Preparation of hemp nanocellulose and its use to improve the properties of paper for food packaging

doi:10.21203/rs.3.rs-1249903/v1

The study describes the preparation of nanocellulose from organosolv hemp pulp (OHP) and its applications in composition of paper for food packaging as an alternative to packaging from petroleum products. OHP was obtained from renewable plant materials hemp fibers by extraction with NaOH solution and cooking using a mixture of acetic acid and hydrogen peroxide. A stable transparent nanocellulose gel was extracted from OHP by acid hydrolysis followed by ultrasonic treatment. It was found that an increase in the consumption of sulfuric acid, temperature and duration of the OHP hydrolysis process improves the quality indicators of hemp nanocellulose. A linear dependence of the tensile strength and transparency of nanocellulose films on their density has been established. Morphological (SEM), structural (FTIR and XRD) and thermal analysis (TGA) of hemp fibers, OHP and nanocellulose were carried out. Nanocellulose films had a density of up to 1.56 g/cm³, a tensile strength of up to 66.7 MPa, a transparency of up to 87.3%. AFM method have shown that the transverse dimension of nanocellulose particles is from 8 nm to 23 nm. Hemp nanocellulose has a higher crystallinity index (87.2%) than OHP (72.0%), but has lower thermal stability. The positive effect of adding nanocellulose from hemp on improving all quality parameters of paper for food packaging has been shown. The addition of 2% hemp nanocellulose makes it possible to obtain paper that exceeds the breaking force requirements of premium grade paper by 40%, and the breaking length increases by 42% compared to paper without chemical additives.

hemp fiber

organosolv pulp

hydrolysis

nanocellulose

paper

One of the urgent problems of humanity is the protection of the environment from plastics and polymers obtained from exhaustible natural resources. Today's problems require solutions using environmentally friendly products from renewable sources, in particular from plant materials, as an alternative to products from oil, gas and coal. Such alternative products include nanocellulose, a new class of nanomaterials with unique properties. Nanocellulose is a biodegradable material with high mechanical strength, stronger than steel, high transparency and chemical resistance, light weight and low coefficient of thermal expansion, large specific surface area and low cost (Jasmani et al. 2018; Dufresne 2019). Therefore, nanocellulose is widely used in industry in the production of biopolymers and biocomposites, packaging and flexible electronics (Reshmy et al. 2020; Zhanga et al. 2020), pharmaceutical science and medical applications (Alavi 2019; Blessy et al. 2020), as well as hygiene and sanitation products (Trache et al. 2020). It is used to increase the mechanical strength and improve the barrier properties of paper and cardboard (Charani et al, 2019; Ehman et al, 2020), polymer and cement composites (Thomas et al. 2018; Baghban et al. 2020), electric batteries and sorbents (Kang et al. 2020; Espíndola et al. 2021).

Several methods are used for extraction of nanocellulose from cellulose, the most abundant biopolymer on Earth. These methods include mechanical, chemical and biological processing, the technological parameters of which depend on the type of the initial cellulose-containing material, the preliminary stages of its processing and determine the properties of nanocellulose. Mechanical methods for the production of nanocellulose use various types of mechanical processing: homogenization, grinding, microfluidization, ultrasonic processing, ball mill and cryogenesis, and they are characterized by significant energy consumption (Yang Y et al. 2019; Kumar et al. 2020). Preliminary enzymatic or chemical treatment of cellulose reduces energy costs for the production of nanocellulose (Phanthong et al. 2018). The enzymatic treatment is usually time-consuming and expensive (Sharma et al. 2019). Therefore, a typical method for extracting nanocellulose from cellulose-containing materials is the use of acid hydrolysis. Mineral acids such as sulfuric, hydrochloric, nitric, hydrobromic and phosphoric acids, or mixtures thereof, are used in the hydrolysis process (Rhim et al. 2014). To obtain nanocellulose, other chemicals are used: organic acids (Li et al. 2015; Biana et al. 2018), deep autectic solvents (Yang X et al. 2019), oxidizing agents: phthalimide-N-oxyl (PINO) radical (Coseri 2009) and TEMPO (Madivoli et al. 2020). But sulfuric acid is most often used for the hydrolysis of cellulose due to the relatively low reaction temperature (up to 60°C) and short reaction time (up to 60 min) and the formation of a gel-like product in water (Habibi 2014). The essence of using sulfuric acid to obtain nanocellulose is to break 1–4 glycosidic bonds in cellulose macromolecules and remove amorphous regions of cellulose (Rhim et al. 2014).

To obtain nanocellulose, bleached wood pulp and microcrystalline cellulose from cotton are more often used as starting materials, which have a negligible residual content of lignin, hemicelluloses and extractives. In the global pulp industry, the dominant production technologies are sulfate and sulfite methods, which pollute the air and water bodies with harmful toxic sulfur compounds (Smook 2003). The use of several classes of organic compounds – alcohols and carboxylic acids, ethers and esters, ketones and amines, makes it possible to replace sulfur-containing cooking liquors and reduce harmful emissions into the atmosphere. For example, acetic acid, due to its relatively low cost, can be considered a potential agent for achieving extensive delignification (Kumar et al. 2013). The use of hydrogen peroxide in cooking with acetic acid improves the delignification of plant materials and increases the whiteness of the pulp. A mixture of acetic acid and hydrogen peroxide forms peracetic acid, which, as a strong oxidizing agent, is characterized by excellent delignification and bleaching properties (Choi et al. 2019). This mixture selectively dissolves lignin and minimally damages cellulose (Karbalaei Esmaeil et al. 2019). The peracetic acid cooking process is applicable to both wood and non-wood plant materials and is carried out at a low temperature, which contributes to low energy consumption (Deykun et al. 2018). Countries with limited forest resources are encouraged to use non-wood plant raw materials, which will help conserve timber stocks and improve the environment.

An alternative to wood for pulp production is drug-free industrial hemp, an annual bast fiber plant of the Cannabis family that is grown for fiber and seeds. The demand for hemp fiber pulp in Europe and the United States already stands at over 6 million tons per year (Erickson 2019). Hemp straw contains about 30% fibers, from which long-fiber cellulose is produced with a yield of up to 80% (Keller 2013). Hemp is used to make a variety of commercial and industrial products, including furniture, automotive, paper, construction, rope, textiles, clothing, shoes, food, bioplastics, insulation, biofuel (Novakiva 2017). Hemp fiber pulp can be used as a raw material for the production of special high-quality white paper (banknote paper, securities, ultra-thin paper, highly porous cigarette paper) and chemical processing (Paulapuro 2000), in particular for the production of hemp nanocellulose (NC).

The aim of the study was to obtain pulp from hemp fibers by the peracetic solution, to investigate the effect of acid hydrolysis parameters on the properties of nanocellulose, and to test the effect of nanocellulose on the quality indicators of paper for food packaging. The relevance of paper production for food packaging is increasing with the need to replace synthetic polymer packaging, as well as the use of biodegradable additives instead of chemical additives from exhaustible petroleum products.

2.1 Materials and chemicals

We used the hemp fibers from the Khmelnitsky region of Ukraine after the harvest in 2020. Before research, the raw material was cut into small sizes 3–5 mm and stored in a desiccator to maintain a constant moisture content and chemical composition. The chemical composition of hemp fibers was determined according to TAPPI standards (TAPPI 2004). Chemicals used for the extraction of pulp and nanocellulose: sodium hydroxide, glacial acetic acid, hydrogen peroxide and sulfuric acid, were of analytical grade, provided by LLC Khimreaktiv (Ukraine). Distilled water was used throughout the experiments.

2.2 Obtaining of hemp pulp

The production of pulp from hemp fibers was carried out according to the method described in previous publications. (Barbash et al. 2020, 2021). Briefly, the chopped hemp fibers were placed in a conical flask, where an alkali solution was added to remove most of the hemicelluloses and minerals and partially remove lignin from the plant material. For this, hemp fibers were extracted with a NaOH solution at a consumption of 5% relative to the weight of absolutely dry material at a liquid to solid ratio of 10:1. The mixture was boiled under reflux on a hot plate with a water bath for 180 minutes. After the reaction time had elapsed, the hemp pulp was filtered in a Buchner funnel and washed with hot distilled water. At the second stage, to remove residual lignin and extractives, organosolv cooking was carried out using a solution of glacial acetic acid and 35% hydrogen peroxide in a volume ratio of 70:30% with a liquid to solid ratio of 10:1 at a temperature of 97 ± 2°C for 180 min. After the end of the cooking time the pulp was washed several times with distilled water to ensure complete removal of residual lignin and non-cellulosic components. The obtained organosolv hemp pulp (OHP) was stored wet in an airtight bag to produce nanocellulose. Never dried pulp is better, as dried samples irreversibly lose access to the surface during the drying process. Using never-dried pulp does not require consumption of energy for drying and grinding since dried cellulose fibers lose the ability to swell and percolate due to irreversible cornification.

2.3 Extraction of nanocellulose

The nanocellulose (NC) has been prepared by acid hydrolysis of OHP followed by ultrasonication treatment. The hydrolysis of OHP was performed with solutions of sulfuric acid with concentrations of 40%, 50% and 60% at a temperature of 40 ^oC, 50 ^oC and 60 ^oC, duration from 30 to 90 minutes using mechanical stirring at approximately 800 rpm. At the end of the reaction time, hydrolysis was stopped by adding excess 10-fold chilled distilled water followed by centrifugation. The resulting nanocellulose was washed three times with distilled water in a laboratory centrifuge at 4000 rpm to remove acidic solution. The precipitate was collected, resuspended in distilled water, and dialyzed in distilled water until a neutral pH value was reached. The nanocellulose suspension was sonicated at 22 kHz using ultrasonic dispersant UZDN − 2t (Ukraine) for one hour to disperse the particles. Sonication was done in a cooling bath to avoid overheating which can cause desulfation of the sulfate groups present on the OHP. The suspension of nanocellulose was poured into Petri dishes and dried at room temperature to obtain films, which were studied by physical and physico-mechanical methods, or the NC suspensions were stored at room temperature in closed containers for use in further research.

2.4 Preparation of handsheets

Standard laboratory handsheet samples of paper for automatic food packaging were prepared using a Rapid-Kothen machine according to TAPPI T205 sp-02. The handsheets of 220 ± 8 g/m² were produced from bleached sulfate softwood pulp. The pulp was beaten in a Valley beater to 40 °SR (Shopper-Rigler). The consumption of alkyl ketene dimer (AKD) was 0.5% or 1.0% and consumption of hemp NC were 0.5, 1.0, 1.5 and 2.0% relative to mass of paper. These sheets were conditioned in a chamber at 23°C and 50% humidity for 24 hours before determining their physical and mechanical parameters and compared with the requirements of the standard.

2.5 Methods of Analyses

Morphological studies of hemp fibers, obtained pulps and NC films were carried out using scanning electron microscopy (SEM) on a PEM-106I microscope (SELMI, Ukraine) at an accelerating voltage of 20 kV. The samples were sputter-coated with a layer of gold using the sputtering technique. Fourier transform infrared spectroscopy (FTIR) spectra of the hemp fibers, pulps and NC films were recorded on Tensor 37 Fourier-transform infrared spectrometer with a 4 cm^− 1 resolution in the 400–4000 cm^− 1 frequency range (BRUKER, Germany).

The X-ray diffraction (XRD) were measured for hemp fibers, hemp pulps and isolated NC with an Ultima IV diffractometer (Rigaku, Japan) using Cu Kα radiation at 40 kV and 30 mA. Samples for testing were placed in a special cuvette, which was located horizontally during the measurement. The diffraction pattern was recorded in stepwise mode, the measurement step was 0.04° at a scan rate of 2°/min in the range of 2θ = 5–40°. Primary processing of X-ray spectra was performed using the PDXL software package of the Ultima IV diffractometer. The baseline for measuring peak intensity was determined by two-point background removal when the baseline connects the two ends of the pattern. The crystallinity index (CrI) was calculated from the heights of the peak of the crystalline phase 200 (I₂₀₀) and the minimum intensity between the peaks 200 and 110, which corresponds to the amorphous phase (I_am,) using Segal's method (Segal et al. 1959):

CrI = [(І₂₀₀ – І_am) / І₂₀₀] x 100%, where I₂₀₀ is the intensity of the (200) reflection for the crystalline phase at 2θ = 22.5° and I_am is the intensity of the amorphous band at 2θ = 18.5°.

Atomic force microscopy (AFM) was used to determine the topographic characteristics of nanocellulose samples. Measurements were accomplished with Si cantilever, operating in a tapping mode on the device Solver Pro M (NT-MDT, Russia). The scanning speed and area were 0.6 line/s and 2×2 µm², respectively. The transparency of the NC films was determined by the electron absorption spectra, which were registered in the range of 200 to 1100 nm. The electron absorption spectra of the NC films in UV and in visible and near infrared regions were registered on two-beam spectrophotometer 4802 (UNICO, USA) with a resolution of 1 nm.

Thermogravimetric analysis (TGA) was used to study the thermal stability of samples of OHP and NC films using a Q50 thermal analyzer (ТА Instrument, USA). The samples were heated at a rate of 20°C/min, from 20 ^oC to 700°C. The weight of samples was within 10 mg, the standard substance for calibrating the temperature scale is nickel, crucible material – platinum. Deviations of weight were registered and processed according to a program involving the use of computer technology. Based on the changes in the gravimetric and differential curves of thermal analysis, the initial temperature of the mass weight loss of OHP and NC samples were determined. TGA scans were performed under nitrogen environment with a purge flow rate of 40 mL/min.

The density of nanocellulose films were determined according to ISO 534:1988. The tensile strength of the nanocellulose films was measured at a 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 2-N load stress. For testing, test strips with 10 ± 2 mm width and 25 ± 5 mm long were used. The tensile strength of the NC films was calculated on five test pieces, expressing the results as an average and standard deviation. Physico-mechanical properties of the paper were determined in accordance with the following standards: tensile strength and breaking length were determined according to ISO 1924-2; ISO 1924-3:2005 was used to determine the relative elongation of paper samples; ISO 535 - to determine the surface absorbency, Cobb₃₀; water absorption - in accordance with EN ISO535: 2017. To determine each of the paper quality indicators, five paper samples were tested and averages and standard deviation were calculated.

3.1 Cooking hemp pulp

Chemical analysis of plant raw materials showed that the most valuable part of it - cellulose in hemp fibers contains 73.9%, which significantly exceeds the content of cellulose in wood (41.0%– 47.8%).

At the same time, the content of lignin in hemp fibers (8.8%) is 2.7–3.5 times less than in wood (21.0% – 28.0%). The content of water-extractable substances in hemp fibers (4.2%) ranges between deciduous (2.2% for birch) and coniferous (6.7% for pine) wood species (Smook 2003). The content of substances extracted with a 1% alkali solution in hemp fibers (20.2%) is close to the value of this indicator for pine (19.4%), almost two times higher than the value of this indicator for birch (11.2%). The content of substances extractable by an alcohol-benzene mixture in hemp fibers (1.9%) is close to the value of this indicator for birch wood (1.8%) and less than for pine (3.4%). Hemp fibers contain more minerals (1.6%) than coniferous and deciduous wood (0.2% – 0.7%). Such values of the main components of hemp fibers a priori indicate the need for a lower consumption of chemical reagents, lower energy costs during their delignification and regeneration of spent cooking liquor compared to obtaining cellulose from wood.

As a result of alkaline extraction and peracetic cooking for 180 min of each treatment, OHP was obtained with a yield of 64.5% relative to the weight of absolutely dry material, which had a residual lignin content of 0.16% and an ash of 0.08%. The alkaline treatment naturally extracts readily soluble polysaccharides (hemicelluloses) and extractives (resins, fats, waxes, minerals) from plant raw materials and insignificantly lignin, the content of which remains at the level of the initial hemp fibers. Carrying out subsequent peracetic cooking leads to significant removal of lignin and residual minerals. The obtained OHP has quality indicators close to those for organosolv pulps obtained earlier from other representatives of non-wood plant raw materials – wheat straw, flax, kenaf, miscanthus, reed (Barbash et al. 2020; 2021). The resulting OHP is suitable for chemical processing, in particular for the production of nanocellulose.

3.2 Pulp morphology

The change in the morphology of the hemp fibers, the hemp pulps and NC was studied by SEM method (Fig. 1).

As can be seen from Fig. 1a, the original plant material consists of several interwoven layers of fibrils with a transverse size in the range of 25–140 µm. In the process of alkaline treatment, hemp fibers swell and partially separate into fibers due to the removal of the main part of hemicelluloses and minerals and partial removal of lignin (Fig. 1b). In this case, there is a separation of layers of fibrils and an increase in the part of individual fibers with a diameter of several microns. Carrying out further peracetic cooking of pulp leads to remove residual lignin and extractives, bleaching and shortening of fibers. As seen from Fig. 1c, this results in almost complete separation into cellulosic fibers with a transverse size of 5–30 µm. Acid hydrolysis of the OHP leads to the rupture of 1–4

glycosidic bonds between pyranose units of cellulose macromolecules, to dissolution of the amorphous part of cellulose, and a significant reduction in the fiber size to nanoparticles (Fig. 1d).

The change in the chemical composition of hemp fiber during its thermochemical processing was confirmed by infrared spectroscopy data. Figure 2 shows the Fourier IR spectra of hemp fibers, pulps after alkaline extraction and peracid cooking and NC.

All spectra have the same typical peaks that characterize stretching vibrations of hydroxyl groups included in intramolecular and intermolecular hydrogen bonds (3000–3750 cm^− 1), valence asymmetric (2920 cm^− 1) and symmetric (2853 cm^− 1) vibrations of the methyl and methylene groups, deformation vibrations of the bonds –CH₂ and –O–H in –CH₂OH groups (1236–1433 cm^− 1), the valence vibrations of the C–O bonds and C–O–C bridge of the glucopyranose ring (1160 cm^− 1, 1112 cm^− 1 and 1058 cm^− 1) and deformation vibrations of C-H bonds (520–667 cm^− 1) of cellulose (Ilyas et al. 2017; Rosli et al. 2021). The band in the spectrum near 1736 cm^− 1 is assigned mainly to the C = O stretching vibration of the carbonyl and acetyl groups in hemicelluloses and in lignin and the ester group of cellulose sulfate in nanocellulose (Silvério et al. 2013). Alkaline extraction removes carbonyl groups from hemicellulose (spectrum b), but subsequent organosolv cooking increases the amount of carbonyl groups due to oxidation of cellulose with hydrogen peroxide (spectrum c), and acid hydrolysis leads to the formation of ester groups in NC (spectrum d). The absence of bands at 1512 cm^− 1 and 1244 cm^− 1 in the spectra b, c and d in Fig. 2 testifies to the removal of lignin from the hemp pulp. Spectral bands in the region of 1645 cm^− 1 are associated with the presence of adsorbed water and characterize the degree of sample moisture (Kumar et al. 2014).

3.3 Pulp crystallinity

The X-ray diffractograms of initial hemp fibers, hemp pulps and NC are depicted in Fig. 3.

Analysis of the X-ray diffractograms shows that the peak with greater intensity at 22.2º–22.9º 2θ reflection belongs to the (200) crystallographic plane of cellulose. The peak with maximum reflection in the range 14.4º–14.9º corresponds to the crystallographic plane (1–10), the peak in the range 15.5–16.2º 2θ reflection assigned to the crystallographic plane (110) and the peak around 34.6º belongs to the crystallographic plane (004) cellulose I (Kumar et al. 2014). Presence of these peaks in all samples indicates that crystalline structure of hemp cellulose had not changed during the thermochemical treatments and belongs to the typical structure of cellulose I. Based on the analysis of the diffraction patterns of the studied samples, their crystallinity index (CrI) was calculated using the Segal equation. Crystallinity index of hemp fibers was 72.0%, CrI of pulp after alkaline extraction was 79.9%, CrI of OHP – 84.7% and CrI of NC – 87.2%.

As can be seen from obtained data, crystallinity index of hemp pulps increases in the following order: initial plant material – pulp after alkaline extraction – OHP – nanocellulose. This is due to the fact that during thermochemical treatments, CrI increases due to the removal of non-cellulose components from the plant raw material and the removal of amorphous regions of cellulose under the action of sulfuric acid. An increase in the CrI values of nanocellulose in comparison with the CrI of the initial cellulose was also found for other representatives of plant raw materials (Lee et al. 2009; Nuruddin et al. 2014; Barbash et al. 2020, 2021).

3.4 Properties of nanocellulose

The influence of the technological parameters of the hydrolysis of OHP on the quality indicators of hemp NC is shown in Table 1.

Table 1

Dependence of the quality indicators of hemp nanocellulose on the technological parameters of the acid hydrolysis process
Hydrolysis temperature ^oC	Duration of hydrolysis, min	Concentration of H₂SO₄, %	Density, g/cm³	Tensile strength, MPa	Transparency, %
40	60	40	0.78 ± 0.01	15.4 ± 0.6	58.2 ± 2.4
		50	0.95 ± 0.01	27.0 ± 0.9	64.3 ± 2.5
		60	1.05 ± 0.02	34.0 ± 1.1	67.2 ± 2.6
	90	40	1.07 ± 0.02	35.8 ± 1.2	71.8 ± 2.8
	90	50	1.26 ± 0.03	48.1 ± 1.8	80.0 ± 3.0
50	60	40	1.18 ± 0.02	47.8 ± 1.7	75.3 ± 2.8
60	30	50	1.16 ± 0.02	44.2 ± 1.6	76.1 ± 2.9
	60	30	1.33 ± 0.03	50.4 ± 2.0	82.3 ± 3.1
		40	1.46 ± 0.04	55.2 ± 2.1	82.0 ± 3.1
		50	1.54 ± 0.05	60.0 ± 2.3	85.5 ± 3.3
	90	40	1.48 ± 0.04	57.8 ± 2.2	83.0 ± 3.2
	90	50	1.56 ± 0.05	66.7 ± 2.5	87.3 ± 3.3

As can be seen from the presented data, an increase in each of the three technological parameters - the concentration of sulfuric acid, temperature and duration of hydrolysis, leads to an increase in the density, tensile strength and transparency of nanocellulose films. Such dependence is explained by the acceleration of the OHP hydrolysis process, which leads to an intensification of the destruction of 1–4 glycosidic bonds between the glucopyranose units of the cellulose macromolecule under the action of both hydronium ions and increased temperature, and an increase in the duration of hydrolysis contributes to further washing out of the amorphous regions of cellulose, which is confirmed by an increase in crystallinity index of nanocellulose.

Figure 4 shows the linear dependence of the tensile strength and transparency of nanocellulose films on its density, which is explained by the formation of stronger hydrogen bonds between nanocellulose particles with a decrease in the distance between nanoparticles with an increase in the values of the above technological parameters of the hydrolysis process.

The hemp NC obtained after acid hydrolysis and sonication had a homogeneous transparent stable gel-like suspension. The stability of a transparent suspension of nanocellulose is explained by the presence of charged groups on the surface of nanocellulose, which are formed interaction of cellulose with sulfuric acid due to the esterification reaction. It should be noted that the nanocellulose suspension is stable during long-term storage at room temperature (more than 6 months) without sedimentation of nanocellulose particles.

Topographic characteristics of hemp NC were determinate by AFM. As can be seen from Fig. 5, hemp NC particles form a multilayer structure between nanoparticles due to the action of hydrogen bonding and van der Waals forces (Poletto et al 2014).

Analysis of the AFM image showed that the hemp NC suspension consists of nanoparticles in the form of needles with a transverse size in the range of 8–23 nm (Fig. 5b), but individual nanofibers have a transverse size of up to 30 nm and a length of several micrometers (Fig. 5a). Such values of the transverse dimensions of hemp nanocellulose are confirmed by the data for nanocellulose extracted by acid hydrolysis of organosolv pulps from other representatives of non-wood plant raw materials: wheat straw (10–45 nm), flax (15–65 nm), kenaf (10–28 nm), miscanthus (10–20 nm) (Barbash et al., 2020).

The change in the transparency of nanocellulose films depending on the hydrolysis conditions is shown in Fig. 6.

A comparison of the samples shows that increasing the acid concentration, temperature and hydrolysis time increase the transparency of the obtained films from 58.2–87.3% by increasing their density.

3.5 Thermal analysis

The effect of temperature on the stability and moisture absorption capability of hemp cellulose and NC was investigated by thermogravimetry (TG) and derivative thermogravimetry (DTG) analysis.

From the dependence of weight loss on temperature on the TG curve (Fig. 7a), it can be seen that when the samples are heated from 25°C to 100°C, an insignificant change in mass is observed due to moisture evaporation.

A further rise in temperature affects the destruction of OHP and NC in different ways. On the TG curve of hemp NC (Fig. 7a), there are two sharp bends at 159°C and at 452°C. The first bend is observed at a lower decomposition temperature than for the OHP, and corresponds to the process of dehydration of chemically bound water and destruction of easily accessible sulfate ester groups of nanocellulose (Silvério et al., 2013). The second sharp bend in the TG curve at 450°C corresponds to the final oxidation and decomposition of charred NC residues. Between these temperatures, a gradual decrease in the mass of hemp NC is observed, which corresponds to the processes of depolymerization and degradation of the main part of the amorphous regions of the NC and the breakdown of glycosidic bonds under the action of sulfuric acid (Yousefi et al., 2013). The TG curve of the OHP indicates the beginning of its degradation at a temperature of 228.8°C and a sharp decrease in weight at a temperature of 351°C due to the depolymerization of glycosidic bonds of OHP. In the temperature range of 351^oC – 500^oC, a gradual decrease in the mass of the OHP is observed due to its pyrolysis, and at 500 ^oC – a sharp oxidation and splitting of charred residues of the OHP. DTG curves (Fig. 7b) show that the decomposition of the NC films begins at a temperature of 159°C and the maximum rate of their decomposition is observed at 452°C, while for OHP, the onset of decomposition began at 228.8°C and the maximum rate of its decomposition is observed at 351°C. The resulting hemp nanocellulose decomposes at a relatively lower temperature than OHP due to the presence of ester sulfate groups on the cellulose surface and a greater number of the free ends of the chains of NC which decompose at a lower temperature (Kumar et al. 2014; Nuruddin et al. 2014). The presence of ester sulfate groups reduces the heat resistance of hemp nanoparticles, since less energy is required to remove sulfuric acid residues from the hydroxyl group at the 6th carbon atom in the pyranose ring of cellulose than to destroy the glucopyranose ring of the OHP (Mandal and Chakrabarty 2011).

3.6 Application of nanocellulose in paper

The results of the use of hemp nanocellulose in the composition of paper for automatic food packaging are shown in Table 2 and Fig. 8.

Table 2

Physical and mechanical properties of paper for automatic food packaging with the use of hemp nanocellulose in its composition
Number of compositions	Density, g/см³	Breaking force, N	Elonga- tion, %	Breaking length, m	Surface absorbency, Cobb_30, g
1. without chemical additives	0.82 ± 0.05	180 ± 10	2.3 ± 0.08	5020 ± 50	136 ± 5.2
2. + 1% AK D	0.82 ± 0.05	184 ± 11	2.5 ± 0.09	5300 ± 55	19 ± 1.0
3. + 0.5% АKD + 0,5% NC	0.83 ± 0.05	186 ± 12	2.8 ± 0.10	5400 ± 60	22 ± 1.2
4. + 0.5% АKD + 1% NC	0.83 ± 0.05	190 ± 12	3.1 ± 0.11	6000 ± 65	17 ± 0.9
5. + 0.5% АKD + 1,5% NC	0.84 ± 0.06	220 ± 13	3.4 ± 0.12	6300 ± 70	16 ± 0.8
6. + 0.5% АKD + 2% NC	0.85 ± 0.07	250 ± 14	3.7 ± 0.15	7150 ± 70	15 ± 0.8
Requirements of the standard	0.7–0.85	not less 78* (150)**	not less 2.4* (3.7)**	-	not more 25
- for the first grade of paper; *- for the premium grade of paper

For comparison with the requirements of the standard, the table shows the quality indicators of paper without chemical additives and with the addition of 1% AKD (alkyl ketene dimer) - a substance that provides hydrophobicity and sizing of paper and is synthesized from petroleum products. The addition of 0.5% or 1% AKD in relation to the bleached softwood sulphate pulp provides paper samples with the required standard values for the first grade of paper in terms of such indicators as: elongation and breaking force by ring compressive (Fig. 8). With the addition of 0.5% AKD and 0.5% NC, we obtained paper samples that also meet the requirements of the standard for the first grade of paper. Adding 0.5% AKD and 2.0% NC allows to obtain samples that meet all the requirements of the standard for the premium grade of this type of product. At the same time, the breaking force exceeds the requirements of the standard for premium grade paper by 40%, and the breaking length increases by 42% compared to a sample of paper without chemical additives.

This dependence is explained by the formation of additional hydrogen bonds between particles of nanocellulose and paper fibers, which improves its properties. The use of nanocellulose makes the paper sheet more durable by filling the voids between the fibers with nanoparticles and increasing the number of fiber-fiber bonds, and, as a result, strengthen the hydrogen bonds in the paper (Charani and Moradian 2019). Samples of paper in terms of such indicators as: density, degree of sizing by a line method, smoothness, whiteness corresponded to the requirements of the standard for all considered types of composition 1–6 (Table 2). The requirements of the standard in terms of water resistance to the action of hot water were fulfilled for paper with the addition of AKD and NC, but not for paper without the addition of chemical additives.

As can be seen from the data presented, the addition of hemp nanocellulose to the fibrous composition makes it possible to obtain paper samples that meet the requirements of the standard for paper for automatic food packaging and significantly reduce the use of synthetic substances from exhaustible oil and gas products.

The hemp fibers have been used as a source of cellulose for NC extraction. The cellulose from hemp fibers (OHP) is obtained by an environmentally friendly organosolv method using a solution of peracetic acid. The effect of acid hydrolysis parameters on the properties of hemp NC was investigated. It is shown that an increase in the concentration of sulfuric acid, temperature and duration of the hydrolysis process intensifies the destruction of the OHP, reduces the proportion of the amorphous part of cellulose, increases the density, tensile strength and transparency of nanocellulose films. A linear dependence of the tensile strength and transparency of hemp NC on the density of nanocellulose films has been established. It was found that the crystallinity index of hemp pulps increases in the following order: initial raw material – pulp after alkaline extraction – OHP – hemp NC. Structural and chemical changes in hemp fibers during thermochemical treatments were studied using SEM, FTIR, XRD, AFM methods. Hemp NC films had a density of up to 1.56 g/cm³, a tensile strength of up to 66.7 MPa, a transparency of up to 87.3%. AFM method have shown that the transverse dimension of nanocellulose particles is from 8 nm to 23 nm, but individual nanofibers are up to 30 nm wide and up to several micrometers in length. Hemp NC has higher crystallinity index (87.2%) than OHP (72.0%), but has lower thermal stability due to the presence of sulfonate groups on the cellulose surface. The positive effect of the addition of hemp NC on the improvement of all quality parameters of paper for automatic food packaging has been shown. The addition of 2.0% NC was found to increase the breaking force of the paper by 40% over the standard requirements for premium grade paper, and the breaking length increased by 42% over a sample of paper without chemical additives. The use of hemp NC makes it possible to improve the properties of composite materials and reduce the consumption of synthetic substances from exhaustible sources of raw materials.

Funding. This work was supported by the Ministry of Education and Science of Ukraine, project No. 2301.

Conflicts of interest. The authors have no conflicts of interest to declare that are relevant to the content of this article.

Ethics approval. This work does not contain any studies with human participants or animals performed by any of the authors. The authors claim the compliance with the ethical standards.

Authors’ contribution statements. Valerii Barbash designed the study, interpreted the experimental data, and drafted the manuscript. Roman Zakharko and Olga Yashchenko extracted organosolv pulp and nanocellulose from hemp fibers and studied its physical and mechanical properties. Olga Yakymenko received samples of the paper and examined its performance. Volodymyr Myshak has investigated the FTIR spectra and TGA of OHP and NC. All authors read and approved the final manuscript.

Affiliations and Author information

Department of Ecology and Technology of Plant Polymers, National Technical University of Ukraine “Igor Sikorsky Kyiv Polytechnic Institute”, Kyiv, Ukraine

V. A. Barbash ORCID 0000-0002-7933-6038, О. V. Yashchenko ORCID 0000-0003-3716-8707, O. S. Yakymenko ORCID 0000-0002-0996-8398, R. M. Zakharko ORCID 0000-0002-8032-4114

Institute of Macromolecular Chemistry of the National Academy of Sciences of Ukraine, Kyiv, Ukraine, V. D. Myshak ORCID 0000-0003-1615-3303

Corresponding author: [email protected]

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Preparation of hemp nanocellulose and its use to improve the properties of paper for food packaging

Status:

Version 1

Abstract

Figures

1 Introduction

2 Experimental Details