Mechanical and morphological characteristic investigations of deinked used newsprint paper via ultra-sonochemistry method

Printed newspaper is one of the print-based media published daily in large quantities. The focus of this research is to remove the ink from used newsprint with alkaline treatment by using ultrasound sonochemistry method. Newsprint sheets collected were characterized by mechanical and morphology tests using FT-IR, XRD, tensile test and SEM. FT-IR spectra analysis shows the absorption of C=C functional groups as the main components of newspapers at 1427 and 1635 cm wavelengths. The treatment with alkaline solution increased the mechanical strength properties of paper. SEM morphology analysis result shows that the surface of paper becomes more rough after ultrasound-alkaline treatment compared to paper without ultrasound treatment (conventional treatment). The crystallinity value decreased with alkaline treatment. The longer the ultrasound duration was, the lower the crystallinity degree became.


INTRODUCTION
Indonesia, as a developing country, has low used paper waste recycling activities. This type of waste mainly comes from used paper where the ink needs to be removed in order to be reused [1]. With the increase of paper production capacity by 5.3% per year [2], Indonesia has the potential to recycle paper waste to reduce the impact of deforestation while meeting the needs of paper [3]. Paper waste mostly comes from daily produced newspapers. Recycled newspapers, however, are not being used back as the base material for printing paper. Until today, they are converted into cardboard papers.
The main constraint of paper recycling is ink removal process. Several processes involve enzymes like laccase [4], cellulase [5] and xylanase [6] obtained from various sources. Although bio-deinking process is categorized as environmentally friendly, the cost required to prepare enzyme in an industrical scale is high due to the big amount of enzymes needed. Meanwhile, the utilization of chemicals, like magnesium hydroxide, is able to produce pulp with better paper brightness [7]. However, the long-term application in the industrial scale damages the environment.
The application of chemical to remove ink from old newspapers does not allow good quality recycling. Additional practices, such as mechanical methods, need to be done. These methods include vibrations and temperature to break physical interractions among lignin, cellulose and hemicellulose [8], [9]. The effective temperature to obtain paper pulp with high brightness is reported to be between 15 o C and 35 o C [10], whereas flotation method without involving vibrations and temperature allows the ink to be deposited [11]. The result of deinking from the combination of ultrasound vibration and alkaline in 5-30 minutes time showed slight improvement in the brightness [10]. This result shows that there is an optimum time for deinking process.
Based on the literature study conducted, investigations of time and deinking output need to be carried out by focusing on physical and *Corresponding Author: s.gea@usu.ac.id morphological characteristics. This study used old newspapers recycled by alkaline method and took longer time to understand the mechanical and morphology properties of the deinking result.

Preparation of paper pulp
The process to convert sample to pulp followed the previous study done by Singh (2012) [12]. In brief, recycled newspapers were cut into 2-3 cm 2 size and soaked in tap water at room temperature for 24 hours. Then, paper sample was washed for 2-3 times using tap water and crushed by using a mechanical stirrer. Crushed sample was filtered and dried in an oven for 50°C to remove water content. Dried sample was kept in a desiccator.

Ink Removal Process by Conventional Method
Ink removal process from recycled newspapers was done by washing in conventional method. 6 g of dried pulp was soaked in 60 mL aquadest. Then, 6 mL of NaOH 2%, Na2SiO3 2% and H2O2 2% mixture was mixed into the suspension by using a magnetic stirrer at 70°C for 2 hours. Then the pulp was cooled and washed until neutral pH was reached [13].

Ink removal process by alkaline-based ultrasound method
Ink removal process by ultra-sonochemistry method started by soaking about 6 g of dried pulp into distilled water. The suspension was treated ultrasonically by an ultrasound device (Elmasonik E 15 H) at 37 kHz frequency at 35 o C with time variations of 30, 60 and 90 minutes. Then, a mixture of 6 mL of NaOH 2%, Na2SiO3 2% and H2O2 2% was added into the suspension and stirred at 70°C for 2 hours. The treated pulp was cooled and washed until neutral pH was reached.
The same steps were repeated for deinking process with Mg(OH)2 substance. In brief, samples with ultrasound treatment at time variations of 30, 60, and 90 minutes were added with 6 mL mixture of Mg(OH)2 2%, Na2SiO3 2% and H2O2 2%. The mixtures were stirred at 70 o C for two hours. Then, they were cooled at room temperature and followed by washing process to reach neutral pH.
Afterwards, all of the samples from both NaOH and Mg(OH)2 treatments were crushed by using a mechanical stirrer. Next, the samples were placed in a paper mold size T150 and dried under the sun to form paper sheets.

Paper Sheet Production
Wet pulps from conventional and ultrasonication ink removal treatments were mashed by using a mechanical stirrer for 5 minutes. Fine pulps were molded and dried. Produced paper sheets were pressed by using an assembly equipment of hot-press with pressure of 50 kg/cm 2 at 105 o C temperature for 5 minutes.

Crystal Structure Analysis for Samples with NaOH Treatment
XRD analysis was done to analyze the crystal structure of paper sheet samples. The diffraction patterns of conventional and ultrasonic-NaOH pulp papers are shown in Figure 1 below. Slight differences were observed.

Figure 1. XRD diffractogram of conventional and ultrasound-NaOH papers
Pulp crystallinity values in this study are presented in Table 1. Table 1 shows that alkali solutions affect crystallinity values. This was observed from the variations of crystallinity values in samples with ultrasound-NaoH treatment. From Figure 1, wide diffraction pattern can be observed from conventional pulp paper due to the presence of cellulose, hemicellulose and lignin. Based on its crystallinity and amorphous patterns, crystallographic peak from cellulose was presented in the range of 21.90 o and 22.20 o , whereas lignin has no crystalline peak [14]. Crystallinity index (CrI) of conventional paper was 86.88% being higher than samples with NaOH-ultrasound treatment for 30, 60 and 90 minutes with values of 74.27%, 74.00% and 73.08% respectively.
Mechanical and morphological characteristic investigations of deinked used newsprint paper … (Saharman Gea, Sri Rahayu, Andriayani, Averroes F Piliang, Noni Oktari) ___________________________________________________________________________________________________   Table 2.  Table 2 shows that the crystallinity of conventional paper is higher at 86.88% than samples with ultrasound-Mg(OH)2 treatment at 72.41%, 71.92% and 71.49% crystallinity values. The decrease was caused by alkaline treatment and the duration of ultrasound. This happened because the alkali solution used in the cooking process had selectively attacked lignin (rather than cellulose) which led to degradation and dissolution. A study reported lignin isolation by using different NaOH concentration left cellulose as the residue from the process [14], [15]. Lignin degradation occurred during the treatment because of the damages in aryl ether chains which comprised 50-70% of total chains in lignin [7].
The data displays amorphous patterns based on the reflection of small angles and 2θ peaks between 25-30 o . There were no changes in the structural patterns of pulp paper. Liquid turbulence and circulation (microstreaming) were produced whenever ultrasonic waves propagated through liquid medium. This has significantly increased mass transfer rate and resulted in the improvement of delignification rate [16]. Figure 3. FTIR spectra of samples treated with NaOH Figure 3 shows FT-IR result of paper pulp, samples with conventional treatment and NaOH-ultrasonication for 30, 60 and 90 minutes. Peaks were observed in FT-IR charts at the transmittance spectra showing the particles interacted with infrared radiation at certain wavelengths. The curves show the bonds of elements in the tested sample. The analysis result can be seen in Table 3.  Mechanical and morphological characteristic investigations of deinked used newsprint paper … (Saharman Gea, Sri Rahayu, Andriayani, Averroes F Piliang, Noni Oktari) ___________________________________________________________________________________________________ main components of newspapers. The treatment with ultrasound-NaOH for 60 minutes resulted in higher intensity, 20.809 at wavenumber 2924 cm -1 , than the intensity in conventional paper, 11.417 at 2900 cm -1 .    11.417 at 2900 cm -1 . The intensity at 3356 cm -1 band was strengthened by ultrasonicationalkaline treatment. The ultrasound energy wave moved to break the molecular bonds, causing inked bubbles to be separated from the fibrous parts [17], [18]. This was correlated with the increase of cellulose content in treated newspapers. Hemicellulose bands were still present but lignin had disappeared. This showed that most lignin had been removed with ultrasound-alkaline treatment.

Samples Morphological Characteristics
Morphological analysis of the surface of papers can be seen in Figure 5. The surfaces of paper pulp and conventional paper were smoother. After the alkaline treatment with NaOH and Mg(OH)2 ultrasound, the surfaces of fibers became rough with the released of some fibers in delignification process. This was in accordance with FTIR results which showed lignin degradation at the surface after alkaline and ultrasound treatments. Other morphological changes, like perforation (small holes in easily torn parts) and cracking, occurred due to hydrolysis by the alkaline. The morphology changes have increased the surface areas of cellulose. When cellulose structure was opened and lignin degradation took place during the procedure, the surface area of cellulose available for alkaline hydrolysis increased [4]. SEM results showed that ultrasonic modified the surfaces of fibers which improved the strength of paper sheets produced. Stress-strain curve of conventional and ultrasound-NaOH papers Figure 6 shows that all 5 samples gave different curves. In this study, the mechanical properties of papers were tested using tensile strength for samples with ultrasound-NaOH treatment for 30, 60 and 90 minutes. The average thickness of paper produced was 1.0 mm with 80 mm length and 25 mm width. The tensile strength of each sample is shown in      The curves in Figure 7 show that the mechanical properties of paper pulp and samples with ultrasonication-Mg(OH)2 for 30, 60 and 90 minutes have slightly different tensile strength.

Mechanical Properties Analysis of Samples with NaOH Treatment
The highest Young's Modulus, 58.12736 MPa, was achieved by the sample with 90 minutes Mg(OH)2 ultrasound, while conventional method produced paper has Young's Modulus of 56,69368 MPa. Tensile strength of each paper sample can be seen in Table 6.
The data above can be concluded that NaOH solution was more effective than Mg(OH)2 in improving mechanic properties of paper. This is because alkali solutions were able to increase carboxyl and carbonyl functional groups content by splitting ester chains in the fibers. Hydrogen bonds with carboxyl and carbonyl groups had higher energy than hydrogen bonds with hydroxyl group only [19]. Hydrogen bonds presented in the paper fibers could increase mechanical properties or tensile strength of the paper produced.

CONCLUSION
The duration of ultrasonication and alkaline solutions used during deinking process of recycled newspapers affect the tensile strength.
The highest Young's Modulus, 58.12736 MPa, was obtained at ultrasound-NaOH treatment for 90 minutes, while conventional paper only achieved 56.69368 MPa. The morphological analysis of paper pulp from conventional method showed a smoother surface. Whereas the surfaces after NaOH and Mg(OH)2 ultrasounds showed rough fiber surfaces. The results aligned with FTIR spectra that showed lignin degradation at the surfaces after alkaline and ultrasonication treatments. There were decreases in crystallinity values in the samples with the addition of alkaline and ultrasonication treatment.