and with OPEFB-TEMPO containing substantial amounts of carboxylic groups, this could contribute to increased swelling, leading to slow drainage. Nevertheless, this drawback can be minimized by adding retention aids such as alum, polyamines, or polyethylene imines (PEI).

Paper Density

Figure 3 shows the relationship between the addition of OPEFB cellulosic material and sheet density. It is evident that even though the densities of all sheets increase with all types of OPEFB cellulosics, the effect varied according to the nature of the cellulose, with OPEFB-B highest followed by OPEFB-MCC and OPEFB-TEMPO.

The high increment upon the addition of OPEFB-B pulp is probably a result of the replacement of the more stiffened recycled fiber base pulp with the active virgin OPEFB bleached pulp. The recycled fibers are more rigid because of hornification, whereby bleached fibers are less flexible, thus unable to conform well to each other leading to bulkier (or less dense) sheets. As these recycled fibers are progressively replaced by the active flexible virgin fibers, the conformability of individual fibers in the fibrous structure increases resulting in denser sheets. In addition, the presence of fines in the OPEFB-B pulp will also contribute through the fillings in the voids between fibers, resulting in an increase in sheet density.