Preparation and characterization of nanocellulose from Albizia lebbeck sawdust and their application in nanocomposites using poly(vinyl chloride) (PVC)

The nanocellulose and its nanocomposites have signicant importance in the most economic sectors of applications. This research involves the synthesis of nanocellulose from Albizia lebbeck wood sawdust. Quantitative evaluations of chemical components present in the sawdust of a native hardwood species Albizia lebbeck were determined by chemical analysis method. The results revealed that the approximate amount of hot water extractives, 96% ethanol extractives, alpha-cellulose, hollocellulose, hemicellulose, lignin and ash content were 6.9%, 7.31%, 40.72%, 65.10%, 24.38%, 25.67%, and 1.10% respectively. Nanocellulose was synthesized from the extracted alpha-cellulose by acid hydrolysis method. Fourier-transform infrared spectroscopy (FTIR) characterizations have been done for Albizia lebbeck sawdust, extracted alpha-cellulose and synthesized nanocellulose respectively. The average diameter of synthesized nanocellulose was found 155.6 nm by particle size analyzer. Nanocellulose with different weight percentages (2%, 4%, 6%, 8% and 10%) were reinforced with poly(vinyl chloride) (PVC) to prepare nanocomposites by solution casting method. The thermal stability of pure PVC, nanocellulose and 2wt% nanocellulose–PVC nanocomposite was investigated and the result revealed that 2 wt% nanocellulose reinforced PVC nanocomposites is thermally more stable than the nanocellulose and pure PVC. The tensile properties of all nanocomposites were counducted by universal testing machine (UTM) and the highest tensile strength was obtained for 2 wt% nanocellulose reinforced PVC nanocomposites. The scanning electron microscope (SEM) images of 2 wt% nanocellulose reinforced PVC nanocomposites also exhibited the uniformly dispersion of nanocelulose in nanocomposite.


Introduction
The research on nanocellulose based nanocomposites has grown exponentially and the research subject of cellulose nanomaterials started in the mid-90s (Oksmana et al. 2016). Nanocomposites have unique properties with incorporation of very small quantities of lling materials. Nanocomposites exhibits improved stiffness, toughness, barreier properties and resistance to re and ignition (Sheltami et al. 2015). A lot of efforts have been devoted to the development of new high performance nanocomposite lms for packaging applications with polymer matrix and nano llers that are completely renewable. Nanocellulose (NC) is made from cellulose and cellulose is the most abundant biopolymer in nature, represents a remarkable emerging class of nature-derived nanomaterial due to having biodegradibility, extraordinary mechanical properties (Dorđević et al. 2016). Almost all types of polymers, such as thermoplastic, thermosets and elastomers have been used to make new materials polymer nanocomposites which have nanosize llers. Applications of eco-friendly, sustainable and degradable nanocomposites offer new technology and business opportunities for several sectors of the aerospace, automotive, construction, electronics, biotechnology and food packing industries. There is a tremendous interest for using bionanoparticles like nanocellulose to be applied in the new era of biocomposites (Gacitua et al. 2005, Camargo et al. 2009, Trache et al. 2020and Chang et al. 2020. Biocomposites refers to composite materials that contain one or more naturally derived content, which can be part of the reinforcement phase or matrix phase or both in a composite system. Various bionanocomposites have been produced using cellulose nanocrystals (CNCs) as nano llers and polymer matrixes from natural and renewable resources (Trache et al. 2020 andChang et al. 2020). Joseph et al. (2020) gives a brief idea about the research and development pertaining to cellulose nanocomposites, characteristics and their biomedical applications. They reported in this review on the nanocellulose and nanocellulose based nanocomposites with natural rubber (NR), Polylactic acid (PLA), poly ethylene oxide (PEO) and polycaprolactone (PCL) matrices. They also reported that nanocellulose is generally considered as potential and excellent reinforcing ller for manufacturing nanocomposites as they are renewable, possess lightweight and is cost effective. Naim et al. (2020) investigated and reported on the polyvinyl chloride (PVC) multiwall carbon nanotubes (MWCNTs) nanocomposite for many industrial applications such as energy storage, chemical sensors and electronics etc. Poly(vinyl chloride) (PVC) was one of the rst polymers to be used in food packaging applications due to its wide acceptance for use in the preservation of foods. These materials have unique cling properties and good barrier properties for the preservation of food (Meena et al. 2017  In this research Albizia lebbeck sawdust were used to synthesize nanocellulose from extracted cellulose of Lebbek sawdust. Albizia lebbeck tree is known as "shirish coroi" wood. Shirish or Lebbek tree is native to Bangladesh, Southeast Asia and Australia. In this work, lebbek sawdust from the local source of Bangladesh was used as a main raw material to produce nanocellulose and nanocomposites. Sawdust is a waste product generated during processing of wood in different types of forest industries. The main objective in this research is to investigate the potential utilization of lebbek sawdust in manufacturing nanocellulose and nanocomposites.

Materials
The main raw material Albizia lebbeck sawdust was collected from Saturia Sawmill Ltd. Saturia, Manikgonj. The chemicals used in this research were acetic acid, sodium hydroxide, sulfuric acid, tetrahydrofuran (THF) and ethanol 96% etc collected from Merck, Germany. Sodium chlorite was collected from BDH, England. The matrix material used in this research was plasticized poly(vinyl chloride) (PVC) from Merck Specialities Private Limited, Mumbai, India. Distilled water was produced and collected from BCSIR Laboratories, Dhaka.

Processing of Sawdust
Sawdust was cleaned manually by removing undesirable particles and then thoroughly washed in tap water and dried in air. Cleaned sawdust was sieved by Analytical Sieve, FRITSCH analysette3, Germany.
The particle sizes of mesh 60+ sawdusts were taken for further works. The components of sawdust were analyzed by chemical methods according to Sultana et al. (2020).

Determination of hot water soluble extractives content of sawdust
The sawdust (10 g) was transferred into a round bottom ask and 500 ml distilled water was added to it. The ask was set on a heating mantle for re ux 3 hrs. After nishing the re uxing, the contents of the ask ltered and dried in an oven at 105 o C temperature. The loss of weight of sawdust was determined as hot water solubility and calculated as percentage.
Determination of 96 % ethanol soluble extractives content of sawdust Extractives content of sawdust were measured by extracting of them in 96 % ethanol (100-120 ml) for 4 hours in a soxhlet apparatus. The loss of weight of sawdust was determined as extractives content and calculated as percentage.

Determination of ash content of sawdust
The ash content of sawdust was measured by using Mu e Furnace, model-CWF1200, Carbolite Ltd, United Kingdom. The samples in silica crucibles were burnt in a gas burner rst to remove the fumes and then placed into the furnace to make ash at 575 ± 25°C for 4 hours. The ash content was calculated as percentage.

Determination of acid insoluble lignin content of sawdust
Sawdust was dried after removing water extractives and 96 % ethanol extractives content. Dried sawdust was taken to treat with 72% sulphuric acid hydrolysis for 2 hrs at room temperature with constant stirring. Then the acid was diluted to 3% by adding distilled water and again re uxes for 4 hours. The ltered residue was washed and dried and calculated as percentage for acid insoluble lignin content.

Determination of hollocellulose content of sawdust
Water and ethanol extractives free sawdust were dried and taken to determine hollocellulose (cellulose + hemicelluloses) content of sawdust. It was treated with sodium chlorite solution at constant pH 4 maintaining by adding buffer solution of sodium acetate and acetic acid. After completion of the treatment, the mixture was ltered, washed with water and dried. The white residue was calculated as percentage for hollocellulose content.
Preparation of Alpha-cellulose from hollocellulose The hollocellulose was treated with 18% NaOH solution for 3 hours at room temperature with occasional stirring. After treatment the alpha-cellulose was separated from hollocellulose as white residue. The white residue was washed and dried and calculated as percentage as alpha-cellulose.
Synthesis of nanocellulose from puri ed alpha-cellulose by acid hydrolysis Puri ed alpha-cellulose was taken to synthesize nanocellulose. It was mixed with 64% sulfuric acid at an initial ratio of 1:8 (w/v). The mixture was hydrolyzed by stirring 300 rpm for 1 h at 45 o C and then the reaction mixture was quenched by adding 10-fold distill water. After quenching, the mixture was allowed to settle overnight in the refrigerator, and the resulting suspension was then centrifuged by Centurion Scienti c Benchtop Centrifuges, Model-K241 until the p H is 6-7. The nanocellulose was separated by centrifugation and then it was sonicated by Bandelin Sonorex Digitec Sonicator, Germany for 10 min. The product is then subjected to freeze drying to get lm by removing water.

Preparation of nanocellulose-PVC nanocomposites
Numerous methods such as solvent casting, melt mixing, in situ polymerization, extrusion and layer by layer formation have been used to prepare NC-based composites (Lasrado et al. 2020). In this work, solution casting method was used to make nanocellulose-PVC nanocomposites. PVC (10 g) was dissolved in THF (30 ml). Dried nanocellulose (0.2 g) was softened by adding few drops of distilled water rst and then THF (10 ml) was added into it and sonicated for 30 min separately. Solutions of PVC and nanocellulose were mixed together and stirred for 3 hours at room temperature using a magnetic stirrer. The solution was cast into a glass petridish and allowed to evaporate at room temperature for 24 hours.

Characterization
Fourier Transform Infrared Spectroscopy (FTIR) analyses The FTIR spectra of the cellulose, nanocellulose and nanocomposites were recorded on a Perkin Elmer-FTIR/NIR, Model-Forntier Spectrophotometer. The FTIR spectra of all samples were obtained in the printed form and presented in the result and discussion section.

FTIR spectroscopic analyses of sawdust and alpha-cellulose
The FTIR spectra of Albizia lebbeck sawdust, extracted alpha-cellulose and prepared nanocellulose are presented in the g.  Nanocellulose was synthesized from extracted alpha-cellulose of Albizia lebbeck sawdust by acid hydrolysis process. The selected conditions of acid hydrolysis are 64% sulfuric acid, 45 0 C and one hour respectively. Various methods are used to synthesize nanocellulose, such as acid hydrolysis using concentrated mineral acids to break down cellulose, ultrasonic technique by application of sound energy to physical and chemical systems to break down cellulose, and enzymatic hydrolysis using enzymes to break down cellulose etc but the acid hydrolysis method is easy and fast to produce nanocellulose that exhibit better properties (Wulandari et al. 2016 andHu et al. 2017). FTIR spectrum and spectral data of nanocellulose are presented in g. 1 (lower part) and table 2 respectively. Most of the peak positions of nanocellulose are not identical but closure to the peak positions of alpha-cellulose, this is due to hydrolysis of alpha-cellulose. It is also showed in g. 1 that there are not new bonds formed in nanocellulose during hydrolysis of alpha-cellulose.
Particle size analysis of nanocelluse from Albizia lebbeck sawdust Nanocellulose was synthesized by acid hydrolysis method and the nanoparticle size of it was measured by particle size analyzer and presented in ( g. 2). It is observed from the gure that the average diameter of prepared nanocellulose is 155.6 nm. Thermal properties of nanocellulose and nanocellulose-PVC nanocomposites The thermal properties of nanocellulose and 2 wt% nanocellulose loaded nanocellulose-PVC nanocomposite have been investigated by simultaneous thermal analysis (STA) and the data of thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) are presented in g. 5-6 and  It is clear from the g. 5-6 and table 3 that 2wt% nanocellulose-PVC nanocomposite is thermally more stable than nanocellulose and pure PVC. The thermal degradation of nanocellulose is faster than the nanocellulose-PVC nanocomposite and pure PVC. The differential peak temperature of the 2wt% nanocellulose-PVC nanocomposite is shifted to the higher temperature side as compared to the nanocellulose and pure PVC. Interaction of nanocellulose with PVC in the nanocomposite material enhanced its thermal properties.

Morphological Analysis by SEM
The SEM images of surface of nanocellulose lm and the tensile fractured surface of 2 wt% nanocellulose-PVC nanocomposite were taken to investigate their morphological character. SEM images of nanocellulose and 2 wt% nanocellulose-PVC nanocomposite are presented in g. 7 and 8 respectively. It is observed from the g. 7 that the surface of nanocellulose lm shows the presence of different sizes of particles on the lm surface. Particle size analysis of nanocellulose was also showed the range of particle size is 24.21-955.4 nm. Almost similar SEM image of nano lm observation was reported by Hernández-Flores et al. (2020) for pure cellulose nanocrystals (CNCs) lm from cotton. Uniform dispersion and distribution of nanocellulose in the PVC phase is also found in the nanocomposites ( g. 8). Nanocellulose lm was dissolved in solvent and mixed with PVC solution to make nanocomposites. So nanocellulose is not found as lm form in the g. 8 but it is found as nanoform and uniformly dispersed in the PVC matrix. Strong interfacial bonding and homogenous blending between nano ber and PVC matrix is observed in the 2 wt% nanocellulose-PVC nanocomposite. Lani  with nanocellulose from oil palm empty fruit bunch ber and nanocomposite lm of 10% polyvinyl alcohol (PVOH)/5% nanocellulose ber (NCF) from agriculture waste of banana plant respectively.

Conclusion
The chemical composition analyses of Albizia lebbeck sawdust showed the presence of alpha-cellulose 40.72%. Nanocellulose was successfully synthesized by acid hydrolysis method from alpha-cellulose. FTIR spectroscopic and size distribution characterization were also evaluated for synthesized nanocellulose. The particle size of the synthesized nanocellulose had an average diameter of 155.6 nm. The acid hydrolysis method can be used for industrial production of nanocellulose due to low cost, easy and fast. So it can be concluded that Albizia lebbeck sawdust can be a suitable source for the extraction of cellulose and synthesize nanocellulose. Nanocellulose reinforced PVC nanocomposites were prepared by solution casting method with different weight percentages of nanocellulose (2-10%). The best tensile strength obtained for nanocomposite containing 2 wt% nanocellulose. The SEM images con rmed the uniform dispersion of nanocellulose and strong interfacial bonding of nanocellulose into the PVC nanocomposite. Nanocomposite containing 2 wt% nanocellulose also exhibited increased thermal stability as compared to the nanocellulose and pure PVC. So, manufacturing of nanocellulose/ nanocomposites from waste sawdust can be pro table for commercial market in the country.

Declarations
The authors declare no con icts of interest.