Effects of different deinking processes on fiber morphology, hydrogen bond models, and cellulose supramolecular structure

Abstract

Deinked pulp fibers produced by three kinds of deinking processes, alkaline deinking, neutral deinking, and enzymatic deinking, were studied by Scanning Electron Microscope (SEM), Fourier Transform Infrared Spectrometer (FTIR), X-ray Diffraction (XRD), and Cross-Polarization Magic Angle Spinning Carbon-13 Nuclear Magnetic Resonance (CP/MAS 13C-NMR). There were remarkable differences in both macroscopic and microscopic structure between the samples. SEM images showed that the effects of deinking processes on fiber morphology were obvious and the influences of alkaline deinking were the most apparent. FTIR analysis indicated that the content of intermolecular hydrogen bonds increased by 22.63%, 9.42%, and 14.40% after the alkaline deinking process, neutral deinking process, and enzymatic deinking process, respectively. XRD revealed that the average width of crystallite size in the (002) lattice plane was decreased after different deinking processes, in accordance with the change tendency of cellulose crystallinity. CP/MAS 13C NMR combined with spectral fitting demonstrated that the content of different cellulose polymorphs changed during deinking processes. The increase of WRV was attributed to changes in the hydrogen bonding patterns and cellulose supramolecular structure.

Full Article

Effects of Different Deinking Processes on Fiber Morphology, Hydrogen Bond Models, and Cellulose Supramolecular Structure

Qinglin Meng,^a,b Jinquan Wan,^b,c,* Yongwen Ma,^b,c and Yan Wang ^b,c

Deinked pulp fibers produced by three kinds of deinking processes, alkaline deinking, neutral deinking, and enzymatic deinking, were studied by Scanning Electron Microscope (SEM), Fourier Transform Infrared Spectrometer (FTIR), X-ray Diffraction (XRD), and Cross-Polarization Magic Angle Spinning Carbon-13 Nuclear Magnetic Resonance (CP/MAS ¹³C-NMR). There were remarkable differences in both macroscopic and microscopic structure between the samples. SEM images showed that the effects of deinking processes on fiber morphology were obvious and the influences of alkaline deinking were the most apparent. FTIR analysis indicated that the content of intermolecular hydrogen bonds increased by 22.63%, 9.42%, and 14.40% after the alkaline deinking process, neutral deinking process, and enzymatic deinking process, respectively. XRD revealed that the average width of crystallite size in the (002) lattice plane was decreased after different deinking processes, in accordance with the change tendency of cellulose crystallinity. CP/MAS ¹³C NMR combined with spectral fitting demonstrated that the content of different cellulose polymorphs changed during deinking processes. The increase of WRV was attributed to changes in the hydrogen bonding patterns and cellulose supramolecular structure.

Keywords: Deinking; Fiber morphology; Hydrogen bond; Supramolecular structure; WRV

Contact information: a: School of Light Chemical and Food Science Engineering, South China University of Technology, Guangzhou 510640, PR China; b: State Key Laboratory of Pulping and Papermaking Engineering, South China University of Technology, Guangzhou 510640, PR China; c: School of Environmental Science and Engineering, South China University of Technology, Guangzhou 510006, PR China; *Corresponding author:mwizard@126.com

INTRODUCTION

With the demand of protecting forest resources and developing a low-carbon economy, secondary fiber recycling has aroused the attention of technologists throughout the world, and the rapid growth of the waste paper utilization has become an important trend of the world’s paper industry. According to the statistics of the China Paper Association, the total consumption of pulp from wastepaper in the paper industry was 44.39 million tons in 2008 in China. In 2011 it increased to 56.60 million tons, accounting for 62% of total pulp consumption (China Paper Association). Recycled cellulose fiber accounted for about 70% of the papermaking materials by the end of the 20^th century (Wan and Ma 2004). This recycled material can help make up for an insufficient supply of fresh papermaking raw materials, can reduce energy consumption in comparison to the pulping of wood, and also can play an essential role in decreasing environmental pollution problems. Deinking is an important stage in secondary fiber recycling. Conventional deinking processes often employ sodium hydroxide in combination with a large amount of other chemicals, including sodium silicate, hydrogen peroxide, and surfactants (Shrinath et al. 1991). This high pH and chemical-intensive process leads to several major problems for the process and recycled paper quality, including a high degree of fragmentation of large sticky contaminants, darkening of pulp fiber, increased chemical oxygen demand (COD) in waste waters, and the need for substantial amounts of acid for neutralization (Lapierre et al. 2002). These disadvantages have hindered the further development of alkaline deinking processes. In this context, a new development of deinking under neutral or weakly acidic conditions (without sodium hydroxide, sodium silicate, and hydrogen peroxide, etc.) has shown promise for alleviating the negative impact of alkaline deinking (Dorris et al. 2003; Lapierre et al. 2006).

With recent advances in biotechnology, enzymes already have become widely used in deinking processes. Compared to the chemical-based deinking methods, enzymatic deinking could reduce the demand for chemicals, enhance ink and stickies removal, and also lower the process costs and environmental impacts (Bajpai and Bajpai 1998; Jeffries et al. 1994). Therefore, enzymatic deinking has been suggested as an environmentally friendly alternative to complement conventional chemical deinking in the recycling of recovered paper. Over the last couple of years, a large number of enzymes, including cellulase, hemicellulases, xylanase, pectinases, amylases, esterases, laccase, and lipase have been evaluated for their potential to replace hazardous chemicals in deinking recycled paper, and some achievements have been made.

During the deinking process, physical and chemical factors significantly influence the fiber structure. At present, most research on the subject has mainly focused on deinking efficiency and the changes of fiber properties, which usually have been indicated by the brightness and strength properties of the paper sheets (Mansfield et al. 1997; Ibarra et al. 2012). However, this change of plant fiber is mainly due to the irreversible structural changes in the fiber wall caused by the deinking method. Different deinking processes with various chemicals may influence the fiber ultrastructure differently. Hence, those studies could not expose the underlying reasons for the changes of paper-making properties, particularly the aspects of fiber structure in a fine level. By contrast, in our research, both the macroscopic and microcosmic structural changes were studied together, including fiber morphology, models to describe the patterns of hydrogen bonding, and cellulose supramolecular structure.

In recent years modern analysis methods have been widely used in pulp and paper areas of application. Scanning electron microscopy (SEM) is one of most popular surface analytical techniques. In the paper industry SEM is mainly used to study or characterize fibers and fillers, including their z-direction distribution in paper structure, by virtue of offering high quality images with high resolution (Chinga 2002; Eriksen et al. 2006; Bennis et al. 2010). So far, several approaches have been improved to carry out the SEM images. As reported, Claramunt et al. (2010) used SEM to investigate morphological changes after drying and rewetting cycles. The effect of laccase treatment on fiber surface was studied by SEM method(Wei et al. 2007). Beyond that, FTIR spectrometry, as a traditional analysis method in wood research, has been used for characterizing the cellulose structural features (Mohkami and Talaeipour 2011; Liu et al. 2005). In addition, FTIR absorption provides useful information related to the change of hydrogen bonding (Maréchal and Chanzy 2000). By using software in combination with a Gaussian distribution function, the FTIR spectra region of 3800 to 3000 cm^-1 was resolved into three or four bands to distinguish the relative contributions of different hydrogen bonds (Maréchal and Chanzy 2000; Oh et al. 2005; Popescu et al. 2009). Furthermore, XRD and CP/MAS ¹³C NMR were used to determine the supramolecular structure of cellulose, including the crystallinity, crystal size, and the content of polymorphs (Wan et al. 2010; Miyamoto et al. 2011; Hult et al. 2002). For example, Newman used solid-state carbon-13 NMR spectroscopy to characterize a bleached softwood kraft pulp in the never-dried state and after cycles of drying and remoistening (2004). Moreover, Rebuzzi and Evtuguin (2006) used XRD and solid-state carbon-13 NMR to determine the cellulose crystallinity of Eucalyptus globulus bleached pulps.

In this work, we used a combination of different analytical techniques, i.e. Scanning Electron Microscope (SEM), Fourier Transform Infrared Spectroscopy (FTIR), X-ray Diffraction (XRD), and Cross-Polarization Magic Angle Spinning Carbon-13 Nuclear Magnetic Resonance (CP/MAS ¹³C NMR) to investigate the changes of fiber morphology, hydrogen bond models, and cellulose supermolecular structure of eucalyptus kraft pulps treated with three kinds of deinking processes. It was possible, based on this work, to clarify the relationships among WRV, hydrogen bonds models, and cellulose supramolecular structure.

EXPERIMENTAL

Materials

Eucalyptus wood chips were cooked in autoclaves according to the conventional kraft process under the following conditions: 17% NaOH and 5% Na₂S, wood-to-liquor ratio of 1:4, temperature ramp-up time of 2 h, cooking temperature of 170 ºC, and time at cooking temperature of 2 h. The pulp was used for deinking treatments, though no ink was used in the study.

Preparation of the neutral deinking agent was carried out as follows: sequentially adding Sodium Alcohol Ether Sulphate (AES), Coconutt Diethanol Amide (6501), and Fatty Alcohol Polyoxy-ethylene Ether (AEO-9) into a three-necked flask (the mass ratios were 2:1:1), mixing when adding materials, in which time intervals for adding the material were more than 10 min. The contents were fully mixed material in a mechanical device at 65 ^oC for 50 min for synthesis of the neutral deinking agent.

The cellulase was mainly supplied by Leveking Bio-Engineering Co. Ltd., and the serial number is LPK-CD06.

Methods

Pulp treatment (Deinking processes)

Three kinds of deinking processes – an alkaline deinking process, a neutral deinking process, and an enzymatic deinking process – were used for deinking in this study. The recipes for different deinking methods were as follows: (1) alkaline deinking process using 2% NaOH, 3% dry Na₂Si₂O₃, 0.5% neutral deinking agent, and 2.0% H₂O₂ for 60 min at 40 ºC and 10% consistency (all based on o.d. paper, the same below), (2) neutral deinking process using 1.0% neutral deinking agent for 40 min at 50 ºC and 10% consistency, (3) enzymatic deinking process using 1.0% cellulase, then adding NaOH to pH 8 for 30 min at 45 ºC and 12% consistency.

Deinking chemicals, 10 g o.d. pulp, and hot water were added into the sealing bag to achieve the required slurry consistency and reaction temperature. Then the sealing bags were put into an electric-heated thermostatic water bath. After reaction, flotation was carried out in ZQS-10 Flotation Cell. The flotation conditions were 0.2% CaCl₂ on pulp, 1% consistency, and certain air pressure and air flow. The time allowed for flotation was 15 min, 20 min, and 40 min, respectively (Wei 2009). Finally, all the pulp samples were exhaustively washed with distilled water through a 200-mesh wire screen. The pulp samples were then transferred into plastic bags for storage.

Morphology

The samples were coated with gold film in order to observe the surface morphology and the microstructure. The instrument was a LEO 1530 VP Scanning Electron Microscope (SEM).

Fourier transform infrared spectrophotometer (FTIR)

Freeze-dried pulp samples (3.5 to 4.0 mg) and KBr (350 mg) were homogenized using an agate mortar and thereafter pressed into a transparent tablet at 200 kgf/cm² for 5 min. Spectra were recorded using a Bruker Vector 33 Fourier Transform Infrared Spectrophotometer (FTIR) set at a resolution of 4 cm^-1 over the range 4000 to 400 cm^-1.

The crystallinity index was calculated from the relative intensities of the infrared bands, finding the ratios of 1372/2900 cm^-1,

N.O’KI=  ×100% (1)

where I₁₃₇₂ represents the intensity (1372 cm^-1) of the band belonging to the CH bending vibration and I₂₉₀₀ is the intensity (2900 cm^-1) of the band belonging to the CH and CH₂bending vibrations.

Determination of cellulose crystallinity by X-ray diffraction (XRD)

The X-ray diffraction (XRD) scattering pattern of the pulp was obtained using a Philipps X’Pert MPD diffractometer with a Cu-Kα source (l = 0.154 nm) in the 2θ range of 4 to 60° and a scanning step width of 0.02°/scan. The crystalline reflections and amorphous halo of samples were defined according to previously described recommendations (Wan et al. 2010 and Liao et al. 2011). The cellulose crystallinity (%) was calculated by means of the following equation (Kim and Hotzapple 2006),

 ×100 % (2)

where I₀₀₂ and I_am are the maximum scattering intensities of the diffraction from the (002) plane at 2θ=22.6° and the diffraction intensity of the background scatter measured at 2θ=18°, respectively.

The average width of crystallites obtained from (hkl) diffraction was determined from the following formula (Bhuiyan and Hirai 2005),

 (3)

where K is the Scherrer constant (0.9),  is the wave length of the X-ray source (0.154 nm), β is the full-width at half-maximum of the reflection hkl measured, and 2θ is the corresponding Bragg angle.

Determination for CP/MAS ¹³C NMR

During sample preparation the pulps for CP/MAS ¹³C-NMR analysis were subjected to a mild chlorite delignification with NaClO₂ (1.5 g/g sample) under acidic conditions at room temperature followed by treatment with 0.1 M NaOH overnight. Between the NaClO₂ and NaOH stages, the samples were rinsed with deionized water to pH 4 to 5. The overall procedure was repeated twice. Afterwards, the samples were hydrolyzed for 8 h in 2.5 M HCl at 100 °C (Hult et al. 2001). Finally, the pulp samples were washed with deionized water to a pH of 4 to 5 prior to drying freely.

All spectra of samples (water content 40 to 60% by weight) were recorded on a Bruker AVANCE AV 400 instrument (at ambient temperature) operating at 9.4 T for ¹³C-NMR. The pulp samples were packed in a zirconium oxide rotor. The MAS rate was 6 kHz. Acquisition was performed with a CP pulse sequence using a 3.3 ms proton 90° pulse, 1500 ms contact pulse, and a 2.0 s delay between repetitions. Glycine was used for the Hartman–Hahn matching procedure and as external standard for the calibration of the chemical shift scale relative to tetramethylsilane (CH₃)₄Si.

Determination of WRV

The water retention value (WRV) is an important property closely associated with cellulose crystallinity and swelling capability (Forsström et al. 2005; Gumuskaya et al. 2003). The WRV value was determined by the centrifugal method (Wistara and Yong 1999) on pulp or paper with a dry weight equivalent of 1.5 g. Centrifugation was stopped after 15 min at 3000 rpm. After centrifugation, the fiber mat was weighed in a pre-weighted weighing bottle and dried in an oven at 105 ± 2 °C for 24 h and then re-weighed. WRV was calculated with the following equation,

 ×100% (4)

where m₁ is the weight of wet pulp after centrifugation and m₂ is the weight of the dry pulp (in grams).

RESULTS AND DISCUSSION

Morphological Characterization

Scanning Electron Microscopy (SEM) is widely used to observe the surface characteristics of fibers after different physical and chemical treatments. In the present study, SEM images of the native and deinked pulp fibers were taken to investigate the morphological changes during different deinking processes. Figure 1 presents the surfaces of eucalyptus pulp fibers treated with three different deinking methods. As shown in Fig. 1A, the fibers of eucalyptus native pulp were thick, the fiber surface was smooth and regular, and the fiber structure was dense. Compared with the fibers of native pulp, the fibers of deinked pulp were damaged differently. After the alkaline deinking process, the fibers appeared to have been damaged, as viewed from the surface (Fig. 1B). The fibers became much thinner, the fiber surface was slightly fibrillated, fines appeared between fibers, and the fiber structure was loosened. The fibers of neutral deinked pulp are shown in Fig. 1C. Compared with the fibers of alkaline deinked pulp, the fibers of neutral deinked pulp were damaged slightly, and the amount of fines was also less than in the case of the alkaline deinked pulp. The fiber surface and structure were similar to the fibers of native pulp. After enzyme treatment, there were more pores on the fibers surface and the interspaces among the fibers increased (Fig. 1D). Those phenomena are mainly attributed to the cellulose hydrolysis on the surface of the fibers, a procedure known as “peeling-off fibers”, which facilitates ink detachment from the surface and leads to a removal of small fibrils.

A

B