Effect of Ultrasonic-assisted Ionic Liquid Pretreatment on the Bleachability and Properties of Eucalyptus Kraft Pulp

Effect of Ultrasonic-assisted Ionic Liquid Pretreatment on the Bleachability and Properties of Eucalyptus Kraft Pulp

Jianmin Peng, Letian Qi

, Guihua Yang

, Ming He, Yu Xue, Jiuachuan Chen Received November 4, 2018; Received in revised form March 26, 2019; Accepted March 27, 2019

ABSTRACT

In this work, the application of ionic liquid (IL) and ultrasonic (Ul) treatment on the bleachability and properties of eucalyptus kraft pulp were investigated. The results showed that ILs protected fibers from depolymerization by improving the pulp viscosity.

The combination of Ul and IL treatment notably reduced the kappa number and improved tensile index, burst index, tear strength index and folding endurance of pulp. Ul and IL treatment reduced the fines content, decrease the water retention value (WRV) and cause the formation of crosslinks between fibers, especially in Ul+[TEA][HSO

] treatment. The XRD and FT-IR analysis illustrated the combination of Ul and IL treatment disturbed the amorphous region of pulp fiber by increasing of crystallinity and no chemical change ap- peared in the treatment process. The Ul assisted IL treatment technology will have a promising application due to its high efficiency and environmental protection character.

Keywords: Ionic liquid, ultrasonic, treatment, bleaching, eucalyptus kraft pulp

• State Key Laboratory of Biobased Material and Green Papermaking, Qilu University of Technology, Shandong Academy of Sciences, Jinan, 250353. P. R. China

† Corresponding Author: E-mail: [email protected]

‡ Co-corresponding Author: E-mail: [email protected]

Printed in Korea http://dx.doi.org/10.7584/JKTAPPI.2019.04.51.2.16

1. Introduction

Elemental chlorine free (ECF) bleaching technol- ogy apply ClO

to instead of Cl

gas in the process.

It avoids the formation of organochlorine com- pounds and reduces the amount of effluent dis- charged, thereby widely adopted in industry.

However, the rigid structure of lignocellulosic fi-

ber

and stable nature of lignin chains

require

high dosage of ClO

during the bleaching process,

which unavoidably causing equipment corrosion

and reducing the physical properties of paper.

Therefore, it would be helpful to develop a treat-

ment technology loosening the lignocellulose fiber

structure, facilitating the bleaching chemicals to

access lignin compounds, and minimizing the im-

pact on the fibers strength. So far, various ap- proaches were investigated for fiber pretreatment such as enzyme, ionic liquid and ultrasonic treat- ment.

Ionic liquids (IL) are molten salts under mild con- ditions. These novel solvents have various merits, such as good solubility, non-volatile, chemical stable, recyclable and tunable.

It has shown par- ticular ability to disturb the H-bond network of the plant fiber. Imidazolium-based ILs were firstly reported to dissolve cellulose through their strong H-bond effect, among them [BMIM][Cl] presented good solubility.

This dissolution endures a de- crease in crystallization, and transformation from Cellulose I into Cellulose II.

Several studies have been done to investigate the effect of ILs with lignocellulosic fibers, and the re- sults showing that ILs can enhancing the swell- ing,

increasing surface area and improving phys- ical strength of paper.

On the other hand, ILs were also applied in the separation of lignocellu- losic components. Imidazolium based ILs have been used on the production of high purity dissolved pulp.

Moreover, [BMIM][Cl] was used to remove low molecular weight lignin compounds, from pulp in order to improves the bleaching performance.

These literature studies indicated that ILs can be designed to lose the rigid plant fiber structure, modify cellulose structures and remove lignin compounds. Therefore, it could assist bleaching process and improve the physical properties of pa- per. However, current studies majorly focused on imidazolium-based ILs and limited to their com- mercial availability, which inhibited further appli- cations. Recently it has been reported that [TEA]

[HSO

] IL has shown great power to dissolve lignin even at high water loading up to 20 wt%.

In ad- dition, this type of IL can be produced at low cost,

which is ideal for plant fiber treatment.

With these merits, [TEA][HSO

] might be suitable for the application in bleaching process.

Ultrasonic (Ul) treatment was considered as a suitable approach to loosen fiber structure.

It has been reported being an efficient technique for hemicellulose extraction,

fiber morphology modifcation

and fibers properties enhancement.

The WRV,

drainability

and accessibility

of fiber have been improved after Ul treatment.

Thereby, when applied along with IL treatment, Ul could enhance the IL treatment effect.

It was re- ported that a combination of Ul and IL treatment improves surface areas of fiber and enhanced the physical strength of handsheets.

However, the effect of Ul assisted IL treatment on bleachability of kraft pulp (KP) have not been investigated yet.

In this work, IL and Ul pretreatment was applied under mild treatment conditions before the bleach- ing of eucalyptus KP. Their effect on bleaching performance was investigated by analyzing the fi- ber qualities and physical strength of handsheets.

2. Materials and Methods

2.1 Materials

Eucalyptus globules wood was obtained from Asia Symbol (Shandong) Pulp and Paper Co., Ltd.

(China). It was air-dried and then cut into pieces in size of 3-4 cm. The kraft cooking of eucalyptus used following process conditions: available alkali 21%, sulfidity 25%, maximum cooking temperature 170℃, and cooking time 90 min at 170℃. And cooking liquor was filtrated and then kraft pulp sealed into polyethylene bag.

Chemicals used in this work including grade and source was listed as following:

1-butyl-3-methylimidazolium hydrogen sulfate

([BMIM][HSO

]) (analytically reagent (AR), 95%)

was purchased from Shanghai Rhawn Chemical

Co., Ltd. Triethylamine (AR, 99%) was purchased

from Tianjin Fuyu Fine Chemical Co., Ltd. Anhy-

drous magnesium sulfate (AR, 99%) and sodium

hydroxide (AR, 96%) were purchased from Tianjin Da Mao Chemical Co. Chlorine dioxide (AR, 1 vol%) was purchased from Shandong Dahua Special En- vironmental Engineering Co., Ltd. Hydrogen per- oxide (AR, 30%) and sulfuric acid (AR, 98%) were purchased from Lai Yang Chemical Co., Ltd. So- dium sulfide (AR, 98%) was purchased from Tianjin Dingshengxin Chemical Industry Co., Ltd.

Triethylammonium hydrogen sulfate ([TEA]

[HSO

]) was synthesized as described by Brandt et al.,

where 5M H

SO

aqueous solution were added stepwise into triethylamine in round-bottom flask under ice bath. The IL generated was dehydrated under rotary evaporation followed by Schlenk line at 70℃ for 24 h.

2.2 Ionic liquid and ultrasonic treatment

25 g oven dried KP was homogeneously mixed with or without the presence of 2.5 g IL, then wa- ter was added until to become pulp consistency of 10%. After that, the mixture was reacted in water bath, with or without 250 W ultrasonic treatment (KQ-250DV, Kun Shan Ultrasonic Instruments Co., Ltd.) for 60 min.

2.3 Bleaching

(O)AD

E

D

bleaching sequence was used, where treated KP was delignified with oxygen treatment (O):0.5 MPa oxygen, 3% Na

O, and 100 for 60 min.

The treated pulp was rinsed with water and ad- justed to 10% pulp consistency in polyethylene bag.

Followed by AD

E

D

bleaching, with the opti- mized condition as following: (1) hot acid bleaching (A): pH 3.5-4.5, 85℃, 120 min; (2) chlorine dioxide bleaching (D

): ClO

7 kg/t pulp, pH 2-3, 70℃, 30 min; (3) alkali extraction with oxygen and hy- drogen peroxide (E

): H

O

12 kg/t pulp, MgSO

6 kg/t, sodium hydroxide aqueous solution (5 wt%) was used for adjustment pH to 11-12, 90℃, 60 min, oxygen pressure 400 kPa; (4) chlorine di- oxide bleaching (D

): ClO

8 kg/t pulp, pH 3.5-4.5,

75℃, 120 min.

2.4 Characterization of pulps

Viscosity was determined according to the ISO 10650:1999 method, and degree of polymerization (DP) of pulp was calculated based on the pulp vis- cosity. The average degree of polymerization was calculated from the intrinsic viscosity (η) using the same manner in literature,

which was shown in Eq. 1.

DP0.905=0.75[η] [1]

Pulp yield were calculated as the mass ratio of sample before and after bleaching. Kappa number was determined according to the ISO 302:2004 method. The mass ratio of samples before and af- ter treatment were calculated as pulp yield. The water retention value (WRV) of fibers was deter- mined according to the ISO 23714:2013 method.

Fiber quality including weight mean length, weight mean width, and fines content was determined ac- cording to the ISO 16065:2007 method using a fiber quality analyzer (FQA-LDA02, OpTest Equipment Inc., Canada).

2.5 Characterization of the handsheets

Handsheets were prepared according to Chen et al.

Treated pulp was dispersed with water and formed 80 g/m

handsheets, then kept at the stan- dard conditions of 23℃ and 50% humidity prior to testing. The tensile index, burst index, tear index, folding endurance and brightness of handsheets were measured according to the ISO methods 15754:2010, 2758:2014, 1974:2012, 5625:1997 and 2470:1999, respectively.

Changes in morphology of the fibers were ana-

lyzed by using a Regulus 8,220 FE-SEM (Hitachi

High-Technologies Corporation, Japan) operated

at 5 kV accelerating voltage, 2,000 times magnifi-

cation.

The Fourier transformed infrared (FT-IR) spectra of the handsheets were recorded using a spectro- photometer (IR Irdison-21, Shimadzu, Japan), with resolution ratio of 4 cm

, scanning speed 32 s

and scanning range of 4,000-500 cm

.

The crystallinity of the pulp fiber was measured by X-ray diffraction (XRD D8-DVANCE Bruker, Germany). The measurements were conducted un- der the conditions of X-Ray 40 kW and 35 mA with the angle from 5 to 60°. The crystallinity was de- termined according to Segal et al.

3. Results and Discussion

3.1 Effect of IL and Ul treatment on the pulp properties

The oxidation of bleaching process destroyed the fiber structures and resulting in undesirable depo- lymerization. The degree of polymerization (DP) was determined basing on viscosity of pulp in this work, where the viscosity and DP of unbleached pulp was 619 mL/g and 884, respectively, and was dropped to 481 mL/g and 669 after bleaching. As can be seen in Table 1, both IL treatments slightly increased the DP of pulp, illustrating a protection of fibers during the process. In addition, both type IL treatment reduced the kappa number, indicating desirable lignin removal. Meanwhile, the yield of pulp and water retention value (WRV) remain un- changed.

Ultrasonic treatment also increased the pulp vis- cosity and DP, however reduced the yield. This could attribute to the mechanical breakage of Ul, resulting in a loss of low-DP fibers during bleach- ing process. This mechanical effect also contrib- uted to fibrillation, which induced the improvement in WRV and helped with lignin removal. While, after Ul assisted IL treatment, the DP increased, and yield of pulp remain unchanged. This phe- nomenon, again, indicated the protection effect of IL during the bleaching process. The Ul treatment promoted the lignin removal effect of IL, as the kappa number reduced notably. Interestingly, the WRV of pulp reduced after Ul+IL treatment in comparison with that in control and Ul test, refer- ring to a lower degradation of fiber as shown in viscosity data. This also pointed out an unexpected inhibition in fiber swelling and modification on microfiber structures. These results firstly identi- fied a fiber protection effect of Ul assisted IL treatment during the bleaching, indicting it might be an ideal combination for bleaching process. It seems that lower WRV value of the Ul+IL treated pulp is due to the lower degradation of fiber i.e.

difference of viscosity of pulp.

3.2 Effect of IL and Ul treatment on fiber structures

Although ILs were widely reported to dissolve lignocellulose, both type ILs tested in this work presented no visible effect on the fiber qualities in

Table 1. Effect of IL and Ul treatment on the pulp properties Treatment

method

Pulp viscosity (mL/g)

Degree of polymerization

Yield

(%) Kappa number WRV

(g/g)

Control 481±1 669 96±0.30 2.25±0.08 1.99±0.06

[BMIM][HSO

] 507±2 709 96±0.25 0.92±0.08 2.00±0.04

[TEA][HSO

] 486±1 677 97±0.46 0.92±0.08 1.99±0.03

Ul 485±1 675 94±0.13 2.16±0.08 2.14±0.02

UI+[BMIM][HSO

] 514±4 720 97±0.24 0.76±0.08 1.96±0.02

UI+[TEA][HSO

] 493±3 688 96±0.22 0.76±0.08 1.87±0.01

Table 2, except [TEA][HSO

] slightly increased the fiber width, and reduced the fines content and kink index, which could contribute to the enhancement of paper strength.

Ul treatment also presented minor effect on the fiber qualities, with slight increase in the fines content value and reduction in curl index, which indicated an improvement in fibrillation. According to Table 2, for the Ul assisted IL treatments, no no- table change was found in fiber length, width and curl index, but the kink index of fibers decreased by 4%, which can improve the strength properties for paper.

However, the combination of IL and Ul treatment notably reduced the fines content of pulp, especially in Ul assisted [TEA][HSO

] treat- ment process, where fines content decreased by 28%. This phenomenon could possibly be caused by the mechanical effect of ultrasonic treatment via modifying the surface of fiber, which improved the accessibility of IL. As a result, micro-fiber compo- nents were modified, and fines content reduced.

3.3 Effect of IL and Ul treatment on the physical properties of handsheets

It was reported that IL could break the hydrogen bond between cellulose and lignin, resulting in an improvement in brightness.

Table 3 showed that IL treatment slightly improved the brightness of paper, but notably enhanced their physical strengths. The tensile index, burst index, tear in- dex and folding endurance of fibers were all im- proved. The reason might be that aqueous IL solu- tion partially dissolved and softened the surface of plant fiber cell wall.

The modification of fibers via the swelling and fibrillation enhanced the in- teractions and crosslinks between fibers, therefore improved the physical strengths.

The folding endurance increased from 115 to 273 in [BMIM]

[HSO

] treatment, and to 333 in [TEA][HSO

] treatment, which was about 3 times of the control samples. Similarly, [TEA][HSO

] treated samples presented a stronger tensile, burst and tear index.

It was reported that [TEA][HSO

] aqueous solution

Table 3. Effect of IL and Ul treatment on the properties of handsheets Tensile index

(N·m·g

)

Burst index (kPa·m

·g

)

Tear index (mN·m

·g

)

Folding endurance (times)

Brightness (%)

Control 4.35±0.01 4.055±0.022 6.746±0.22 115±7 75.2±0.2

[BMIM][HSO

] 4.60±0.03 4.433±0.021 7.274±0.05 273±13 75.8±0.1

[TEA][HSO

] 4.72±0.03 4.454±0.01 7.461±0.14 333±18 75.8±0.1

Ul 4.64±0.06 3.964±0.01 7.537±0.11 186±11 75.3±0.2

Ul+[BMIM][HSO

] 4.68±0.04 4.335±0.019 7.955±0.23 305±15 76.0±0.1 Ul+[TEA][HSO

] 5.09±0.07 4.196±0.013 8.058±0.04 369±14 76.0±0.1 Table 2. Effect of IL and Ul treatment on the fiber qualities

Weight mean length (mm)

Weight mean width (μm)

Fines content

(%) Kink index Curl index

Control 0.586±0.002 14.0±0.1 20.25±0.10 55.4±0.35 15.7±0.15

[BMIM][HSO

] 0.583±0.001 14.0±0.2 20.46±0.17 55.6±0.20 14.8±0.10 [TEA][HSO

] 0.590±0.005 14.4±0.1 18.92±0.31 54.1±0.32 15.0±0.10

Ul 0.587±0.001 14.0±0.1 20.99±0.54 54.6±0.45 14.8±0.15

UI+[BMIM][HSO

] 0.596±0.004 13.9±0.2 16.79±0.95 55.3±0.50 15.5±0.25

UI+[TEA][HSO

] 0.596±0.002 14.6±0.2 15.04±0.76 53.2±0.46 16.0±0.35

promoted lignin removal,

which contributed to the fibrillation and swelling. In addition, the removal of lignin also improved the flexibility of pulp fiber, which can contribute to the important of paper strength.

Ul treatment also improved the tensile, tear and folding strength properties of paper, but presented lower reduction in burst strength. It provided the similar effect in beating process, assisting the loose of plant fiber.

The modification on the surface of fiber improved the accessibility, thereby enhanced the physical strengths and brightness. Ul treat- ment mechanically broke the fiber and reduced the length of fiber, leading to a reduction in burst in- dex. The combination of IL and Ul treatment nota- bly improved the physical strength of paper. This phenomenon was also reported by Chen et al.

where Ul improved the accessibility of IL. The swelling of fiber improved, and crosslinks en- hanced, both of which improved the strength of paper via Ul and IL pretreatment.

3.4 Morphology of the handsheets

The morphology of fibers was shown in Fig. 1, where IL treatment changed the morphology of the fiber. Minor cracks were observed on the fiber wall surface after the [BMIM][HSO

] treatment (Fig.

1-b), Meanwhile [TEA][HSO

] provided more better separation influence, and a significant modification on fiber could be viewed in Fig. 1-c. Formation of adhesive microfibers on the pulp fiber surface were observed. This refers to the fact that the fines content value dropped after IL and Ul treatment, while the yield of pulp and water retention value remain unchanged. In addition, the viscosity of pulp have slightly increased via Ul+IL treatment.

All these could possibly due to the formation of microfibers, which has more surface area and in- duces cross-links between fibers and resulting in an enhancement of paper strength.

Fibers became more fibrillated with the assistance

of Ul treatment. A sign of mechanical breakage was observed in Fig. 1-d. Applying Ul combined with IL treatment caused a notable modification on fiber surface. And the fibrillation notably im- proved, and more adhesive microfibers formed.

The morphology of handsheets backed up with previous analysis, where ILs treatment changed the fiber surface, provided more cross-links. Es- pecially in Fig. 1-f, with Ul assisted [TEA][HSO

] treatment, a lot of cross-links formed. In the Ul assisted IL approach, Ul treatment provided me- chanical breakages, improved the accessibility of IL to fibers, thereby improved the IL treatment performance. Thus, Ul assisted IL treatment nota- bly improved physical properties of paper.

3.5 Fourier transform infrared (FT-IR) analysis

Fourier transform infrared (FT-IR) spectroscopy of the treated pulp were shown in Fig. 2, where the Fig. 1. SEM images of the treated and untreat-

ed pulp. Note: (a) control, (b) [BMIM]

[HSO

], (c) [TEA][HSO

]; (d) Ul, (e) Ul+[BMIM][HSO

] and (f) Ul+[TEA]

[HSO

].

a

c

e

b

d

f

treated pulp showed a similar spectrum, indicating no significant change in functional group of the pulp occurred during the treatment. The peak po- sition and assignment bonds were listed as follows:

the broad absorption band at 3,400 cm

was asso- ciated with H-bond OH groups on cellulose and lignin;

the peak detected at 2,940 cm

indicated a symmetric C-H stretching vibration; the band at 2,850 cm

was attributed to the symmetric C-H stretching vibration; the band at 1,670-1,720 cm

was attributed to the -COO stretching vibration;

the band at 1,506-1,596 cm

was attributed to the vibration of aromatic ring; the peak around 1,160 cm

was attributed to the C-O-C bending vibrations in cellulose, while the peak near 1,060 cm

was C-O-C stretching vibration.

Compared to untreated pulp, the FT-IR spectra of treated pulp in Fig. 2 observed no new peak, which indicates no chemical change in the IL and Ul treatment process. It should be mention that, the intensity of H-bond peaks (3,400 cm

)

strengthened after Ul and IL treatments. The hy- drogen bonds (3,000 cm

to 3,600 cm

) in polymer was evaluated use the same manner as in litera- ture,

and the area of hydrogen bonds after aque-

ous Ul+IL treatment decreased to 17.62%

(Ul+[TEA][HSO

]) and 17.59% (Ul+[BMIM][HSO

]), whereas the other functional groups in pulp were nearly unchanged. Breaking down the hydrogen bonds in lignin structure by Ul and IL treatments reduced the intermolecular forces.

This may con- tribute to a better crosslink between fibers, im- proving the physical properties of handsheets. This is another evidence of this treatment has an effect on lignocellulosic fibers, which supports the previ- ous SEM results.

3.6 X-ray diffraction (XRD) analysis

As shown in Fig. 3, XRD results showed no new diffraction peak in addition to the enhancement of intensity. The crystallinity of cellulose I was calcu- lated in this work. According to Table 4, the crys- tallinity of control samples is 43.7%. Both [BMIM]

[HSO

] and [TEA][HSO

] treatment improved the crystallinity of pulp. [TEA][HSO

] performed slightly better, improved to 46.7%. Ul treatment improved the crystallinity of fiber by 2.5-3.0%.

Among all the treatments, [TEA][HSO

]+Ul treat- ment showed the highest crystallinity of 48.0%.

Higher crystallinity degree indicates the IL and Ul

Fig. 2. FT-IR spectra analysis of the treated and untreated sheet. Note: (a) control, (b) Ul, (c) [BMIM][HSO

], (d) Ul+

[BMIM][HSO

], (e) [TEA][HSO

] and (f) Ul+[TEA][HSO

].

Fig. 3. XRD analysis of the treated and un- treated sheet. Note: (a) control (b) UI, (c) [BMIM][HSO

], (d) Ul+[BMIM]

[HSO

], (e) [TEA][HSO4] and (f)

Ul+[TEA][HSO

].

treatment promoted the dissolution of amorphous region of fiber.

[TEA][HSO

] could form strong hydroxyl bond with fiber,

thereby disturbed the H-bond network in amorphous region. In addition, Ul mechanically loosened the fiber structure, im- proved the accessibility of ILs to the fibers.

Thereby, the combination of Ul and IL treating technique contributed to the dissolving of amor- phous region, increased the crystallinity, resulting in the improvement in physical strength of paper.

4. Conclusions

Combination of IL and Ul treatment had obvious influence on the bleachability of eucalyptus kraft pulp. ILs treatment protected fibers from depo- lymerization in the bleaching process by improving the viscosity. The Ul assisted IL treatment notably reduced the kappa number. Enhancement in phys- ical strength of pulp was observed due to the sig- nificant decrease of the fines content of the pulp treated by combination of IL and Ul treatment.

Fines content of the pulp treated by Ul and [TEA][HSO

] decreased by 28%, while the yield of pulp remains unchanged. The modification of mi- cro-fibers resulted in a formation of adhesive mi- crofibers, improved fiber crosslinks. The XRD and FT-IR analysis showed that the Ul assisted IL treatment disturbed the amorphous region of pulp fiber by increasing the crystallinity without any

effect properties and chemical structure of pulp fi- bers. Thereby, the Ul assisted IL treatment tech- nology has a promising application in pulping and papermaking industry for its high efficiency and environmental protection character.

Acknowledgement

The authors are grateful for the financial support from the National Key Research and Development Program of China (Grant no. 2017YFB0307900), the financial support from the National Natural Science Foundation of China (Grant no. 31770628), the Taishan Scholars Program, Foundation (No.

ZR201711) of Key Laboratory of Pulp and Paper Science and Technology of Ministry of Education/

Shandong Province of China，and a Project of Shandong Province Higher Educational Science and Technology Program (Grant no. J18KA111).

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