- Nano Express
- Open Access
The Effect of Mechanochemical Treatment of the Cellulose on Characteristics of Nanocellulose Films
© The Author(s). 2016
- Received: 24 November 2015
- Accepted: 12 September 2016
- Published: 20 September 2016
The development of the nanomaterials with the advanced functional characteristics is a challenging task because of the growing demand in the market of the optoelectronic devices, biodegradable plastics, and materials for energy saving and energy storage. Nanocellulose is comprised of the nanosized cellulose particles, properties of which depend on characteristics of plant raw materials as well as methods of nanocellulose preparation. In this study, the effect of the mechanochemical treatment of bleached softwood sulfate pulp on the optical and mechanical properties of nanocellulose films was assessed. It was established that the method of the subsequent grinding, acid hydrolysis and ultrasound treatment of cellulose generated films with the significant transparency in the visible spectral range (up to 78 % at 600 nm), high Young’s modulus (up to 8.8 GPa), and tensile strength (up to 88 MPa) with increased ordering of the packing of the cellulose macromolecules. Morphological characterization was done using the dynamic light scattering (DLS) analyzer and transmission electron microscopy (TEM). The nanocellulose particles had an average diameter of 15–30 nm and a high aspect ratio in the range 120–150. The crystallinity was increased with successive treatments as shown by the X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) analysis. The thermal degradation behavior of cellulose samples was explored by thermal gravimetric analysis (TGA).
- Mechanochemical treatment
- Nanocellulose film
- Young’s modulus
- Tensile strength
The development of the nanomaterials with improved functional characteristics is a challenging task at present because of the growing demand in the fields of optoelectronics, materials for energy saving and storage [1–5]. Nanocellulose is a group of nanomaterials that consists of the nanosized cellulose particles. Characteristics of nanocellulose particles depend on properties of plant raw materials and methods used in the production [6, 7]. Nanocellulose that is produced from the renewable lignocellulose materials has improved mechanical properties, such as high surface area-to-volume ratio and high aspect ratio [8–10]. Nanocellulose often replaces well-known material such as glass and certain polymers, which are not biodegradable at ambient conditions, in order to create new specific nanocomposites, adsorbents, and functional materials for the electrodes in the chemical sources of power and optoelectronic devices [11–16], biodegradable plastics and paper with special characteristics [17–19].
Cellulose is one of the most abundant biopolymers in our planet with the annual production of up to 1011 tons . During the process of photosynthesis, cellulose macromolecules form nano- and microfibril structures stabilized by the hydrogen bonds . Nanocrystalline cellulose, which consists of rod-shaped crystals with 2–20 nm cross-section diameter of and length from 100 nm to several micrometers , is prepared by removing amorphous regions from the cellulose. Nanocrystalline cellulose can form stable aqueous suspensions with chiral nematic properties and, as cholesteric liquid crystals, retains optical properties in films even after solvent evaporation . Nanocrystalline cellulose is characterized by the strength five times higher than that of steel and a coefficient of thermal expansion of less than that of quartz . The network formation of nanosized particles in nanocrystalline cellulose films determines its optical transparency. In this network, the diameter of these particles is much smaller than the light wavelength, which significantly decreases light scattering as compared to the regular fibers .
Nanocellulose is prepared by mechanical, chemical, and enzymatic methods. Mechanical methods employ various forces to reduce the size of the natural cellulose fibers to nanoscale. This approach includes multiple passages of the cellulose fibers through a high-pressure homogenizer and leads to the significant energy consumption (above 25 kW/kg) . Spence et al.  have shown that the homogenization process is the most expensive method for the nanomaterial isolation. Chemical methods are based on the cleavage of 1–4 glycosidic bonds of the cellulose chains and an isolation of the cellulose nanocrystals eliminating the amorphous cellulose part . Enzymatic methods generate nanocellulose through the biosynthesis from monosaccharides or fermentation of the cellulose fibers. The enzymatic methods are time-consuming and require expensive reagents. In some instances, the initial enzymatic treatment of the cellulose prior to the mechanical grinding can decrease the energy consumption required for the preparation of nanocellulose . For these reasons, a pre-treatment of the fibrous material is usually performed in order to decrease the size of the cellulose fibers and to ease the fibrillation and the process of nanocellulose preparation.
In the present study, the effects of the treatment of the bleached softwood sulfate pulp to nanocellulose followed by mechanochemical treatment on the optical (transparency in the visible spectrum range) and mechanical properties (Young’s modulus, tensile strength) of the nanocellulose films were investigated. The grinding process using the standard for pulp industry equipment was used as a pre-treatment of the cellulose to decrease the size of the fibers and to reduce acid consumption in the process of the hydrolysis. Additionally, in comparison to the high-pressure homogenization processes, it does not have the blocking problems generated by larger fibers . We used the bleached softwood sulfate pulp as a starting material since it is highly produced by the paper industry.
Mechanical treatment of the bleached softwood sulfate pulp (Arkhangelsk CPF, Russia) was performed using laboratory grinding complex LRK-1 (UkrSRIP, Ukraine) with the setting of diamond garniture from 0.1 to 0.4 mm for reaching 93 Schopper-Riegler (SR) degrees freeness. Measurements of the beating rate were carried out by SR-2 device (CSRIP, Russia). The pulp contained approximately 92.3 % cellulose, 5.7 % hemicelluloses, 0.23 % lignin, and 0.21 % ash. The chemical composition of the pulp was determined by the methods described by TAPPI standards .
Hydrolysis of the grinded cellulose was carried out by sulfuric acid solutions with different concentration (from 18 to 64 %) at the liquid-to-solid ratio of 44:1 during 5–60 min. The calculated amount of sulfuric acid with the corresponding concentration was slowly added into the flask with the cellulose suspension, and the required volume of the acid with concentration above 50 % was added drop-wise. The temperature of the reaction was maintained in the range from 20 ± 1 to 60 ± 3 °C. Upon expiration of the reaction time, the hydrolysis was stopped by tenfold dilution with distilled water and cooling of the suspension to the room temperature.
The hydrolyzed cellulose was washed three times by the centrifugation (8000 rev/min) and subsequent dialysis until reaching neutral pH. Ultrasound treatment was performed using ultrasound disintegrator UZDN-A (SELMI, Ukraine) for 5–60 min. The cellulose dispersion was placed in an ice bath to prevent overheating during treatment. Eventually, the suspension had taken the form of a homogenous gel-like dispersion.
The prepared dispersions were poured into Petri dishes and dried at room temperature in air to obtain cellulose films. Their density was determined according to the ISO 534:1988. The degree of polymerization (DP) was determined by the viscosity of the samples dissolved in copper ethylene-diamine solution according to ISO 5351.
The determination of particle diameter’s distribution for cellulose dispersions was performed by dynamic light scattering (DLS) using analyzer Zetasizer Nano (Malvern Instruments, UK). Transmission electron microscopy (TEM) images were obtained using electron microscope TEM125K (SELMI, Ukraine) operating at a potential of 100 kV. A delute suspension (0.1 wt.%) was dropped onto a thin scaffoldings Lacey Formvar/Carbon, 400 mesh, copper approx. grid hole size 42 μm (TED PELLA, Inc, USA). Electron absorption spectra of the nanocellulose films in UV, visible and near infrared regions were registered on two-beam spectrophotometer 4802 (UNICO, USA) with resolution of 1 nm. Fourier transform infrared spectroscopy (FTIR) spectra were measured using spectrophotometer IFS66 (Bruker, USA) with resolution of 2 cm−1. X-ray diffraction patterns of the different cellulose samples were obtained by Ultima IV diffractometer (Rigaku, Japan). The method proposed in  was used to determine the crystallinity degree (CD) of the samples, in terms of which CD = (I 200 − I am)/I 200 × 100 %, where I 200 is an intensity of (200) reflex about 23°, I am intensity of amorphous scattering at 18.5°.
Tensile properties of the nanocellulose films were measured at controlled temperature (23 ± 1 °C) and humidity (50 ± 2 %) according to ISO 527-1. Tension tests were performed at a crosshead speed of 0.5 mm/min on the TIRAtest-2151 (Germany) instrument equipment with a 2N load stress. For testing, test strips with 10 ± 2-mm wide and 25 ± 5-mm long were used. The data reported are tensile strength and Young’s modulus. Each composition was tested with a minimum of five specimens to extract an average and standard deviation for each property.
The thermal degradation behavior of cellulose samples was explored by heating using Netzsch STA-409 thermoanalyzer. The samples were heated at a rate of 5 °C/min, from 25 to 450 °C.
The increase in the transparency of the cellulose films suggests that the mechanochemical treatment followed by the sonication procedure results in the decrease of the hydrolyzed cellulose particle size through more dispersivity of nanocellulose. The decrease of the cellulose particle size and the increase of its dispersivity were assessed by measuring the changes in the degree of polymerization (DP). Thus, DP of the initial pulp was 1037; DP pulp grinded in laboratory grinding complex up to 93 °SR was 635; DP after hydrolysis with 43 % sulfuric acid was 305; DP after additional ultrasound treatment was 110; and DP after hydrolysis with 64 % sulfuric acid was 53. The decrease in the DP confirmed the degradation of the cellulose during the mechanochemical treatment.
The effect of sulfuric acid concentration, temperature, duration of hydrolysis, and ultrasound treatment on density and mechanical properties of the nanocellulose films
Concentration H2SO4, %
Duration of hydrolysis, min
Duration of the ultrasound treatment, min
Tensile strength, MPa
Young’s modulus, GPa
1.01 ± 0.04
26.1 ± 1.4
2.1 ± 0.18
1.03 ± 0.02
28.0 ± 2.2
2.5 ± 0.11
1.14 ± 0.05
31.9 ± 1.7
3.1 ± 0.12
1.27 ± 0.03
38.7 ± 1.6
3.7 ± 0.14
1.13 ± 0.03
61.2 ± 3.9
6.3 ± 0.33
1.16 ± 0.06
63.1 ± 3.6
5.9 ± 0.29
1.23 ± 0.04
63.5 ± 2.4
5.7 ± 0.14
1.25 ± 0.03
69.0 ± 5.0
5.5 ± 0.31
1.26 ± 0.02
72.8 ± 4.3
5.7 ± 0.19
1.56 ± 0.05
88.0 ± 7.4
8.8 ± 0.16
1.43 ± 0.04
77.2 ± 6.3
7.3 ± 0.25
1.51 ± 0.04
84.1 ± 5.3
7.8 ± 0.37
1.45 ± 0.02
78.3 ± 4.6
7.1 ± 0.48
1.41 ± 0.03
74.3 ± 5.8
6.7 ± 0.35
1.34 ± 0.04
60.3 ± 4.8
5.5 ± 0.36
1.31 ± 0.05
64.5 ± 4.2
5.2 ± 0.45
1.33 ± 0.03
66.7 ± 4.1
5.1 ± 0.33
1.38 ± 0.04
73.8 ± 3.4
6.2 ± 0.41
1.11 ± 0.03
50.0 ± 4.4
4.4 ± 0.38
1.12 ± 0.03
51.3 ± 5.3
4.6 ± 0.32
1.42 ± 0.06
56.7 ± 5.6
4.5 ± 0.29
1.36 ± 0.03
67.4 ± 3.6
6.8 ± 0.35
1.45 ± 0.04
83.8 ± 6.4
7.3 ± 0.43
Further increase of the acid concentration (above 50 %) used in the hydrolysis leads to a sharp decrease of all strength properties. The hydrolysis with low concentration of sulfuric acid affected predominantly amorphous regions of the cellulose, compared to crystalline ones, and led to an increase of the mechanical properties of the films. The hydrolysis with high concentrations of sulfuric acid (50–64 %) affected both amorphous and crystalline regions of the cellulose and led to the formation of films with the brownish color mentioned above.
Next, the cellulose samples were analyzed by FTIR. Previously published data  suggest that the degree of crystallinity of cellulose changes symbatically to the ratio between the intensity of the bands at 1430 and 900 cm−1 (Fig. 3). In our dataset, the ratio between the intensity of the bands at 1430 and 900 cm−1 for the initial cellulose, the hydrolyzed cellulose, and the sonicated cellulose was 1.74, 1.82, and 1.86, respectively. These data correlate well with the data from X-ray diffraction described above.
In the present study, the effect of the mechanical treatment, hydrolysis, and sonication of the bleached softwood sulfate pulp on the optical and mechanical properties of the transparent nanocellulose films was assessed. It was established that the method of subsequent grinding, acid hydrolysis, and ultrasound treatment of the cellulose produces films, which are characterized by the transparency in the visible spectral range (up to 78 % at 600 nm), high Young’s modulus (up to 8.8 GPa), and tensile strength (up to 88 MPa) due to the increase in the order of the packing of the cellulose macromolecules.
The authors declare that they have no competing interests.
VB designed the study, interpreted the experimental data, and drafted the manuscript. OY has performed the mechanical treatment and hydrolysis of the bleached softwood sulfate pulp, prepared the nanocellulose films and analyzed its mechanical properties. SA has determined the transparency of nanocellulose films and carried out ultrasound studies. AK has investigated the TEM images and FTIR spectra. OP designed the study, interpreted the experimental data, and drafted the manuscript. VK has made substantial contributions to the design of the study and data analysis. All authors read and approved the final manuscript.
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