Cellulose nanocrystals obtained from office waste

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Oct 18, 2017 - Anhydrous ethanol (AR) was purchased from Fuyu Fine. Chemical Co. ..... cule was between 20 and 140 μm (Hu et al., 2008), much larger than.
Carbohydrate Polymers 181 (2018) 376–385

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Carbohydrate Polymers journal homepage: www.elsevier.com/locate/carbpol

Cellulose nanocrystals obtained from office waste paper and their potential application in PET packing materials

T



Wanqing Leia, Changqing Fanga,b, , Xing Zhoua,b, Qian Yinb, Shaofei Panb, Rong Yangb, ⁎⁎ Donghong Liuc, , Yun Ouyangd a

School of Mechanical and Precision Instrument Engineering, Xi’an University of Technology, Xi’an 710048, PR China Faculty of Printing, Packing Engineering and Digital Media Technology, Xi’an University of Technology, Xi’an 710048, PR China c Fuli Institute of Food Science, Zhejiang University, Hang Zhou 310058, PR China d Beijing Key Laboratory of Packaging and Printing New Technology & Key Laboratory of Printing Environmental Protection Technology, China Academy of Printing Technology, Beijing 100000, PR China b

A R T I C L E I N F O

A B S T R A C T

Keywords: Office waste paper Cellulose nanocrystals Transparent cellulose film Orientated nanofibers Coating materials

Annually a tremendous amount of office waste paper (OWP) is discarded creating environmental pollution. Therefore, how to make this paper from waste to wealth and use it in new approaches have become a meaningful and challenging work. In this work, OWP being a cellulose rich biomass was employed for the production of cellulose nanocrystals (CNCs) by acid hydrolysis with different acid concentrations but without subjecting OWP to alkali and bleaching treatments. The testing results showed that CNCs prepared using sulfuric acid concentration of 59% with respect to OWP had the highest crystallinity and this concentration was the transition concentration for the production of opaque CNCs film with convoluted nanofibers to transparent one with orientated nanofibers. Besides, CNCs prepared using acid concentration of 65% coated on PET sheet not only had a better water vapor barrier property but also was on a par with the transparency of PET, which was hopeful to be used as coating materials in packaging materials.

1. Introduction In recent years, solid wastes generated by different industrial, mining, domestic and agricultural activities have raised serious environment issues (Joshi, Naithani, Varshney, Bisht, & Rana, 2017). Among all solid wastes, huge amount of waste paper which comes from office plays an important role in damaging the ecology of the earth. It is usually underutilized and finally achieves to burning or slow biodegradation (Su et al., 2017). As reported, recycling of one ton of waste paper can save 17 trees and 7000 gallons of water (Joshi et al., 2015), which has great environmental and economical benefits. Therefore, how to make this paper from waste to wealth and use it in new approaches, i.e. recycling of office waste paper (OWP), have become a meaningful and challenging work. Since the recycled paper is an integral part of paper and pulp production, the paper to paper recycling has been carried out. Despite recycling efforts, the quality of paper produced by the waste paper is far poorer than that made from virgin pulps due to a shortening of the fiber length and reduction in tensile strength. Meanwhile, Ikeda, Park, & Okuda (2006) has reported that the maximum ratio of paper to paper recycling is to be 65% and this causes ⁎

the production of large quantities of by products which ultimately have to be disposed. With higher cost of producing paper from recycled pulp, and disposal of waste fibres unfit for use, finding alternative options to recycle wastepaper is a necessity (Danial et al., 2015). Recently a number of attempts have been taken in new approaches to utilizing waste paper including pyrolysis (Méndez, Fidalgo, Guerrero, & Gascó, 2009) eco-composite manufacturing (Das, 2017), bioethanol synthesis (Ribeiro, De Melo Órfão, & Pereira, 2017; Byadgi, & Kalburgi, 2016), cyanoethyl and carboxymethyl cellulose synthesis (Joshi et al., 2017; Joshi et al., 2015) and biocompatible cellulose aerogels (Feng, Nguyen, Fan, & Duong, 2015). In particular, waste paper being a cellulose rich biomass provides a potential source for the production of various green and useful specialty end products such as cellulose nanocrystals (CNCs). CNCs are a promising new material with unique properties, including nanoscale dimension, high specific strength and modulus, high surface area, high crystallinity and unique optical properties, etc. (Csiszar, Kalic, Kobol, & Eduardo de Paulo, 2016). They are produced from several starting materials such as microcrystalline cellulose (Bondeson, Mathew, & Oksman, 2006), wood (Chen et al., 2011), cotton fibers (Li, Li, Zou, Zhou, & Lian, 2014), tunicate (Anglès, & Dufresne,

Corresponding author at: Faculty of Printing, Packing Engineering and Digital Media Technology, Xi’an University of Technology, Xi’an 710048, PR China. Corresponding author: Fuli Institute of Food Science, Zhejiang University, Hang Zhou 310058, PR China. E-mail addresses: [email protected] (C. Fang), [email protected] (D. Liu).

⁎⁎

http://dx.doi.org/10.1016/j.carbpol.2017.10.059 Received 20 July 2017; Received in revised form 19 September 2017; Accepted 16 October 2017 Available online 18 October 2017 0144-8617/ © 2017 Elsevier Ltd. All rights reserved.

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2.2. Deinking and defibering of OWP

2000), bacteria (Roman, & Winter, 2004), ramie (Peresin, Habibi, Zoppe, Pawlak, & Rojas, 2010), and sisal (De Rodriguez, Thielemans, & Dufresne, 2006) by removal of the amorphous phase with acid hydrolysis usually followed by an ultrasonic treatment in order to disintegrate the aggregates of liberated crystalline cellulose particles. In spite of the abundant availability of raw materials, the development of processes using waste or residual biomass as CNCs source is of practical interest. In recent years, a small number of people has focused on the extraction of CNC from waste paper and published some achievements in rare articles. Danial et al. (2015), Tang et al. (2015), and Maiti et al. (2013) extracted CNCs from old newspaper, old corrugated container and waste tissue papers, respectively. The production of CNCs from waste paper would provide an alternative to waste paper recycling and possibly address the issue of by products arising from paper to paper recycling. To the best of our knowledge, the use of OWP as the raw material for the extraction of CNCs has rarely been reported. Only Ander Orue, Santamaria-Echart, Eceiza, Peña-Rodriguez, & Arbelaiz (2017) extracted CNCs from OWP by using an alkali solution and a subsequent acid hydrolysis process. The effect of different concentrations of alkali solution on the properties of CNCs extracted from OWP was studied. CNCs have also been successfully extracted from OWP in this work, however, OWP was not subjected to alkali and bleaching treatments before the acid hydrolysis, which made the extraction process simple. Conventional physical chemical characterizations including attenuated total reflectance-fourier transform infrared spectroscopy (ATR-FTIR), xray diffraction (XRD), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and morphological investigations have been carried out to study the formation, crystallinity, thermal properties and morphology of CNCs. Furthermore, in order to find out the transition concentration for the production of opaque CNCs film to transparent one, the acid-catalyzed hydrolysis was conducted with different acid concentrations in low gradient. Besides, poly(ethylene terephthalate) (PET) is a very popular packaging material because of its high transparence to exhibit products, relatively good water, oxygen and carbon dioxide barrier properties to protect products, and lighter weight to reduce the environmental impact during transport (Fang, Lei, Zhou, Yu, & Cheng, 2015; Dombre, Rigou, Wirth, & Chalier, 2015), therefore, we have made attempt to associate CNCs film with PET, i.e. CNCs coated on PET sheets, in order to apply CNCs derived from OWP in packaging material. Water vapor permeability and transparency testing showed that CNCs as coating material on the surface of PET could improve water vapor barrier properties and transparency of packaging material PET. In a word, the study of this paper is significant in waste paper management, new approach towards the production of CNCs and even application of CNCs in packaging material as coating material.

OWP (30 g) was collected from our laboratory and shreded into pieces of 15 mm × 4 mm by paper shredder. Then deinking agents including 1.5 wt% NaOH, 3 wt% H2O2, 5 wt%NaSiO3, 1.5 wt% SDBS and 1.5 wt% OP-10 were dissolved in the tap water of 1000 ml and stirred by the glass rod. The prepared deinking agents and paper pieces were added into the laboratory repulper and soaked for about half an hour. Afterwards, the repulper started to defibrize at the speed of 10000 r/min for approximately 10–15 min. In the cooperation of chemical agents and mechanical forces ink fell off the waste paper and defibered paper was obtained. Subsequently, defibered paper was washed for several times through a fine-mesh for the purpose of removing the ink and impurities from the OWP pulp. Finally, OWP pulp was dried in the oven at 80 °C for 24 h and smashed with the high-speed universal pulverizer for 10–30 s to obtained fine deinked OWP pulp fibres (DP), since the mechanical treatment resulted in disintegration of the cellulose structure into microcrystalline cellulose particles. 2.3. Preparation of CNCs The most commonly used method for the preparation of CNCs is acid hydrolysis of cellulosic materials using sulfuric acid ca 64% (w/w) (Filson, Dawson-Andoh, & Schwegler-Berry, 2009). To study the influence of acid concentration on properties of CNCs acid concentrations of 55%, 60%, 63% and 65% were used to extract CNCs, which were named as HP55, HP60, HP63 and HP65. However, some dried cellulose films were observed to be dark and transparent. In order to find out the transition concentration for the production of opaque CNCs film to transparent one, the acid-catalyzed hydrolysis was further carried out with acid concentrations of 57%, 58%, 59%, and 59.5% (HP57, HP58, HP59 and HP59.5). In this paper, CNCs were prepared by acid-catalyzed hydrolysis of OWP pulp with a similar method used by some researchers (Danial et al., 2015). Briefly, DP of 2 g were mixed with 100 ml H2SO4 (with different concentrations) and stirred under a constant agitation of 270 r/min at 45 °C for 1 h. The hydrolysis was quenched by adding 10-fold excess cold water (1000 ml) to the reaction mixture. The suspension liquid was standing for 1 h and then washed for several times with water until the solution was neutral. 2.4. Preparation of different CNCs samples 2.4.1. Preparation of CNCs suspensions The hydrolyzed cellulose reached a constant pH by being washed for several times, and then air pump filtration was adopted to remove water. Afterwards, hydrolyzed cellulose was dispersed in deionized water of 600 ml and ultrasound dispersed 2 h to obtain CNCs suspensions.

2. Experimental section 2.4.2. Preparation of the dried CNCs The hydrolyzed cellulose reached a constant pH by being washed for several times, and then air pump filtration was carried out to remove the water. Afterwards, hydrolyzed cellulose was dispersed in deionized water of 60 ml and ultrasound dispersed 1 h. Finally, these suspensions were transferred into the Petri dish and dried in the oven at 80 °C for 2–3 h.

2.1. Materials OWP was collected from our laboratory. Hydrogen peroxide (H2O2) (AR), surfactant sodium dodecyl benzene sulfonate (SDBS) (AR), sodium hydroxide (NaOH) (AR) were supplied by Tianli Chemical, Tianjin, China. Sodium silicate (NaSiO3) (AR) was purchased from Baishi Chemical Co., LTD, Tianjin, China. OP-10 (CP) was purchased from Yongsheng Fine Chemical Co., LTD, Tianjin, China. Sulfuric acid (H2SO4) (GR) was supplied by Sichuan Xilong Chemical Reagent Co., Ltd. China. Anhydrous ethanol (AR) was purchased from Fuyu Fine Chemical Co., LTD, Tianjin, China. Tap water and deionized water were used in the process of cleaning PET sheet and cellulose derived from OWP. All chemicals were used as received without further treatment.

2.4.3. Preparation of CNCs-coated PET sheets (HP/PET) For the purpose of testing water vapor permeability, water contact angle and transparency, acid hydrolyzed cellulose CNCs-coated PET sheets were prepared as follows. Firstly, PET sheets were washed with anhydrous ethanol to clean up dirt and then were further cleaned by water. Afterwards, PET sheets were dried at room temperature and cut into PET wafers with the diameter of 80 mm and PET strips with the size of 50 mm × 10 mm, respectively. Subsequently, CNCs suspensions 377

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1 kV. Before scanning all the dried cellulose were prepared with gold plating. Transmission electron microscopy (TEM) was performed to investigate the shape and length of CNCs using a JEM-3010 microscope with a Gatan894 CCD camera operating at an accelerating voltage of 300 kV. CNCs suspensions were ultrasound dispersed evenly before TEM.

of 7.5 ml were coated on PET wafers (for water vapor permeability measurement) and of 1 ml were coated on PET strips (for water contact angle and transparency measurements). Finally, these CNCs-coated PET sheets were dried in the oven at 80 °C for 1–2 h. The dried CNCs-coated PET sheets were placed in the desiccator for the further measurements. 2.5. Characterization 2.5.1. Attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) DP and acid hydrolyzed cellulose CNCs prepared from OWP using different acid concentrations were analyzed with a Bruker Vertex7.0 v FTIR spectrometer. The dimand crystal was used as infrared transmission crystal. FTIR was performed to confirm the presence of cellulose, and FTIR spectra of all samples were recorded in the wave band ranging from 400 to 4000 cm−1 using ATR technique.

2.5.6. Water contact angle measurement The water contact angle (WCA) was carried out on an OCA 20 (Dataphysics, Germany) to evaluate the hydrophilic performance of CNCs-coated PET sheets. During testing, water drops of 2 μl were controlled by the syringe system of the tester and dispensed drop by drop onto the surface of the samples. The contact angle values were calculated using the contact angle meter software on basis of the droplet shape in the image. For each sample, measurements were repeated at several different positions, and its contact angle was finally determined by averaging these contact angle values from various measurements.

2.5.2. X-ray diffraction (XRD) An x-ray diffractometer instrument (XRD-7000, SHIMADZU LIMITED, Japan) was used to analyze the degree of crystallinity and crystallite size of CNCs prepared from OWP using different acid concentrations. A scanning of 2θ angles between 10 and 40° below the scan speed of 8.0000 deg min−1 was carried out.

2.5.7. Measurements of water vapor permeability and transparency Water vapor permeability was tested by PERMETM W3/060 (Jinan Languang, China). The water vapor transmission rates (WVTR) (g m−2 day−1) for PET wafer and CNCs-coated PET wafers were measured at a constant atmosphere of 38 °C and 90% relative humidity. Before testing samples were cut into circles with the diameter of 74 mm and the actual tested area was 33 cm2. The thickness of PET wafer and CNCs-coated PET wafers was measured for five times and the mean value of the thickness was imported to the tester to calculate the water vapor permeability. The PET wafer and CNCs-coated PET wafers were sealed on the cup containing distilled water of 10 ml. The gas drying unit of PERMETM W3/060 containing molecular sieve was used for monitoring the relative humidity at 38 °C. After 24 h the tester gave a constant value and the water vapor transmission rates were obtained. Transparency was carried out by ultraviolet and visible spectrophotometer (Hitachi U-3900, Japan).

2.5.3. Thermogravimetric analysis (TGA) TGA was performed under nitrogen atmosphere with NETZSCH TG209F3. DP and acid hydrolyzed cellulose CNCs prepared from OWP using different acid concentrations weighing between 4 and 10 mg were placed in an alumina ceramic crucible and heated from 30 to 700 °C with the nitrogen flow of 30 ml min−1 and heating rates of 10 °C min−1. During the heating period, the weight fraction and temperature difference were recorded as a function of temperature. 2.5.4. Differential scanning calorimetry (DSC) DSC analysis was performed under nitrogen atmosphere with NETZSCH DSC200F3 equipment to acquire thermograms of DP and acid hydrolyzed cellulose CNCs prepared from OWP using different acid concentrations. The heating rate was 10 °C min−1 and the sample weight was approximately 5–10 mg. DSC was performed twice to ensure a consistent thermal history with a temperature range from room temperature to 400 °C on the second run.

3. Results and discussion 3.1. ATR-FTIR analysis The FTIR spectra were carried out to characterize the chemical structure by identifying the functional groups present in each sample. The FTIR spectra of DP and CNCs prepared using different acid concentrations were shown in Fig. 1. The spectra of DP and CNCs exhibited a broad band in the region from 3500 to 3200 cm−1 standing for the

2.5.5. Morphological investigation Field emission scanning electron microscope (FESEM) was performed using a SU-8010 microscope working at accelerating voltage of

Fig. 1. ATR-FTIR spectra of DP and CNCs prepared using different acid concentrations derived from OWP (a) the whole region from 400 to 4000 cm−1; (b) the extended main region for CeH, CeOeC, and CeC groups from 800 to 1200 cm−1.

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Fig. 2. XRD graphs of (a) DP and CNCs prepared from OWP using lower acid concentration of 55%, 57% and 58%; (b) CNCs prepared from OWP using relative higher acid concentration of 59%, 59.5%, 60%, 63% and 65%.

(Segal, Creely, Martin, & Conrad, 1959)

Table 1 Crystallinity index and apparent crystallite size of DP and CNCs prepared using different acid concentrations. Samples

2θ (amorphous) (∘)

2θ (crystalline) (∘)

Crystalline index (CI) (%)

Apparent crystallite size (nm)

DP HP55 HP57 HP58 HP59 HP59.5 HP60 HP63 HP65

15.5 15.5 15.4 15.7 15.9 15.9 15.3 15.3 15.3

22.5 22.5 22.7 22.9 22.5 22.6 22.6 22.6 22.5

45.5 48.7 54.1 61.2 74.1 60.1 65.7 56.9 54.5

4.4 3.8 5.1 5.2 4.6 4.5 4.7 4.4 4.1

CI (%) = (I002 − Iam ) Iam × 100

(1)

where I002 is the intensity of the 002 crystalline peak at around 22° and Iam is the intensity for diffraction of non-crystalline material which is taken at 2θ angle of about 15°. The crystallite size was estimated from XRD patterns using Scherrer equation (Normand, Moriana, & Ek, 2014)

Dhkl = Kλ β1 cos θ 2

(2)

where Dhkl is the crystallite dimension in the direction normal to the (hkl) lattice planes, K is the Scherrer constant usually taken to be 0.9, λ is the radiation wavelength (λ=0.154 nm for Cu Kα), θ is the diffraction angle and β1/2 is the peak width at the half-maximum intensity in radians. The crystallite size was determined using the diffraction pattern obtained from 002 lattice planes. Changes in CI and crystallite size perpendicular to 002 planes were summarized in Table 1. CI of CNCs samples was higher (maximum 74.1%) than that of DP (45.5%). Acid hydrolysis of cellulose caused bond cleavage, i.e., hydrolytic cleavage of glycosidic bonds between two anhydroglucose units. This action was counteracted by rearrangement of the tangling chain ends, which was favoured by release of internal strain (Das et al., 2009). Thus the amorphous portion got dissolved by the acid hydrolysis, leaving behind the crystalline regions, and led to the higher crystallinity of the resultant CNCs samples. In contrast, CI of CNCs prepared in this work was lower than that reported in previous studies (Chen et al., 2011; Tang et al., 2014). Since, except for the dissolution of amorphous phase caused by mechanical and chemical treatment, the chain length of cellulose molecules could also be reduced by mechanical and chemical treatment and the degree of crystallinity of the resultant product would decrease (Filson and Dawson-Andoh, 2009). With the gradually increasing acid concentration used for hydrolysis the CI increased proving that high sulfuric acid concentration could improve the degree of crystallinity and at an appropriate concentration the CI would reach the optimum such as 59%, however, the CI did not increase unboundedly with the further increasing concentration. Instead, the CI decreased when the acid concentration was higher than 59%. A possible reason was that the excessive high acid concentration not only disintegrated the amorphous phase but also caused dissolution of crystalline phase. Compared with the crystallite size in DP (4.4 nm) a slight increase was observed in case of CNCs (maximum 5.2 nm) except for HP55, HP63 and HP65. Das, Ray, Bandyopadhyay, and Sengupta (2010). and Tang et al. (1996) have reported the similar increase in crystallite size on acid

free OeH stretching vibration of the OH groups in cellulose molecules. The spectra of all samples showed the characteristic CeH stretching vibration around 2900 cm−1 (Garside & Wyeth, 2003). Besides, two other peaks observed in the spectra of all samples around 1055 cm−1 and 900 cm−1 were associated with the CeOeC pyranose ring (antisymmetric in phase ring) stretching vibration and CeH rock vibration of cellulose (anomeric vibration, specific for β-glucosides). Furthermore, the CeC ring breathing band about 1160 cm−1 and the CeOeC glycosidic ether band around 1105 cm−1 arose from the polysaccharide component. The FTIR spectra of all samples were similar in all wave numbers, which indicated that the main cellulose structure has been maintained after the acid hydrolysis. 3.2. XRD analysis The degree of crystallinity and crystallite size of DP and CNCs prepared using different acid concentrations derived from OWP were established by XRD. Major peaks appearing around 15° and 22° in Fig. 2 for all samples were characteristics of the 101 and 002 lattice planes, respectively, in cellulose type I. The highest diffraction peak ca. 2θ = 22° corresponded to the crystalline structure of cellulose I and was used for the calculation of the crystallinity index (CI), whilst the low diffraction peak ca. 2θ=15° represented the amorphous background. The small and broad peak at about 34° represented the contribution of the 040 plane (Tang et al., 2015). This result suggested that all hydrolysis reactions at different acid concentrations had a limited effect on the polymorphism of cellulose I for the CNCs sample produced. The degree of crystallinity or CI for all samples was determined using the peak height method and calculated by the following formula 379

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temperature, the finally unchanged residual weight of the majority of CNCs samples was found to be more than DP. The higher char residue was probably due to the presence of a larger amount of intrinsically flame resistant crystalline cellulose I. Besides, it could also be ascribed to the intrinsically flame resistant sulfated amorphous introduced by the hydrolysis of sulfuric acid. The decomposition temperature peaks for all samples could be seen in the derivative weight loss curve DTG (Fig. 3b). These temperatures were designated as Tmax and summarized in Table 2. All the recycled paper cellulose exhibited multiple step of thermal degradation except for DP, which only exhibited a major degradation peak of cellulose at 353 °C. However, the degradation of CNCs appeared to follow a different mechanism and this was manifested by the presence of two or more degradation peaks in DTG curve. HP55, HP57 and HP58 showing an additional large hump or shoulder around 270 °C (quite apparent in the DTG curve) might correspond to the degradation of more accessible and therefore more highly sulfated amorphous regions, whereas broader degradation range showing a peak around 355 °C was related to the breakdown of unsulfated crystal interior. HP59, HP59.5 and HP60 had a similar variation tendency of DTG curves and three degradation peaks occurred. Compared with DP, it was observed that the major degradation peak shifted to lower temperature around 180 °C due to the evaporation of loosely bound moisture on the surface of samples (Haafiz, Eichhorn, Hassan, & Jawaid, 2013). In the case of CNCs the increased portion of moisture may be assumed to be due to the higher extent of solvation or ionic association around the sulfated cellulose molecules by the water molecules. The sulfated amorphous region in case of HP might also provide for enhanced interchain spaces where moisture can get entrapped. Besides, a weak degradation peak around 270 °C appearing in DTG of HP59, HP59.5 and HP60 might correspond to the drastic reduction in molecular weight of hydrolysis which made it more susceptible to degrade when temperature increased. It also might be due to the degradation of more accessible and therefore more highly sulfated amorphous regions (present only in case of acid hydrolyzed specimen) in particular. The third degradation peak, i.e. the depolymerization of cellulose, was very weak, which might be because that hydrolysis of cellulose not only dissolved the amorphous regions, but also some crystalline regions. With the continuously increasing acid concentration such as HP63 and HP65, there were more degradation peaks appearing, which were listed in Table 2. It proved that more kinds of products were generated because of the higher acid concentration. Meanwhile, the degradation peak of cellulose in the range of 360–400 °C was higher than that of the DP which might be due to the higher crystallinity of CNCs. If the thermal stability was evaluated by the char residue, then it could be said that CNCs prepared using acid concentrations of 57%, 59% and 60% showed higher thermal stability than HP58 and HP63, highest being in case of HP59.5 followed by HP65. The lower residue of HP58 and HP63 at 700 °C, which even continued to break down after 700 °C, could be largely related to the increased split hydrogen bonds (Meyabadi, Dadashian, Sadeghi, & Asl, 2014). In brief, CNCs showed lower thermal stability than deinked paper cellulose. Among all CNCs samples, HP57 and HP65 had a better thermal stability by considering every factor affecting the thermal stability such as the onset of major thermal degradation temperature and char residue.

Fig. 3. (a) TG-curves; (b) DTG-curves of DP and CNCs prepared using different acid concentrations derived from OWP.

hydrolysis and inferred that celluloses with loose structure favoured the changes in the size of the crystallites during acid hydrolysis. However, with the further increasing acid concentration from 63% to 65%, the crystallite size gradually decreased. This was ascribed to the probability of stronger hydrolysis at higher concentration, which removed both the amorphous domains and even part of crystalline regions resulting in the lower crystallite size (Samir, Alloin, Paillet, & Dufresne, 2004). 3.3. TG analysis The inherent characteristics of samples and the molecular interactions between the different macromolecules determine the thermal stability of a polymeric material. The chain cleavage or the bond dissociation of the macromolecules occurs when the supplied thermal energy exceeds the bond dissociation energy of the respective chemical bonds. The recycled paper cellulose was subjected to TGA. The loss in weight of DP and CNCs derived from OWP with the rise in temperature and their rates of degradations were shown in Fig. 3. The thermal degradation data of all samples were tabulated in Table 2, using the temperature at 5% weight loss as the onset degradation temperature T(onset) and the onset temperature at the final unchanged weight as T(residues). As seen in Table 2 the onset of major thermal degradation temperature and the temperature at 10% weight loss T10 of CNCs were lower than that of DP. The obtained CNCs showed lower thermal stability. This was probably due to the introduction of sulfate groups into the cellulose crystals through hydrolysis by sulfuric acid. The sulfate groups introduced to the outer surfaces of cellulose during acid hydrolysis caused dehydration of cellulose fiber to reduce the thermal stability (Reddy & Rhim, 2014). In spite of the losses in weight at lower

3.4. DSC analysis Fig. 4 showed DSC diagrams of DP and CNCs prepared using different concentrations derived from OWP. Obviously, DP, HP55, HP57, HP58 had a similar tendency of DSC curve while the others had another similar DSC structure. All samples exhibited a distinct endotherm peak within the range of temperature studied and the nature of endotherms, however, was quite characteristic of the composition of the material and differed from each other. The endothermic peak was an indication of the fusion or melting of the crystalline packed cellulose. Moreover, 380

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Table 2 Thermal stability of DP and CNCs obtained from acid hydrolysis at different concentrations derived from OWP. Samples

a

DP HP55 HP57 HP58 HP59 HP59.5 HP60 HP63 HP65

291 125 229 231 132 177 127 147 208

a b c d

T(onset) (°C)

b

c

318 219 254 256 185 182 177 227 233

– – – – 189 184 182 225 210

T10 (°C)

Tmax (°C)

d

Char residue (%)

388 632 637 696 548 586 600 697 695

19.7% 28.5% 30.2% 14.4% 34.7% 39.2% 34.9% 13.5% 37.3%

T(residues) (°C)

– 253 273 275 275 272 270 269 230

– – – – – – – 277 271

353 357 356 355 370 376 379 362 400

onset degradation temperature. temperature at 10% weight loss. DTG peak temperature. the onset temperature at the final unchanged weight loss (residue content of PUE).

conclusion, it was easy to found a significant acid concentration 59%, since it was the demarcation point for the tendency of DSC curve. 3.5. Morphological analysis The morphology of DP and CNCs prepared using different acid concentrations derived from OWP was evaluated with SEM. The samples used in SEM were from the section 2.4.2. All samples presented a fibrous morphology. Fig. 5(a) DP displayed long and flat cellulose fibers with size distribution in the micron range 3–40 μm but after acid hydrolysis of the cellulose fibers, one can recognized cellulose nanofibrils with distribution of diameter in nanometer range 10–25 nm (Fig. 5(c)(j)). Generally, to produce CNCs, cellulose has to be treated with sulfuric acid. This acid hydrolysis not only helps to further disintegrate the cellulose fibers but also facilitates defibrillation of the fibers on a nanoscale level (Kargarzadeh et al., 2012). However, HP55 also showed cracked and short microcellulose fibres except for long and convoluted nanocellulose fibres (Fig. 5(b)) suggesting the incomplete acid hydrolysis reaction. The lowest degree of crystallinity for HP55 (48.7%) among all acid hydrolyzed cellulose in XRD also proved the incomplete acid hydrolysis reaction, since the amorphous part of cellulose underwent preferential acidic hydrolysis when compared to crystalline domains (Kargarzadeh et al., 2012). Fig. 5 (a)-(h) showed a great amount of long nanocellulose fibrils indicating lower breakdown of the cellulose chains. Meanwhile, the TEM (Fig. 5(k), (l)) exhibited the length of CNCs. Although diluted suspensions of all samples were ultrasound dispersed evenly before TEM, the higher degree of aggregation of the nanocrystals still prevented the length of CNCs from being evaluated for the majority of samples. The well dispersed cellulose of HP57 showed that the length was in the range from 220 to 770 nm and HP59 had a relative shorter length from 56 to 140 nm. In Fig. 5 (i), (j) some nano spherical particles and lamellar structure were observed in HP65 and especially for HP63, except for rod-like or needle-shaped cellulosic nanoparticles. It was inferred that higher acid concentration might lead to dehydration and carbonization of the cellulose. XRD showed that with the increase in acid concentrations from 63 to 65%, crystallite size gradually decreased, which proved that the stronger hydrolysis at higher concentration not only removed the amorphous domains more effectively but also destroyed the crystalline domains. As seen the diameter of the CNCs fibers did not show any significant evolution as the acid concentration increased from 55% to 65%, whereas the CNCs fibers tended to rearrange according to a certain orientation, especially CNCs prepared using acid concentration higher than 59%. By contrast, CNCs fibers of HP55, HP57 and HP58 had convoluted structures without any oriented rearrangement. This might be associated with the changed appearance of the dried CNCs samples, i.e. CNCs prepared with acid concentration higher than 59% could from the transparent dried film. Differences of the appearance might be attributed to the degree of hydrolysis. Higher acid concentration resulted in higher degree of

Fig. 4. DSC diagrams of DP and CNCs prepared using different acid concentrations derived from OWP.

there was an earlier onset of fusion in CNCs, compared to DP. This might be because that the accessible hydroxyl groups as present in CNCs were now sulfated and relatively bulky sulfate groups led to the expected increase in interlayer spacing, therefore, a further rearrangement in crystal compactness took place. Meanwhile, similar sulfation also happened in the amorphous regions. This change in orientation and the simultaneous breakdown in molecular weight or degree of polymerization resulted in an earlier onset of fusion in CNCs. Besides, it could be easily seen that the onset of cellulose fusion peak gradually moved to lower temperature from DP, HP55, HP57, HP58, HP59.5 to HP60 (i.e. from 357 °C, 267 °C, 252 °C, 233 °C, 192 °C, 189 °C to 181 °C). It proved that the breakdown in molecular weight become more serious and more non-cellulosic constituents were produced with the increasing acid concentration. Obviously, the width of the fusion endotherm was narrower in CNCs with respect to DP, which might be attributed to the sulfation effect influencing the increasing proportion of amorphous region. Importantly, an exothermic peak appeared in the range of 180–230 °C in DSC diagrams in CNCs samples prepared using acid with higher concentrations, i.e. higher than 59%. This peak might be the transition process for cellulose from amorphous regions to crystal nanocelluloses, i.e., amorphous structure coexisted with the crystal in this temperature range. This peak could be named as crystallization temperature of cellulose, i.e. the rearrangement in crystal compactness. Although the onset of cellulose fusion peak gradually shifted to lower temperature, the area of the endothermic peak became larger for samples without the exothermic peak, i.e. from DP, HP55, HP57 to HP58, demonstrating the higher thermal enthalpy (△H), which was consistent with the gradually increasing crystallinity in XRD. As 381

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Fig. 5. SEM images of (a) DP; (b) acid hydrolyzed microcellulose at 55% acid concentration; (c) acid hydrolyzed nanocellulose at 55% acid concentration; (d)-(j) CNCs prepared using acid concentrations of 57%, 58%, 59%, 59.5%, 60%, 63% and 65%; TEM images of (k) CNCs prepared using acid concentration of 57%; (l) CNCs prepared using acid concentration of 59%.

fibres. If the cellulose particles were not in nanoscale, the transparency was lower and the haze was higher due to the increased light scattering (Hietala, Mathew, & Oksman, 2013), which agreed well with the special microstructure of HP55 (SEM image in Fig. 5(b), (c)), i.e. HP55 not only had a nanoscale fibrous morphology, but also had cracked and short microcellulose fibres. Besides, acid hydrolyzed cellulose coated on PET strips seemed to be less transparent and homogeneous than PET strip, especially for CNCs prepared using lower acid concentration, which conformed with the values of transparency. Fig. 6 confirmed the visual perception and revealed a great increase in clarity and decrease in haze with the increasing acid concentration, especially for HP65/PET, which could be on a par with the transparency of pure PET strip. Associated the SEM images with transparency, it was easily found that CNCs tended to rearrange according to a certain orientation and the transparency of CNC film has been improved with the increasing acid concentrations for hydrolysis. This might be because that when CNCs tended to rearrange according to a certain orientation, the convoluted

hydrolysis and more CNCs were produced. In the final drying process, water evaporation led to the gradual reduction in the dispersion liquid and the final lost of it. The distance between the hydrogen bonding in CNCs was shorter and the intensity of the hydrogen bonding became stronger, which caused CNCs to tend to rearrange according to a certain orientation. 3.6. Transparency Fig. 6 showed the transparency of PET strip and acid hydrolysed cellulose using different acid concentrations coated on PET strips. It was obvious that all acid hydrolyzed cellulose coated on PET strips (HP/ PET) were flat, smooth, and colourless except for HP55/PET. HP55/ PET was cloudy and many small white aggregates of CNCs were perceptible in it, which was in accordance with the negative value of transparency in Fig. 6. It was well known that transparency was widely used as an indirect measure of size and dispersion of cellulose nano

Fig. 6. Transparency of PET strip and CNCs-coated PET strips.

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Fig. 7. (A) Water contact angle of (a) PET strip; (b)-(i) CNCs prepared using different acid concentrations including 55%, 57%, 58%, 59%, 59.5%, 60%, 63% and 65% coated on PET strips; (B) Histogram of water contact angles of all samples.

(i), (j)). Interestingly, CNCs-coated PET with acid concentration of 55% used for hydrolysis (HP55/PET) had a dynamic water contact angle varying from 100.5°, 96.2°, 92.4° to the finally constant value 78.9° while other samples had a constant water contact angle. It could be said that HP55/PET changed from hydrophobicity to hydrophilicity, which might be associated with the special microstructure of HP55. HP55 not only had long and convoluted nanocellulose fibres, but also showed cracked and short microcellulose fibres (seen in SEM in Fig. 5 (i), (j)). Hence, the interspaces among these nanocellulose and microcellulose fibres were diverse and capillary effect in HP55 was different from other samples resulting in the dynamic water contact angle of HP55/ PET.

structures would reduce and the light would pass through easily, and the transparency could be improved. 3.7. Water contact angle To investigate the hydrophilic performance of CNCs prepared using different acid concentrations derived from OWP. CNCs-coated PET strips were prepared and water contact angle experiment was performed. As shown in Fig. 7, PET had the largest contact angle of 82.1° while CNC-coated PET had the contact angles below 78.9° proving that the hydrophilic coating successfully covered the whole PET strips. The water contact angles of CNCs-coated PET strips were below 90°, suggesting that CNCs films were hydrophilic (Zhou et al., 2016). This result might have arisen from the existence of hydrophilic hydroxyl groups in CNCs derived from the acid hydrolysis of OWP cellulose. The hydrophilicity of CNCs-coated PET strips strongly increased from HP55/PET, HP57/PET, HP58/PET, HP59/PET, HP59.5/PET to HP60/PET as the contact angle notably decreased (Fig. 7), indicating that there were greater hydrophilic hydroxyl groups on the film surfaces. Since with the increasing acid concentration to hydrolyze DP more anhydroglucose was produced and more hydrophilic hydroxyl groups appeared leading to the increase in the hydrophilicity. In addition, for the hydrophilic cellulose the direction of capillary pressure pointed to PET strip accelerating the diffusion of water into cellulose and reducing the water contact angle. However, with the further increasing acid concentration, such as 63% and 65%, the water contact angle of HP63/PET and HP65/PET increased. This might be caused by the dehydration and carbonization of the cellulose due to the higher acid concentration resulting in lower amount of hydrophilic groups. It could be proved by the present nano spherical particles or lamellar structure in HP63 and HP65 observed in SEM images (Fig. 5

3.8. Measurements of water vapor permeability Water vapor transmission rate (WVTR) (g m−2 day−1) for PET wafer and CNCs-coated PET wafers was determined gravimetrically. As shown in Fig. 8 (b) CNCs-coated PET wafers had smaller WVTR compared with PET wafers. According to section 3.7 it was known that the water contact angle of PET was below 90°. Thus, when PET wafers were placed above the water vapor, the water vapor was absorbed on PET wafers due to the larger intermolecular forces between water molecules and PET, and molecular diffusion happened from water molecule to PET wafer. Subsequently, the water molecule was desorbed on the other side of PET wafer and the water vapor permeability ended. However, there were still further processes for CNCs-coated PET wafers. As seen in Fig. 8(a) that part of water vapor became condensed water when encountering PET and was absorbed on the cellulose film attempting to penetrate it, however, an average diameter of condensed water molecule was between 20 and 140 μm (Hu et al., 2008), much larger than crystallite sizes of cellulose in nanoscales, so that it was difficult for the 383

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larger interspaces between micro cellulose fibres might make water vapor pass through the cellulose film easily compared with the compact nano structures. 4. Conclusions In this work, CNCs have been successfully extracted from OWP without subjecting OWP to alkali and bleaching treatments before the acid hydrolysis with different concentrations, which made the extraction process simple. The results showed that the acid concentrations greatly influenced properties of CNCs. 59% was found to be a significant concentration, since CNCs prepared using it had the highest crystallinity (CI59 = 74.1%) and could form a transparent cellulose film consisting of oriented nanofibres. Besides, 59% was also the transition concentration for the production of opaque CNCs film with convoluted nanofibers to transparent one with orientated nanofibers. Morphological analysis showed that hydrolyzed cellulose using acid concentration of 55% not only had a nanoscale fibrous morphology with the size distribution of 10–25 nm, but also had cracked and short microcellulose fibres. Interestingly, because of the special microstructure of HP55, HP55/PET had a dynamic water contact angle varying from 100.5°, 96.2°, 92.4° to the finally lowest constant value 78.9°, i.e. HP55/PET changed from hydrophobicity to hydrophilicity. Finally, HP65/PET was confirmed to have not only a better water vapor barrier property than PET but also be on a par with the transparency of PET, which was hopeful to be used as coating materials to improve the shelflife of soft drinks or other food packed with PET. Acknowledgements This work was supported by the National Natural Science Foundation of China [grant number 51372200]; Program for New Century Excellent Talents in University of Ministry of Education of China [grant number NCET-12-1045]; Special Program for local serving from Education Department of Shaanxi Provincial Government [grant number 2013JC19]; Program for Innovation Team in Xi’an University of Technology [grant number 108-25605T401]; Ph.D. Innovation Fund Projects of Xi’an University of Technology [grant number 310252071501]; Scientific Research Plan Projects of Shaanxi Education Department [grant number 16JK1551], China Postdoctoral Science Foundation Funded Project [grant number 2016M592824] and Key Program for China Academy of Printing Technology.

Fig. 8. (a) The process of water vapor penetrating CNCs-coated PET wafers; (b) Water vapor transmission rate (WVTR) of PET wafer and CNCs-coated PET wafers.

condensed water molecule to diffuse in the cellulose film and the water vapor permeability become bad. Meanwhile, the remaining uncondensed water vapor could still pass through the cellulose film due to the smaller average diameter 0.0004 μm (Hu et al., 2008) and gave a certain poor water vapor permeability of CNCs-coated PET wafers. There was not a strict trend between WVTR and CI, i.e., WVTR did not increase or decrease consecutively with the increasing CI. It could be roughly given a variation tendency of WVTR with the varied CI. Among all samples CNCs prepared using acid concentrations of 57% (CI: 54.1%), 63% (CI: 56.9%) and 65% (CI: 54.5%) coated on PET wafers (HP57/PET, HP63/PET, HP65/PET) had much lower water vapor permeability. It was allowed the acid hydrolyzed cellulose CNCs to be used as coating materials to improve water vapor barrier properties of PET, and consequently the shelflife of soft drinks or other food packed with PET was improved. Besides, CNCs prepared using acid concentrations of 58% (CI: 61.2%), 59% (CI: 74.1%), 59.5% (CI: 60.1%) and 60% (CI: 65.7%) coated on PET wafers (HP58/PET, HP59/PET, HP59.5/PET, HP60/PET) had higher value of WVTR. It seemed that CNCs with relative high CI coated on PET wafers had the relative high water vapor permeability. It might be because more amorphous regions were hydrolyzed and more hydrophilic groups appeared, therefore, the hydrogen bonding between water molecules and hydrophilic groups made more water vapor penetrate the cellulose film. Interestingly, HP55 had lower CI but the water vapor permeability of HP55/PET was higher which might be attributed to the special microstructure. HP55 had both micro cellulose and nano cellulose fibres seen in SEM image and the

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