Improving the Mechanical Strength of Paper Sheets Made from Phosphorylated Fibers Through the Use of Forming Agents

ABSTRACT

The fabrication of paper or cardboards sheets with phosphorylated pulp fibers has proven to be a difficult task using traditional papermaking techniques. This problem is mainly attributed to the very high anionic charge of phosphorylated fibers. In the present work, we propose the use of cationic polymers as a neutralizing agent to attenuate the detrimental effect of the very high anionic charge of phosphorylated fibers and to promote the formation of microflocs that can be agglomerated into a uniform sheet with sufficient strength. The results obtained show that using positively charged retention agents significantly improves the sheet formation. The Kaptra formation index (KFI) was reduced from 133 to 17, which is a very good value when compared to unmodified kraft pulp fibers, which have a formation index of 35. The addition of cationic polymeric additives during the preparation of phosphorylated fiber sheets resulted in an improvement of mechanical properties over unmodified kraft pulp fibers, especially for tensile energy absorption and elongation. This study demonstrates that the use of cationic retention agents reduces the impact of the charge of the phosphorylated fibers on the overall sheet quality.

HIGHLIGHTS

• The use of retention agents overcomes the strong repulsion between phosphorylated fibers due to their high charge.

• The addition of polymeric retention agents allows a significant improvement in the formation index of phosphorylated sheets.

• The polymeric retention agents yield higher strength to the sheets prepared from PKF1 and PKF2, even outperforming sheets made from kraft pulp only.

摘要

用磷酸化纸浆纤维制造纸张或纸板已被证明是使用传统造纸技术的一项艰巨任务. 这个问题主要归因于磷酸化纤维的阴离子电荷非常高. 在目前的工作中，我们建议使用阳离子聚合物作为中和剂，以减弱磷酸化纤维的非常高的阴离子电荷的有害影响，并促进微生物群落的形成，这些微生物群落可以聚集成具有足够强度的均匀片材. 所获得的结果表明，使用带正电的保持剂显著改善了片材的形成. Kaptra形成指数（KFI）从133降低到17，与形成指数为35的未改性硫酸盐浆纤维相比，这是一个非常好的值. 在磷酸化纤维片材的制备过程中添加阳离子聚合物添加剂，与未改性的硫酸盐浆纤维相比，提高了机械性能，尤其是拉伸能量吸收和伸长率. 这项研究表明，阳离子保留剂的使用减少了磷酸化纤维的电荷对整体片材质量的影响.

KEYWORDS:

• Coagulant
• flocculant
• formation
• kraft pulp fibers
• phosphorylation
• physical properties
• retention

关键词:

• 凝固剂
• 絮凝剂
• 组成
• 硫酸盐浆纤维
• 磷酸化
• 物理性质
• 保持

Introduction

In recent decades, the substitution of fossil-based products with biomaterials such as natural fibers has attracted increasing interest due to their recyclability, biodegradability and renewability. However, their high flammability and low thermal stability pose serious problems for certain applications. Therefore, chemical or physical modifications are required to impart a flame-retardant character to fibers. Phosphorylation is one of the most known modifications conferring a flame-retardant character to lignocellulosic fibers. Phosphorylated fibers are produced by adding phosphorus-containing groups to reactive cellulose hydroxyls, as well as to hemicelluloses or even, to a lesser extent, lignin. Although these fibers have advantageous properties, such as their potential to be used as flame retardant materials (Fiss et al. 2019; Li et al. 2020; Rol et al. 2019), for ion exchange (Oshima et al. 2008; C. Shi, Qian, and Jing 2020) and for insulation (Ghanadpour et al. 2015, 2018), they are not yet widely commercialized. Phosphorylated fibers are capable of achieving high electrostatic charges, in excess of 2500 mmol/kg (Y. Shi et al. 2015), making them particularly useful for insulation, where strong interfiber repulsion is required to make a resilient voluminous material. Nevertheless, the repulsion between fibers can be problematic when it comes to producing materials such as sheets. The use of phosphorylated fibers in the manufacture of sheets using traditional methods creates several challenges. First, the fibers bear a high negative charge causing a strong repulsion between them, making the production of high-quality sheets a difficult task. Also, the modified fibers tend to stick to various metallic parts of the paper machine, leading to production issues such as equipment downtime and low sheet quality. Additionally, modifying the fibers can cause a reduction in certain mechanical properties of the paper, such as stiffness, which can have an adverse effect on the sheet formation process.

To overcome these problems, we propose to use cationic polymeric additives to neutralize PF anionic charge before sheet formation. These additives are widely used in papermaking, in particular to improve the retention of fines and charges in the wet-end of the paper machine. In this work, the cationic polymeric additives will be added not to improve charge retention or fine retention, but to counter the strong interfiber repulsion occurring in the phosphorylated fibers suspensions, because of their unusually high anionic charge.

Given the highly anionic charge of phosphorylated fibers, our study investigates the feasibility of manufacturing sheets using a cationic-cationic polymeric retention aid system comprising of a coagulant (polyDADMAC) and a flocculant (cationic polyacrylamide (cPAM)). To fully characterize the phosphorylated fibers, we employed a combination of techniques, including SEM-EDX, fiber length distribution analysis (FQA), thermogravimetric analysis (TGA), and zeta potential measurements. Additionally, we used conductometric titration to accurately quantify the total charge density of phosphorylated fibers. The Kaptra Formation Index (KFI) and reconstructed images were used to evaluate the uniformity of sheet formation. Furthermore, the mechanical properties of phosphorylated sheets produced using the retention aid system were examined and compared to those of sheets made from unmodified fibers.

Experimental

Materials

The pulp material used in this study was an unbeaten bleached softwood kraft pulp fibers (referred to as KF) provided by Kruger Wayagamack Inc. (Trois-Rivières, Canada). All chemical reagents were used as received from different suppliers: 1-decanol, polyphosphoric acid and phosphorous pentoxide (Sigma-Aldrich), and urea (Alfa Aesar).

The typical cationic polyelectrolytes used in this study were provided by Ciba. The coagulant is a poly (diallyldimethylammonium chloride) (polyDADMAC, Alcofix 111) with a molecular weight of 600kDa and a very high charge density. The flocculant is a cationic polyacrylamide (Organopol 5032) with a high molecular weight and a charge density of 1.2 mmol/g.

A mono n-decyl phosphate (referred to as phosphate ester or PE) was used to phosphorylate kraft fibers (KF). It was synthetized using 1-decanol, polyphosphoric acid and phosphorus pentoxide. Decanol (100 g) was heated to 70°C under stirring for 15 min. Polyphosphoric acid (26.9 g) was added in small portions until total dissolution, followed by phosphorus pentoxide (26.9 g). The reaction mixture was stirred for 7 hours at 70°C until a brown viscous product was obtained. No further purification has been made.

Handsheet preparation

KF were phosphorylated using the previously synthesized phosphate ester. The modification was carried out in molten urea according to the method described by Shi et al (2014). The molar ratio of urea:anhydroglucose units was set at 17:1 Three different molar equivalents of PE: anhydroglucose units were used: 3, 2 and 1:1. Modified fibers were identified as PKF1, PKF2 and PKF3 according to the molar ratio of PE. To simplify the calculation of the AGU content of the fibers, KF were assumed to contain only cellulose.

Handsheets with a target basis weight of 60 g/m² were prepared according to a slightly modified version of standard method TAPPI/ANSI T 205 2018, sp-18. First, 24 g of dry pulp were soaked overnight in deionized water. The pulp suspension was disintegrated in the standard disintegrator at 3000 rpm until all fiber bundles were dispersed. In order to accurately control the contact time between fibers and polymers, the original tank of the handsheet machine was replaced with a 14 cm deep plastic tank, and a dynamic drainage jar (DDJ) was installed on top of it (Figure 1). The sheets were prepared by adding first 800 ml of fiber suspension (0.15% consistency) into the DDJ and stirring for 20 seconds. Next, the coagulant was prepared according to the dosages given in Table 1 and poured into the DDJ tank while stirring for 60 seconds. The flocculant solution, prepared according to the dosages given in Table 1, was injected into the DDJ tank while stirring for 3 minutes. The valve was opened to drain the suspension at the end of the mixing step. The injection sequence of the polymers is very important. Indeed, the two-component system requires a certain contact time between the different components and the fibers. The injection sequence is summarized in Table 2.

Figure 1. Modified handsheet machine.

Table 1. Experimental conditions.

Coagulant, polyDADMAC (mg/g fibers)	1.14
Flocculant, C-PAM (mg/g fibers)	6.25
Consistency (%)	0.15
pH of suspension	9
Rotational speed (rpm)	1000

Table 2. Cationic polyelectrolytes injection sequence.

Time (s)	Task
0	Introduction of the suspension in the Dynamic Drainage Jar
20	Injection of coagulant
60	Injection of flocculant
180	Stop agitation

The prepared sheets were stored in an air-conditioned environment (temperature of 23 ± 1°C and relative humidity of 50 ± 1%) according to standard method TAPPI/ANSI T 402 2021, sp-21 for further optical and mechanical tests.

Characterization of modified fibers

Electrostatic properties

The total charge of phosphorylated fibers was measured by conductimetric titration according to the SCAN-CM 65:02 2002 method using a Thermo (USA) Orion conductometer (Model 150) and a Metrohm Brinkmann (USA) automated titrator (Dosimat 765) under an inert nitrogen atmosphere.

The Zeta potential was measured with a Mütek SZP-06 device, as a pH-dependent function. Firstly, a fiber buffer was prepared and introduced into the measuring cell. Then, the phosphorylated fiber suspension was added and diluted to 0.05 wt%. Finally, KCl was added to adjust the ionic strength of the system to 1 mM.

Water retention value measurement

Water retention value measurement (WRV) is an analytical technique that uses centrifugation to determine the amount of water retained in a fiber pad under conditions well defined in the norms. According to the TAPPI UM 256 (2015) method, the procedure is carried out by dispersing 1 g of fibers in 30 ml of deionized water for 24 h at room temperature. Then, the wet fibers are centrifuged with a relative centrifugal force of 3000 g for 15 min to remove excess water. After centrifugation, the samples are measured and dried at 105°C for 12 hours to determine the dry mass. The water retention value is calculated according to equation 1.

(1) $\mathrm{WRV}=\mathit{\hspace{0.17em}}\frac{m_w-m_d}{m_d}$(1)

where m_w and m_d are, respectively, the wet and dry substrate masses (g).

Elemental analysis

The determination of the phosphorus content of modified fibers was carried out by a two-step method. In the first step, fibers are digested in concentrated sulfuric acid and hydrogen peroxide. In the second step, the phosphate ions present in solution are quantified by UV-Visible spectrophotometry. To determine the phosphorus content of the fibers, a calibration curve was obtained by preparing solutions with known phosphorus concentrations from 3 to 60 ppm. These solutions are prepared from mixtures of KH₂PO₄ (150 ppm), 1 N sulfuric acid, 0.055 M ammonium heptamolybdate, and 10% w/v sodium sulfite (Mahadevaiah Kumar et al. 2007). The absorbance of solutions was measured at a wavelength of 715 nm with a Hach FN 1280× spectrophotometer.

Scanning electron microscopy (sem)/energy dispersive X-ray (EDX)

Scanning electron microscopy (15 kV, variable pressure) coupled to energy dispersive X-ray spectroscopy (SEM/EDX, Hitachi SU1510 with Oxford X-max 20 mm²) was used to track structural changes occurring at fiber surface during the phosphorylation reaction, and to evaluate the phosphorus content and its distribution on the surface of treated fibers. Samples were directly mounted on a conductive double-sided tape, without metallization. The phosphorus degree of substitution (DS_P) was calculated with the following equation:

(2) $\mathit{D}{S}_P=\frac{162\times \mathit{P}(\mathrm{\%})}{3100-80\mathit{P}(\mathrm{\%})}$(2)

Fiber length distribution

A Lorentzen & Wettre Fiber Tester Plus fiber quality analyzer (FQA) was used to determine the mean arithmetic fiber length, length weighted percentage of fines, and fiber width. The analyzer takes the sample and calculates the length of each fiber for about 3 minutes. By comparing unmodified and modified fibers, the extent of fiber degradation occurring during phosphorylation can be evaluated.

Sheet characterisation

The Kaptra Vision 9000 Formation Tester was used to evaluate sheet formation. The system determines the intensity of light transmitted through the sheet and produce a reconstructed image of showing low- and high-density areas and calculates a formation index (KFI) that reflects sheet uniformity. Lower KFI values indicate a better formation.

The physical properties of the sheets were measured according to TAPPI standard methods: the tensile index was measured with and Instron 4201 (TAPPI/ANSI T 494 2022, om-22), the tear index was evaluated using an Elmendorf tearing tester (TAPPI/ANSI T 414 2021, om-21), and the burst index was measured according to TAPPI/ANSI T 403 2022, om-22. Brightness was determined with standard method TAPPI T 452 2018, om-18 using a Technibrite TB-IC.

Results and discussion

Characterization of modified fibers

Figure 2 shows SEM images of KF and PKF obtained with different PE molar ratios. The FQA results clearly show that the phosphorylation reaction slightly affects fiber length with three moles of PE (PKF3). The SEM results confirm the mild reaction conditions used lead to a very limited fiber cleavage while also preserving their surface aspect. Table 3 gives the average fiber length, surface charge and total charge, measured before and after phosphorylation. As expected, after phosphorylation with 3 moles of PE, the small amount of microfibrils present on the surface of the unbeaten kraft pulp fibers disappeared. This can be attributed to an increase in the acidic character of the reaction environment (acidolysis), which affects exposed microfibrils.

Figure 2. SEM images of fibers: (a) unmodified KF; KF modified with (b) 1 mol PE, PKF1; (c) 2 mol PE, PKF2; (d) 3 mol PE, PKF3.

Table 3. Water retention value, charge density and dimensional properties of fibers before and after phosphorylation.

Sample	Molar ratio PE: KF	Charge content (mmol/kg)	Surface charge (mmol/kg)	Average fiber length (mm)	Average fiber diameter (µm)	WRV (g water/g fibers)
KF	-	230 ± 15	30 ± 5	2.041 ± 0.03	25.7 ± 0.2	1.01 ± 0.01
PKF1	(1 :1)	2833 ± 20	350 ± 15	1.99 ± 0.01	31.9 ± 0.1	1.07 ± 0.01
PKF2	(1 :2)	3618.8 ± 40	470 ± 10	1.75 ± 0.02	32.4 ± 0.3	1.68 ± 0.01
PKF3	(1 :3)	3719.7 ± 30	477 ± 10	1.61 ± 0.01	32.8 ± 0.1	1.88 ± 0.01

A conductimetric titration with a 0.1 N NaOH solution was used to estimate the total charge of KF and PKF. There are generally very few sulfonate or carboxylate groups in kraft pulp fibers. The V-shaped curve in Figure 3a of the native kraft fibers (KF) clearly indicates a low presence of acid groups such as sulfonate and carboxyl. The negative charge of approximately 230 μeq/g observed during the conductometric titration of the unmodified fibers is a result of the pulping procedure used in the production of kraft pulp fibers (Ablouh et al. 2021; Y. Shi et al. 2015). In the case of PKF1 (Figure 3b), the total charge value was found to be 2833 mmol/kg, which is 11 times higher than that of the unmodified fibers. Typically, phosphorylation with 3 equivalents of phosphate ester (PKF3) results in a significant increase in the total charge, reaching an optimal value of around 3719 mmol/kg. We can assume that the increased anionic charge in PKF is essentially due to the addition of phosphates. In consequence, the different parts of the titration curve of PFK represent the neutralization of both protons of the cellulose phosphate monoester. The titration curve for PKF1 is shown in Figure 3b. Diluted HCl (20 mL, 0.1 M) was added to the pulp suspension to facilitate the detection of the first endpoint. In this first section of the curve (Zone 1), strong acidic groups are neutralized and a strong decrease of the conductivity is observed. For PKF1, the first endpoint is found at 21.28 mL of added 0.1 M NaOH, indicating a very low amount of strong acid groups. We can assume that the neutralization of both protons of the phosphate groups occurs in the second part of the curve (Zone 2) (from 21.28 to 36.02 mL). The total amount of anionic charge detected (in mmol/kg fibers) should correspond to twice the amount of phosphorus found in PKF (also in mmol P/g fibers) since each phosphate groups bears two protons. Finally, after the second endpoint (Zone 3), the conductivity rises sharply due to the presence of excess hydroxide ions.

Figure 3. Conductimetric titration curve of native kraft fibers and phosphorylated kraft fibers (PKF1).

Table 3 shows the average fiber length, water retention value, and charge content, measured before and after phosphorylation. The water retention value for the studied samples is 1.01 for kraft pulp and 1.07, 1.68 and 1.88 for PKF1, PKF2 and PKF3 respectively. PKF3 exhibited the highest WRV among the three phosphorylated fiber types with an average. This can be explained by its higher degree of substitution compared to the other samples. Furthermore, the presence of phosphate groups on the surface of cellulosic fibers increases the hydrophilicity of the fibers and their counter-ion is able to break the hydrogen bonds, which promotes the penetration of water into fiber pores.

The increase in diameter of PKF compared to unmodified KF (an increase from 25.7 to 32.8 µm) can be explained by the increase of the internal repulsive effect in the structure of the fibers which leads to their swelling.

Figure 4 illustrates changes in zeta potential of kraft fibers as a function of pH, both before and after phosphorylation modification, in order to better understand the surface charge characteristics of these fibers. It is obvious that PKF1, the phosphorylated kraft fiber, exhibits a stronger anionic character than the unmodified fiber, with zeta potentials of −50.1 mV and −3.2 mV, respectively. The presence of phosphate groups in PKF1 imparts a negative charge, which can be observed across the entire pH range studied. These zeta potential findings are consistent with prior results indicating that PKF1 is more anionic than unmodified fibers.

Figure 4. Effect of pH on the zeta potential for unmodified kraft fibers and phosphorylated kraft fibers.

Table 4 shows the results of the analyses performed to determine the phosphorus content of the fibers, using EDX and UV-visible spectroscopy, on the different samples prepared. The DS_p was calculated using two different methods to establish a comparison between the relative amount of phosphate found on the surface and in the whole fibers. The DSp calculated by elemental analysis ranged from 0.23 to 0.3 and the DSp obtained by UV-visible spectroscopy titration varied from 0.11 to 0.14. The two methods of analysis give values that agree with the molar ratio used. The difference between the two methods is that the elemental analysis gives the percentage of phosphorus present on the surface of the fibers, while the UV-visible spectroscopic titration gives the percentage of phosphorus by mass (the fibers are digested in a concentrated acid).

Table 4. Phosphorus content of fibers.

Sample	Phosphorus
	EDX (Surface)		UV-visible spectroscopy (Total)
	P (%)	DSP	P (%)	DSP
KF	-	-	-	-
PKF1	4.1 ± 0.1	0.23 ± 0.02	2.2 ± 0.1	0.11 ± 0.01
PKF2	4.4 ± 0.1	0.26 ± 0.02	2.5 ± 0.1	0.14 ± 0.01
PKF3	4.9 ± 0.1	0.3 ± 0.1	2.6 ± 0.1	0.14 ± 0.02

Characterization of handsheets

The formation quality of a paper sheet can be assessed visually by looking through it. Figure 5 illustrates the visual aspect of 60 g/m² handsheets prepared with KF, PKF1 without polymeric additives, and PKF1 with polymeric additives. Several light and dark spots of varying sizes are visible, where the black spots are areas with many agglomerated fibers and the light areas are zones with fewer fibers or even holes. The quality of the formation is expressed by a single number, the KFI index, reflecting the overall uniformity of the sheet. Using positively charged retention agents improves the sheet formation index significantly. KFI is reduced from 133 to 17 (Figure 6), which is an excellent value compared to unmodified kraft pulp fibers, which have a KFI of 35. As mentioned before, cationic coagulants and flocculants were not used to increase the retention of fines or mineral charges, but to promote the formation of microflocs that can be agglomerated and thus obtain a good uniformity of sheet formation. When manufacturing sheets from phosphorylated fibers without the addition of retention agents, their formation is poor and the sheet adheres strongly to the metal plate upon drying, as it is clearly shown in Figure 5b.

Figure 5. Visual aspect of 60 g/m² handsheets prepared with (a) KF, (b) PKF1 without polymeric additives, and (c) PKF1 with polymeric additives.

Figure 6. Reconstructed image and KFI value of (a) KF, (b) PKF1 without polymeric additives, and (c) PKF1 with polymeric additives.

Sheets made from phosphorylated fibers without the addition of retention agents have such poor properties that it is almost impossible to compare them to KF sheets. In consequence, the optical and mechanical properties of only PKF sheets with additives were measured and compared to the unmodified pulp control KF handsheets. Results are shown in Table 5 for samples obtained with 1 to 3 moles of PE. The mechanical properties of formed sheets are dramatically improved when phosphorylated kraft pulp fibers are treated with 1 or 2 moles of PE. This is expected because increasing the number of PE moles causes an increase in the acidity of the reaction medium, which primarily affects the exposed microfibrils, resulting in a slight decrease in fiber length (see Table 3). The polymeric retention agents provide better strength to sheets prepared from PKF1 and PKF2, even outperforming sheets made from unmodified KF. On the other hand, the tear index decreased by almost 70%. However, it is important to note that is the only tested property that depends on the intrinsic strength of fibers rather than interfiber bonds. It was not possible to measure all mechanical properties for PKF3. TEA, break index, and elongation were not determined because of the brittleness of the sheet, which is due to a significant reduction in the average size of the fibers, making them unsuitable for paper or cardboard manufacturing.

Table 5. Physical and optical properties of sheets.

Sample	Thickness (mm)	ISO brightness (%)	Burst index (kPa m^2/g)	Tear index (mN.m^2/g)	Break index (N.m/g)	TEA (J/m^2)	Elongation (%)
KF	0.175	86.8	1.42	3.2	22.14	14.88	1.49
PKF1	0.18	77.08	3.02	0.96	37.8	41.3	2.375
PKF2	0.175	72.05	2.38	0.97	30.88	30.33	1.772
PKF3	0.27	63.29	0.58	0.68	n.a.*	n.a.*	n.a.*

n.a.* = not analyzed.

The addition of polymeric retention agents improves the mechanical properties of the paper. It holds the fibers together, resulting in a more interconnected network and a more mechanically stable structure. This, in turn, improves sheet formation.

Finally, the increase in PE molar ratio during phosphorylation significantly reduces sheet brightness. The effect is a typical effect of phosphorylation that could represent a drawback for standard paper products, but should have limited impact on other applications where color is not a critical property.

Conclusions

This work presents a technique for producing sheets from highly charged phosphorylated fibers, a subject that has not been previously explored. This approach offers an environmentally friendly alternative to certain products. The use of cationic polymers, acting as a coagulant and flocculant, during the manufacturing process, has been found to be an effective method for improving the formation process and ensuring the uniformity of the sheets. The inclusion of these polymers in the pulp suspension during sheet preparation has shown to decrease the KFI value from 133 to 17.

The mechanical properties of phosphorylated fiber sheets prepared with forming agents is improved over KF, especially elongation and tensile energy absorption (TEA). This result suggests that the addition of forming agents has a positive impact on mechanical properties. However, we are still lacking a comparison with a phosphorylated fiber sheet obtained without forming agents. As mentioned above, such sheet cannot be prepared using a standard handsheet former. As a result, the proposed method in this study provides an efficient means of creating sheets from phosphorylated fibers that exhibit favorable physical, mechanical, and optical properties.

Acknowledgement

The authors would like to acknowledge the financial support of the Natural Sciences and Engineering Research Council of Canada (NSERC) through the Collaborative Research and Development (CRD) grant program and the Consortium for Research and Innovation in Industrial Bioprocesses in Quebec (CRIBIQ).

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by the financial support of the Natural Sciences [NA]; Consortium for Research and Innovation in Industrial Bioprocesses in Quebec [NA]; Collaborative Research and Development [NA]; Engineering Research Council of Canada [NA].