Moderate surface acetylation of nanofibrillated cellulose for the improvement of paper strength and barrier properties

Abstract

This study evaluated the effect of using acetylated nanofibrillated cellulose (ANFC) and acetylated pulp (AP) fibers to modify strength and barrier properties of paper. Nanofibrillated cellulose (NFC) was produced using an ultra-fine friction grinder. The NFC and pulp fibers were modified by the heterogeneous acetylation process. Chemical modification was characterized by Fourier transform infrared spectroscopy together with titration. The values of density, burst strength, tensile strength, and air resistance were increased by adding NFC and ANFC to the paper combination. Addition of NFC to the non-acetylated pulp had no significant effect on the water absorption of made paper (p > 0.05), while addition of ANFC to the non-acetylated pulp led to a decrease in the water absorption of about 23.1%. The results indicated that addition of partially acetylated NFC to the pulp caused an improvement in both the air and water barrier and mechanical strength properties of the paper, simultaneously.

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1. Introduction

Paper can be produced with very diverse properties depending upon its application such as writing and printing, cleaning, construction, packaging, and so on. The packaging industry is one of the most important consumers of paper, in that about one third of materials used in the packaging industry are paper and paperboard.¹ In general, packaging papers need to have high barrier and mechanical properties, simultaneously. The required strength and barrier properties of packaging papers depend on the nature of the packaging content, transport, and storage conditions.² Conventionally, strength and barrier properties of these papers are usually improved by applying a synthetic reinforcing agent and petroleum-based chemicals, respectively, due to their ease of processing, excellent strength and barrier properties, and also low-cost.¹ Regarding the importance of sustainable development and debates about using petrochemical-based polymers in food packaging, there is an increasing interest in replacing synthetic and fossil-based polymers with more renewable and sustainable materials.^3–6 A renewable biomaterial that may be a good alternative for improving strength and barrier properties of paper and composites is nanofibrillated cellulose (NFC). NFC refers to cellulose fibers fibrillated to achieve agglomerates of cellulose microfibril units. NFCs have a nanoscale (less than 100 nm) diameter with a typical length of several micrometers.⁷ The NFCs have unique properties such as renewability, biodegradability, potential reinforcing properties, and a broad capacity for chemical modification to improve barrier properties.⁸ Therefore, it opens up new windows toward intense and promising research on nanocellulose-based material with an expanding area of potential applications, such as nanopaper production, and reinforcing agent for paper,^8–11 nanocomposite materials,^12,13 packaging materials with high mechanical properties, and low gas permeability.^1,14,15 Some of the previous studies using NFC showed great effects on improving mechanical strength and gas barrier properties of paper and composites.^9,11,16–19 Furthermore, it should be noted that high water barrier property as well as low air permeability and high mechanical strength can be important target properties for packaging paper depending on the composition and properties of the products that are in contact with them.^2,20

The water barrier properties of paper are limited due to the free hydroxyl groups of cellulose and hemicellulose in fibers structure, along with the porous structure of the paper fiber network. Paper can easily absorb water from the environment and the products that are in contact with it; therefore, its strength properties and application efficiency can be drastically decreased.²

Using NFC as a reinforcing agent did not show desirable effect on improving water barrier property of paper compared to the mechanical strength and air barrier characteristics. The NFCs have a high specific surface area and plentiful surface hydroxyl groups, so they have a high hydrophilicity and affinity with water.⁷ In addition, they can fill the voids between fibers in paper network, which leads to an increase in paper density;^11,18 therefore, they can act as a physical barrier against water and water vapor. Hassan et al.¹⁸ reported that addition of 2.5–20% NFC to unbeaten softwood fibers resulted in an increase in water absorption of paper sheets. Lavoine et al.²¹ studied mechanical and barrier properties of MFC-coated cardboard samples and reported that water absorption of samples was increased by increasing coating process steps. Missoum et al.¹ reported that water absorption of paper was not significantly affected by increasing 5–50% neat NFC to the pulp slurry.

One of the methods to overcome this problem is chemical surface modification of pulp microfibers or NFC nanofibers. Among the modification reactions, acetylation is widely used, and acetylated cellulose fibers have commercial potential.^14,22 Acetylation is an inexpensive and eco-friendly method to make thermoplastic biopolymers and has been widely used in wood modification.^23,24 There are two acetylation mechanisms including of fibrous (heterogeneous) and solution (homogeneous) acetylation, that can be recognized depending on whether a non-solvent is applied or not. The fibrous acetylation process is performed in the presence of non-solvents, such as toluene, benzene or carbon tetrachloride. The reaction product (the cellulose acetate) is insoluble and thereby, this process preserves the fiber morphological structure. In the solution acetylation process, cellulose acetate is dissolved during the reaction.^25–27 To preserve the core of the cellulose, most of the modifications have been performed using heterogeneous reactions.²⁸ The acetylation process causes to substitute hydroxyl groups (–OH) of cellulose with acetyl (CH₃CO) groups; therefore, hydrophobicity of cellulose fibers increases and water absorption of paper decreases.^29,30 In addition, substituting hydroxyl groups with acetyl groups causes to decrease in the paper sheet mechanical strength.^31,32 As NFC has a high specific surface area and plentiful surface hydroxyl groups compared with cellulose fibers, it is expected that in the same conditions of acetylation process, the decrease in bondability of ANFCs is lower than acetylated fibers. In some previous studies, partially acetylated NFCs were successfully used to make NFC film with improved barrier properties without affecting mechanical strengths.^14,33 Hence, it was assumed that using partially acetylated NFC may simultaneously improve mechanical strength, air barrier, and water barrier properties of paper sheet.

In this study for the first time, the ANFCs prepared by heterogeneous acetylation process without any catalyst were added to the pulp. Also, addition of the NFC to suspension of the acetylated pulp fibers was evaluated.

2. Materials and methods

2.1. Materials

Commercial long fiber pulp (American southern pine pulp) was supplied from Linter Pak Co., Iran. Cationic polyacrylamide (C-PAM) (Farinret K325, Degussa Co.) with medium cationic charge and high molecular weight were used as a retention aid. Acetic anhydride ((CH₃CO)₂O > 98.5%), ethanol (C₂H₅OH > 99.8%), acetone (CH₃COCH₃ > 99.8%), toluene (C₆H₅CH₃ > 99.5%), sodium hydroxide (NaOH ≥ 99.0%), and hydrochloric acid (HCl, 37%) were purchased from Merck Chemical Co., Germany, and used without further purification.

2.2. Methods

2.2.1. Preparation of pulp. Based on the TAPPI T 277 om-04 standard, the freeness of as-received softwood pulp was assessed 770 CSF (Canadian Standard Freeness). The freeness of pulp tuned on 350 CSF according to the TAPPI T 248 sp-00 standard, using PFI refiner (VI Hamar Co., Norway). The refining process resulted in fibrillation of the fibers. This pulp was used for handsheet making.

2.2.2. Production of NFC. In order to produce NFC, first the supplied commercial long fibers were washed to remove the contaminations. Then, the water slurry with 1 wt% purified softwood fibers was passed three times through a disk grinder (MKCA6-3; Masuko Sangyo Co., Ltd, Japan) at 1500 rpm to produce NFC.^8,34,35

2.2.3. Heterogeneous acetylation. The acetylation of NFC and pulp was performed according to a method as described by Rodionova et al.³³ Successive solvent exchange using water, acetone, and toluene was respectively undertaken before the acetylation reaction. The reaction vessel was placed in an oil bath on a magnetic stirrer and heated to 70 °C. The moist nanofibrillated cellulose in toluene (40 g) was placed in the reaction vessel. The desired amount of acetic anhydride (AA) (60 mL) was then added and the stirrer engaged. Water condenser system was used to prevent the evaporation of the liquid phase. Different reaction times (0.5, 1, and 3 h) were used. After the acetylation, the reaction mixture was cooled down and centrifuged for 30 min at 750 RCF (g) to remove the rests of acetic anhydride. After centrifugation, the obtained pellet was re-suspended in toluene and centrifuged again (two more times). After the reaction, the NFC suspension was carefully solvent exchanged back to water phase. The fibers were acetylated using the same method.

2.2.4. Handsheets preparation. The made handsheets were presented in Table 1. Handsheets (60 g m⁻²) were prepared according to TAPPI T 205 sp-02 standard. C-PAM was used as a retention aid during the production of handsheets reinforced with NFCs. C-PAM was diluted to a solution concentration of 0.05% (w/w) before using. The NFC and pulp dry content in the systems was kept constant at 0.3% (w/w), while the relative composition of the component was constant (10% NFC to 90% pulp). The amount of C-PAM added was constant (0.3% (w/w)). The retention of fines and NFCs was assessed using a Britt Dynamic Drainage Jar based on the TAPPI standard method (T 261 cmp-00). The C-PAM was added to the fiber suspension and after 15 min mixing at 500 rpm, 10% NFC was added.¹¹ After a further 15 min of mixing, handsheets were formed using a laboratory sheet former according to the TAPPI standard method (T 205 sp-02). The pH of the resulting suspensions (pulp + NFC + C-PAM) was between 7.5 and 8. Handsheets were stored at 23 °C and 50% relative humidity (RH) for at least 3 days before testing.

Table 1 The series of handsheets produced in this study

Handsheet	Composition	Acetylation time (h)	Abbreviation
Unmixed paper (100% pulp)	Pulp	0	UMP
	Acetylated pulp (UMAP)	0.5	UMAP-0.5
		1	UMAP-1
		3	UMAP-3
Mixed paper (90% pulp + 10% NFC + C-PAM)	Pulp + NFC (MP)	0	MP
	Acetylated pulp + NFC (MAP)	0.5	MAP-0.5
		1	MAP-1
		3	MAP-3
	Pulp + acetylated NFC (MANFC)	0.5	MANFC-0.5
		1	MANFC-1
		3	MANFC-3

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2.3. Measurements

2.3.1. Measurement of fiber dimensions. The fiber dimensions including length, thickness, and wall thickness were measured using an optical microscope (Olympus Co., Japan).

2.3.2. Characterization of NFC. Morphology of NFC was determined using an atomic force microscope (SII Nanonavi E-sweep, SII Nanotechnology, Inc., Japan) in a phase mode at 25 °C and a RH of 45–55%. The Si probe was an SI-DF20 (SII Nanotechnology, Inc., Japan) with a spring constant of 18 N m⁻¹ and a frequency of 138 kHz.

2.3.3. Chemical characterization of NFCs and pulp fibers.
2.3.3.1. Determination of the degree of substitution (DS) by titration. The degree of substitution (DS) was determined according to the method proposed by Kim et al.³⁶ First, 110 mg of the acetylated pulp fibers and the acetylated NFCs from each reaction time were placed in conical flasks consequently, 40 mL of 75% ethanol was added and the flasks were kept at 50 °C for 30 min. Finally, 40 mL of NaOH (0.5 M) solution was added to the mixture and heated to 50 °C for 15 min. The mixture was kept at room temperature for 48 h under constant stirring. The excess of NaOH was titrated with HCl (0.5 M) until a pH of 7 was obtained.
2.3.3.2. Fourier transform infrared (FTIR) spectroscopy. To determine changes in the functional groups which may have been caused by the treatments, FTIR studies were performed using a Perkin-Elmer Spectrum RXI (USA). The NFC films of approximately 44 μm thick were used for the direct FTIR analysis.³³ The NFC films were prepared by vacuum filtration using a polyester filter membrane, as Ernest-Saunders et al. reported elsewhere.¹⁴ Then, the prepared films were dried at room temperature for 3 days. Thickness measurements were performed by using a PTA thickness tester N1101 (Germany). All the prepared films were dried at room temperature and conditioned for at least 3 days prior to measurements at 23 °C and 50% relative humidity (RH).

Prior to the analysis of pulps, 1.0 mg pulp fibers were ground and mixed with 100 mg potassium bromide (KBr). The resultant powder was pressed into transparent pellets. The FTIR analysis was carried out using 64 scans with a resolution of 4 cm⁻¹, in absorbance mode within the range of 4000–400 cm⁻¹. In order to compare the obtained FTIR spectra, all spectra were normalized based on absorption peak at 1059 cm⁻¹ wavenumber associated with C–O stretching.

2.3.3.3. Dynamic absorption test (DAT). The surface properties of papers made from unmodified and acetylated pulp fibers and unmodified and acetylated NFC films were evaluated by dynamic contact angle (DCA) measurements at different check times. The measurements were performed using a contact angle goniometer (Data physics OCA 15 plus, Germany) at 23 °C and 50% RH. Contact angles were measured 0.2 and 2 seconds after the application of a 4 μL drop of water. In this investigation, the measurements were repeated 3 times at different locations on the surface of handsheets and NFC films and an arithmetic mean was calculated.

2.3.4. Structural analysis. The FE-SEM images of NFC films were acquired using a Hitachi S-4160 (Japan) field-emission scanning electron microscope at 20 kV acceleration voltage. Also, the SEM images of the handsheets were obtained using a Pemtron PS-230 (Korea) scanning electron microscope at 10 kV acceleration voltage. In order to avoid sample charging, prior to imaging, the samples were sputter-coated with gold.

The images were analyzed with the (freeware) program ImageJ v.1.48. First, noise of the images was reduced using Noise Despeckle within the Process menu, then a thresholded image was created using threshold within the Image menu. The image analysis was performed using analyze particles within the Analyze menu. The area percentage of pores was calculated for different images. The obtained area percentage data from analyze particles method were analyzed using a completely randomized design, one-way ANOVA, to find out the statistical significance of the various treatments effect on the changes of pores area percentage.

2.3.5. Paper handsheet characterizations.
2.3.5.1. Physical and mechanical properties. The density of the handsheets was measured by dividing handsheet base weight by its thickness. The thickness of the handsheets was measured using a PTA thickness tester N1101 (Germany). The bursting strength of specimens was measured according to the TAPPI T 403 om-02 standard using a digital burst-tester (FRANK-PTI GmbH, Germany). The tensile strength of the handsheets was measured using a horizontal tensile tester (L & W Tensile Strength Tester, Sweden), according to the TAPPI method T 494 om-01 standard. All measurements were done at 23 °C and 50% RH.
2.3.5.2. Barrier properties.
2.3.5.2.1. Air resistance. The air resistance test was performed using a Gurley tester 4110 (genuine Gurley™, USA) digital timing attachment according to the TAPPI standard method (T 460 om-02).
2.3.5.2.2. Water absorption. The water absorption measurements, Cobb₆₀ tests, were performed according to the TAPPI standard method (T 441 om-04) using a ring of 10 cm², and all samples were cut around the ring in order to avoid errors associated with the capillarity. Deionized water (100 mL) was added into the ring for 60 s, following the TAPPI T 441 om-04 standard. Then, “wet samples” were pressed once between two absorbent papers with a roll of 10 kg in order to remove residual water and weighted with a digital balance four digit. Cobb index expresses the ratio between mass of absorbed water and the wet area (g m⁻²).

2.4. Statistical analysis

The experimental followed a completely randomized design with three replication. Some data were analyzed by one-way analysis of variance (ANOVA). Means separation was conducted using Duncan multiple comparison at p < 0.05. All statistical analyses were performed using SPSS 18.0 software (IBM Corporation).

3. Results and discussions

3.1. Fiber and NFC characterization

Atomic force microscopy (AFM) image of the nanofibers is shown in Fig. 1. The nanofibers displayed fibrillar structure with a diameter around 32 ± 10 nm. The diameter of nanofibers was determined by digital image analysis (ImageJ) of AFM picture (a minimum of 50 measurements was performed). Also, the average length and diameter of softwood fibers measured using an optical microscope were around 3374 ± 456 μm and 34 ± 8 μm, respectively. These data show that the softwood fibers with diameter of 34 ± 8 μm were downsized to NFC with diameter of 32 ± 10 nm through fast and simple process of grinding. Because of network-like highly entangled NFC, the individual nanofibers are not contained clear head and tail, so an exact length of NFC could not be mentioned neither in the current investigation, nor in the literature. With respect to FE-SEM and transmission electron microscope (TEM) micrographs, the length of NFC was estimated to be longer than 10 μm.^8,34,37

Fig. 1 Atomic force microscopy image of the extracted NFC from the bleached soft wood pulp.

3.2. Chemical characterization

The most prevalent method for explanation of structural cellulosic production features is FTIR spectroscopy.³⁸ Therefore, FTIR spectroscopy was used to evaluate the effect of acetylation on fibers and NFCs chemical characteristics. The normalized FTIR spectra of unmodified and acetylated fibers and NFCs are shown in Fig. 2A and B, respectively. The absorption peaks between 2900 and 2950 cm⁻¹ for examined fibers and NFCs are due to stretching of C–H groups of cellulose.⁴⁸ The peak observed in the range of 1368–1379 cm⁻¹ is attributed to the bending vibration of C–H in –O(C=O)–CH₃.^14,39,40 The bands between 1053–1062 cm⁻¹ are assigned to the stretching of C–O bonds for all the evaluated materials.³⁹

Fig. 2 Normalized FTIR-spectra of (A) unmodified and partially acetylated pulp fibers, and (B) unmodified and partially acetylated NFCs.

The results show that acetylated fibers and NFCs exhibited three new bands characteristic to acetyl group vibration at about 1735–1740, 1368–1375, and 1259–1277 cm⁻¹. The peaks located at 1735–1740 cm⁻¹ are attributed to the C=O stretching of carbonyl in the ester bonds.^39,41–43 The peaks located at 1368–1375 are referred methyl C–H symmetric bending.^41,44–47 The vibration peaks, between 1259 and 1277 cm⁻¹, are referred to C–O stretching of acetyl groups.^39,48 These three peaks confirmed successful acetylation of the fibers and NFCs. As expected, the lack of a peak at the region 1840–1760 cm⁻¹ in the spectra demonstrated that the product is free of the unreacted acetic anhydride. The absence of absorption in 1700 cm⁻¹ for a carboxylic group implied that the products are also free of the byproduct of acetic acid.

The peak intensity of acetylated fibers and NFCs was used to investigate the effect of the reaction time on the degree of acetylation. As shown in Fig. 2, an increase in reaction time from 0.5 to 1.0 h and to 3.0 h resulted in an increment in the intensity of the three peaks in the regions 1735–1740, 1368–1375, and 1259–1277 cm⁻¹, but a decrease in OH stretching, showing a raise of acetylation. As seen from the FTIR results, the low intensity of absorbance peaks in the regions 1735–1740, 1368–1375, and 1259–1277 cm⁻¹ indicates that the fibers and especially NFCs have a low degree of acetylation.⁴⁹

Progress in acetylation of pulp fibers and NFCs with increasing reaction time was estimated from the IR spectra by computing a ratio, called R₁, which is defined as the ratio between the intensity of the acetyl C=O stretching band of ester at 1735–1740 cm⁻¹ and the intensity of C–O stretching vibration of the cellulose backbone at about 1059 cm⁻¹, that is, R₁ = I₁₇₄₀/I₁₀₅₉.^39,41,47,50 Also, the peak ratio I₃₃₅₉/I₁₀₅₉ (I_O–H/I_C–O) was used to evaluate the decrement of hydroxyl groups with increasing acetylation time. The peak ratios (I₁₇₄₀/I₁₀₅₉ and I₃₃₅₉/I₁₀₅₉) for unmodified and acetylated pulp fibers and NFCs were shown in Table 2. The obtained results showed that an increase in reaction time caused to increase absorption intensity of the band located at 1740 cm⁻¹ and decrease absorption intensity of the band located at 3359 cm⁻¹, assigned to the stretching of the hydroxyl group. Partial substituting the hydroxyl groups with acetyl groups in acetylated fibers and NFCs leads to increase absorption intensity of carbonyl group band and decrease absorption intensity of hydroxyl group band.

Table 2 Degree of substitution, FTIR results, and contact angle for unmodified and acetylated fibers and NFCs

Specimen	Acetylation time, h	Abbreviation	Degree of substitution (by titration)	I1740/I1059 (IC=O/IC–O)	I3359/I1059 (IO–H/IC–O)	Contact angle at 0.2 s
Pulp fibers	0	Pulp	—	—	1.05	38.71 ± 4.0
	0.5	AP-0.5 h	0.73 ± 0.08	0.23	0.97	40.91 ± 3.7
	1	AP-1 h	0.95 ± 0.05	0.38	0.95	44.78 ± 3.2
	3	AP-3 h	1.06 ± 0.06	0.42	0.93	46.57 ± 4.1
NFCs	0	NFC	—	—	0.85	43.43 ± 3.1
	0.5	ANFC-0.5 h	0.12 ± 0.02	0.014	0.81	48.34 ± 2.3
	1	ANFC-1 h	0.20 ± 0.04	0.021	0.76	63.47 ± 1.8
	3	ANFC-3 h	0.29 ± 0.05	0.031	0.73	66.94 ± 2.6

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Titration, as a usual method, was performed to determine the degree of substitution (DS) of the acetylated fibers and NFCs. The obtained results are presented in Table 2. The results demonstrated that the DS was increased with increasing treatment time both in the fibers and in the NFCs. The DS value for the acetylated pulp fibers was also more than the acetylated NFCs, even if the samples were acetylated using the same procedure. A possible explanation for lower DS of acetylated NFCs could be the effect of association of nanofibrils by hydrogen bonding (in the presence of toluene), which could lower the number of accessible OH groups for chemical modification.

In addition, contact angle measurements were performed in order to point out the acetylation treatment effect on the hydrophobic behavior of the pulp fibers and NFCs (Table 2). As expected, the obtained contact angle values of a drop of water deposited on the surface of the sheets made from acetylated fibers or acetylated NFCs were higher than those found for the samples of unmodified fibers or NFCs. This confirms that the successful chemical surface modification. The highest contact angle values registered for acetylated fibers and NFCs after a 3 h treatment were 46.57° and 66.94°, respectively.

As can be seen in Table 2, the water contact angle values of acetylated NFC films were higher than the paper made from acetylated pulp fibers. Water contact angle generally depends on structural properties of cellulosic based materials surface such as porosity, voids size, and interactions that occur between the surface and the water due to the free hydroxyl groups of cellulose and hemicellulose in fibers structure.⁵¹ In this study, the lower porosity and extremely smaller voids could be considered as explanation of the higher water contact angle values of the NFC films compared to the paper. Furthermore, as expected, by increasing DS, the hydrophobicity and water contact angle increased. By increasing the acetylation time, the DS and the water contact angle were increased both in the fibers sheets and in the NFCs films. But the high DS value of pulp fibers led to decrease in bondability of fibers and weaken the paper network compared to the acetylated NFC film network (Fig. 3B and 4A). This led to increase in the porosity and voids size and ultimately caused to increase in paper water absorption.

Fig. 3 FE-SEM images of the film surface of (A) NFC, and (B) ANFC-3 h.

Fig. 4 SEM images of paper surface of (A) UMP, (B) MP, (C) MANFC-3, and (D) MAP-3. The surface images were acquired at low (left) and high (right) magnification.

3.3. Structural characterization of the NFC films and handsheets

The FE-SEM micrographs of NFC films and the SEM micrographs of papers are shown in Fig. 3 and 4, respectively. The visual assessment showed that the compaction of ANFC film is less than the unmodified ones (Fig. 3). The compaction of ANFC film was decreased because of decrease in hydrogen bonding as result of blocking hydroxyl groups by acetyl groups during acetylation process. Adding unmodified NFC/C-PAM or ANFC/C-PAM to the paper structure caused the closure of pores because the nanofibrils were acting as an improving agent of relative bonding area in paper fiber network (Fig. 4B and C). It is worth to mention that such a structural development, which is a result of unmodified or modified NFC addition, has been studied previously.^1,10,22,52 Furthermore, ImageJ analysis showed that the pores of MANFC-3 and MAP-3 papers were more than the MP papers. Acetylation of fibers or NFCs reduces the hydroxyl groups as a result of blocking by acetyl groups and therefore effective surface interaction among the micro and nanofiber components decreases. However, the unmodified and acetylated NFCs form network structures among the microfiber components.

3.4. Retention characterization

C-PAM was used to retain the NFCs during the handsheets making and it is expected to bind fines and NFC material to the fibers. In fact, the C-PAM neutralizes the negative fines and decreases double-layer forces and consequently the fines and NFC start to flocculate, so the amount of NFCs retention will increase.^10,53

The amount of the added C-PAM was 0.3% (w/w) for all the handsheets reinforced with unmodified and acetylated NFC. The retention of the total amount of NFC and fines in the fiber suspension during handsheet preparation was measured using a Britt Dynamic Drainage Jar according to the TAPPI standard method (T 261 cmp-00). The results are represented in Fig. 5. As can be seen, the highest and lowest retention was obtained for MP and MAP papers, respectively. Considering the fixed amount of C-PAM, acetylation caused to decrease the retention of the total amount of NFC and fines in the made papers. Acetylation process causes to increase the fibers and NFCs negative charge,⁵⁴ and that it probably decreases the effect of C-PAM to neutralize their negative charges.

Fig. 5 Effect of acetylation treatment on NFC and fine fractions retention in mixed papers.

3.5. Handsheets physical and mechanical properties

The handsheets density values are presented in Fig. 6A. As can be seen, the highest and lowest density values were obtained for MP and UMAP papers, respectively. The density was increased with the addition of unmodified and acetylated NFC to the pulp about 16 and 8%, respectively. Furthermore, density was increased by adding the NFCs to the acetylated pulp about 12.6%. NFCs with high specific surface area are able to fill paper micro and nanocavities, along with extending hydrogen bonding between fibers in paper network structure which leads to a decrease in paper pore volume and increasing paper density. Moreover, acetylation of NFC decreased its ability to increase the paper sheet density. Furthermore, the density of papers was decreased by increasing the acetylation time; it could be because of more substitution of hydroxyl groups with acetyl groups after acetylation process that led to a decrease in fibers and NFCs bondability. Similar results have been reported in related to the ANFC film.¹⁴

Fig. 6 Effect of acetylation treatment on (A) density, (B) burst index, and (C) tensile index of the different made papers.

The effect of adding NFC and ANFC on burst strength index of made papers is demonstrated in Fig. 6B. With the addition of unmodified and acetylated NFC to the pulp, the burst index increased about 29.5 and 20%, respectively. The highest and lowest burst index was obtained for MP and UMAP papers, respectively. It decreased about 14% in the UMAP papers compared with the reference paper (UMP). By adding NFC to UMAP paper, the burst index increased about 24%.

As can be seen in Fig. 6C, the changing trends in tensile index were similar to the burst index.

Relative bonding area (RBA) and bonding strength as well as single fiber strength are the main factors that affect the tensile and burst strength and other strength properties of paper.⁵² The Addition of NFCs to the paper led to an increase in mechanical entanglements of fibrils, increase in number of inter-fiber bonds, and increase in mechanical strength of the paper.^10,55 Similar results have been reported in previous studies.^{1,9–11,17,55}

Density results reveal that acetylation of NFC decreased its potential of improving burst and tensile indices to some extent that could be attributed to the substitution of hydroxyl groups with bulky acetyl groups and lowered nanofibers bondability. As this substitution continued by passing time, burst and tensile indices of papers decreased. This observation is a direct indication of the key role of the surface hydroxyl groups on the strength of the material. Similar results have been obtained for NFC film and paper sheet previously.^14,31,33,56

3.6. Handsheets barrier properties

3.6.1. Air resistance. Air resistance of paper is related to the density and porosity of paper, the ratio of pore volume to total volume of a sheet.⁵¹ According to Syverud and Stenius,⁴ NFC is a good choice to improve gas barrier properties of paper because of partial change of microporous to nanoporous in paper network, according to the amount, and the way of its application. The NFC causes to form better bonding network among the microfibers by creating mechanical entanglements between them in a similar manner to fibrils after refining that it leads to better physical and mechanical properties.⁵⁷

The air resistance of the made handsheets is presented in Fig. 7A. As shown, the highest and lowest air resistance values were obtained for MP and UMAP papers, respectively. The air resistance was increased by adding both unmodified and acetylated NFC to the pulp about 1090 and 537%, respectively. Moreover, these results were confirmed by the SEM images of papers surface (Fig. 4). NFCs mainly act as a physical barrier and also make homogeneous structures by filling porosities. In comparison with reference paper (UMP), the obtained air resistance for UMAP paper decreased about 81%. In fact, by acetylation of the pulp, air resistance strongly dropped down because of decrease in amount of surface hydroxyl groups, bonding area, and increase in structural porosities. By adding NFC to UMAP, the paper sheet air resistance increased about 1593%. Using NFC, paper porosity was reduced due to increase in the number of inter-fiber bonds. Furthermore, the air resistance of made handsheets was decreased by increasing the acetylation time. This observation is attributed to the fact that more acetylation time increased more disruptions of hydrogen bonds among fibers and fibrils, which increased the paper porosity. Missoum et al.¹ found similar results by adding modified NFC to the paper composition. Finally, it is worth mentioning that the paper made from pulp reinforced with ANFC showed better air resistance compared with that made from acetylated pulp reinforced with the same amount of NFC.

Fig. 7 Effect of acetylation treatment on (A) air resistance, and (B) water absorption of the different made papers.

3.6.2. Water absorption of paper. The results of water absorption are shown in Fig. 7B. The highest and lowest water absorption values appertained to UMAP and MANFC papers, respectively. As shown, addition of the unmodified NFC had no significant effect on the water absorption of MP paper (p > 0.05), while adding acetylated NFC for 0.5 h to the pulp led to a decrease in the water absorption about 23.1%. Furthermore, water absorption was decreased by adding unmodified NFC to the acetylated pulp about 18.5% in comparison with UMAP paper. By increasing in treatment time, the water absorption increased. In general, the water absorption of paper depends on the porous structure of paper sheet and the nature of the interactions that occur between fibers and the water. Partial acetylation in this research decreased the paper water absorption due to the substitution of hydroxyl hydrophilic groups with acetyl hydrophobic groups. However, acetylation progress of fibers causes to increase paper porosity due to decrease in bonding area and therefore, the water absorption of paper increased in longer time of acetylation. NFC is a cellulosic fraction that has a high-surface area and plentiful hydroxyl groups compared with the originating microfibers. This is why ANFC keeps its reinforcing character more than acetylated fiber. Modification of NFC with partial surface acetylation leads to partial substitution of hydroxyl groups with acetyl groups that it causes to increase NFC hydrophobicity, meanwhile NFC keeps its reinforcing character due to having high surface area simultaneously.

4. Conclusions

The main aim of this work was to simultaneously improve mechanical strengths and water barrier properties of conventional papers using partially acetylated NFCs. NFCs (produced using ultra-fine friction grinder) and the bleached commercial pulp were partially treated by heterogeneous acetylation process at different times. Results obtained from the titration, the FTIR analysis, and water contact angle measurements proved successful modification and showed that the acetylation intensity was increased by increasing the reaction time. Degree of substitution for the acetylated pulp fibers and NFCs after a 3 h treatment was calculated 1.06 and 0.29, respectively. Acetylation of the pulp fibers to improve water barrier property of paper did not produce good results and on the contrary, this treatment led to a drastic increase in the water absorption of the UMAP paper and at the same time a severe decrease in the strength properties and the air resistance of this paper. Addition of NFC to the acetylated pulp led to an improvement in the water barrier property of MAP paper about 18.5%. But MAP paper still showed more water absorption in comparison with the reference paper (UMP). Moreover, it is worth mentioning that the mechanical strengths and the air resistance of MAP paper were acceptably improved compared with the reference paper. The obtained results showed that the mechanical and barrier properties of the MAP papers were weaker than the MANFC papers. This could be due to the fact that DS of the acetylated pulp fibers was significantly higher than the acetylated NFCs, even if the samples were acetylated using the same procedure. The partial acetylation of NFCs and addition of ANFCs to the pulp led to an improvement in water barrier property of the MANFC paper significantly compared to all other treatments and the reference paper. Furthermore, because of the filling porosities and increasing in mechanical entanglements of fibrils together with increased number of inter-fiber bonds, the mechanical strengths and the air resistance of the MANFC paper was enhanced about 1.13 times and 6.4 times, respectively, in comparison with the reference paper (UMP). It should be mentioned that the mechanical strengths and the air resistance of MANFC paper decreased to some extent compared to the reinforced paper with the same amount of unmodified NFC (MP paper). However, low water barrier is main disadvantage of the MP paper. This was acceptably improved by adding ANFC to the paper composition.

In this study, for the first time, partially acetylated NFC was added to the paper bulk before forming the web-like structure. Simultaneous improvement in both air and water barrier and mechanical strength properties of the made paper was confirmed using various performed analysis. Simultaneous access to the strength and barrier properties has been considered as a big challenge and the authors believe that this new achievement promises novel potential applications in the paper industry to make special high-performance paper for packaging, printing, and so on.

Acknowledgements

We would like to acknowledge the Iran Nanotechnology Initiative Council for its financial support. We also thank Nano Novin Polymer Co. (Iran) for their help in producing cellulose nanofibers.

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