Structural, Morphological and Thermal Properties of Nano Filler Produced from Date Palm-Based Micro Fibers (Phoenix dactylifera L.)

In this century, the development of nano-sized filler from biomass material has become the main focus of industries in achieving their final green composite product for a wide range of applications. From a commercial and environmental point of view, fragmentation and downsizing of waste lignocellulosic fibers without chemical treatments into small size particles is a viable option. In this study, an attempt was made to produce nano-sized lignocellulosic fillers from date palm micro fibers via mechanical ball milling process at intense 99 cycles run (equivalent to 25 h). The resultant nanofillers as well as the microfibers were characterized in details by various analytical techniques, including transmission electron microscopy (TEM), scanning electron microscopy (SEM), particle size analysis (PSA), Energy Dispersive X-Ray (EDX), X-ray diffraction (XRD), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC) to assess their structure—property relationship. From microscopy examination, the nanofillers showed a heterogeneous mix of irregular shaped particles, and while having a size ranging of 30–110 nm in width and 1–10 mm length dimensions. Also, the crystallography analysis revealed the crystallinity had mildly declined from microfibers (71.8%) to nanofiller (68.9%) due to amorphization effect. As for thermal analysis, the nanofillers exhibited high heat resistance at 260.8 °C decomposition temperature. Furthermore, the nanofillers also had stable thermo-changing behavior by presenting low heat enthalpy change (40.15 J/g) in its endothermic reaction for breaking organic bonds. The thermal results suggest its suitability for composite fabrication process at high temperature. Thus, the produced nanofillers can be used as a low cost reinforcing agent in the future for versatile polymer-based composite systems.


Introduction
Due to the growing awareness regarding environmental concerns and the reduction of finite petroleum products, alternatives to synthetic materials are being actively sought. Larger carbon footprints, pollution, energy intensive fabrication processes, recycling and waste management are some of the issues associated with synthetic fibers and subsequent composites. Therefore, natural materials are considered as one of the most crucial components in various applications. Considerable activities have been devoted towards research and development of such materials [1]. In order to minimize environmental impacts, research studies are focusing on eco-friendly natural lignocellulosic fibers owing to characteristics such as biodegradability, low density, low cost, non-toxic nature, adequate mechanical properties and abundant availability [2]. Besides, new policies are strongly focusing on the renewables alternatives materials to reduce the environmental issues associated with synthetic materials. These policies are supporting the effective use of bio-composites produced from various natural fibers. The future prospects for bio-composites are based on the balance between performance, economics and sustainability [3].
Despite all these advantages and progress in bio-composites, there are many limitations associated with them. These limitations include poor fiber/matrix adhesion, hydrophilicity, agglomeration of fibers and lack of fabrication processes. These shortcomings lead to the loss in required mechanical properties and hindrances in commercialization of these composites. Different physical and chemical techniques are employed to overcome these issues. These technique treatments modify the surface of fibers to have better adhesion and compatibility between fiber and matrix and reduce the agglomeration effect [22]. Bio-composites have a vast range of applications which include construction, textile, packaging and automobile industries. Automobile industries have been successfully using bio-composites to make dashboard parts, cupholders, door panels and exterior parts. In the construction industry, bio-composites are used to manufacture doors, window panels, matting and ceilings [23,24].
Date palm trees (Phoenix dactylifera L.) are rich in lignocellulosic fibers. Date palm trees are normally grown in tropical and sub-tropical regions. The Arab countries are rich in date palm trees and are the larger sources for natural fibers. Their growth is dependent on environmental conditions as well as the quality of the soil. Natural fibers can be obtained from several different parts of the tree include stems, mesh, leaflets and the midribs. The midribs are center part that connects leaves while the mesh is bark or the surface of the tree as shown in Fig. 1.
From reported works, the obtained fibers can be incorporated in various synthetic matrix materials such as epoxy, vinyl ester, phenolic, polypropylene, and biodegradable polymer matrix materials such as Poly-lactic acid (PLA) are also used [26][27][28][29][30][31][32]. There are several parameters to control the compatibility of fibres with matrix materials include type, loadings, surface area and aspect ratio. These factors also determine the possible applications for final applications. Reinforcement of date palm tree fibers in various polymer matrix materials increases acoustical, thermal and mechanical properties of bio-composite. These properties are additional improved through surface treatments of fibers. However, properties of bio-composites are further enhanced by inclusion of nano-sized fibers. Nano-sized fibers, also known as nanofillers are commonly obtained from microfibers by treating them with strong acids to disintegrate the fibers structure into nano-scale dimension. With the hydrolysis treatment, amorphous regions of fibers are expelled and only crystalline regions remain, which impart a high degree of crystallinity to the nanofillers which in return increase mechanical properties of the resultant composite. Other techniques include mechanical and alkaline extraction methods have also been used to produce nanofillers from microfibers [33][34][35].
According literature study, various research works have been conducted on the use of date palm tree fibers as reinforcement for bio-composites. Similarly, different approaches were also reportedly employed for producing small size cellulosic fiber fillers. One of the recent study by Alothman et al. [36], had successfully prepared cellulose nanocrystals from date palm tree fibers using combined acetic/sulphuric acid hydrolysis treatments. From their results, good aspect ratio of cellulosic nanocrystals was obtained, suggesting the cellulose nanocrystals as well as other different micro-or nano-particles could be produced from date palm fibers, which considerably have great potential used for various applications. Moreover, some recent publications have interested on fabricating low cost on both nano-and micro-fibres/fillers for enhancing the strength properties of bio-materials [37,38]. As far as we know, there is no study dealt with characterization for micro/nano-fillers. Therefore, in present work, the novelty focuses on the using of ball milling to prepare of nano-fillers from date palm micro-fibers. Both obtained nano-and micro-fibres/fillers were well characterized to comprehensively evaluate their potentials to be used as reinforcing agent for our future work in biocomposites application.

Preparation of Nanofillers
In this study, the mesh fiber part source of date palm was collected from the farms outskirt of Riyadh, Saudi Arabia. This residue, firstly cleaned by tap water in order to remove attached impurities, and dust particles. It was then kept in large container filled with fresh water for another one week for retting at room temperature. Finally, it was dried at 90-100 °C in ordinary oven for 3 days for removal of moisture [39]. It was then crushed with industrial grade crushing machine to obtain fine powder (microfibers). This powder was sieved by using 38 μm ASTM type sieve and then collected for ball milling. The microfibers was reduced to nanoscale size (nanofillers) by dry milling with a planetary ball mill (Pulverisette 7 Premium, Fritsch Co. Germany). The milling process was conducted according to Saba et al. [39] method with some modification along with numerous trial and fail attempts. It was carried out in 15 ml zirconia container with a 10 mm zirconia balls, and material to ball ratio kept 1:10. A total ninety-nine cycle of 15-min were maintained with 15 min of pause time between each cycle. The RPM was kept constant i.e. 300 throughout the process. The digital images for microfibers and nanofillers are shown in Fig. 2. The obtained microfibers and nanofillers were also used for further characterization.

Analysis of Nanofillers Size and Chemical Composition
The size of the nanofillers was determined by Zetasizer Nano-ZS (Malvern Instruments, UK) at the scattering angle of 90° and 25 °C temperature after appropriate dilution (1:200) of the samples with Milli-Q water, sonicated and filtered through 0.45 micron membrane filter. Additionally, the chemical composition of each fibre was analyzed to determine their contents of α-cellulose with TAPPI T203cm-99, holocellulose with TAPPI T249-75, and lignin with TAPPI T222 om-88, whilst the hemicellulose was determined by deducting with α-cellulose from holocellulose.

Morphological Analysis
Transmission electron microscopy (TEM) imaging was performed to study the nanofillers structure by using a JEM-1400 (JEOL, Japan) field-emission electron microscope operating at an accelerating voltage of 120 kV. In sample preparation, the nanofillers was mixed with ethanol and deposited on copper grid substrate for drying before viewing. In addition, another morphological characterization, including the elemental analysis was also carried out through a scanning electron microscopy (SEM) coupled with Energy Dispersive X-ray (EDX) facility (JEOL, JSM-6360A, Japan). A small amount of dried powder was dispersed in ethanol, sonicated for about 30 min and drop applied on the conductive carbon tape attached to the sampling stub. All the samples were gold sputtered prior to observation.

Wide Angle X-Ray Diffraction (XRD) Analysis
Wide angle X-ray diffraction (XRD) analysis was carried out to investigate the crystalline behavior of the samples. A computer-controlled wide-angle goniometer coupled to a sealed-tube source of Cu-Ka radiation (λ = 1.54056 Å) was used. All samples were scanned at 5°/min and 2θ range from 5° to 60°.

Thermal Analysis
Thermogravimetric analysis (TGA) was performed using Shimadzu thermal analyzer (Model: DTG-60H). The alumina pan was filled with ~ 10-15 mg of the sample. Subsequently, the samples were heated from room temperature to 900 °C at heating rate of 20 °C/min. The analysis was done under a nitrogen atmosphere with a flow rate of (50 cm 3 / min) and accordingly, the corresponding weight loss was recorded.
In order to study the thermo-molecular behavior of samples, a differential scanning calorimetry (DSC) (Shimadzu DSC-60, Japan) was used. The samples were heated from room temperature to 300 °C at a rate of 10 °C/min under nitrogen purge condition.

Effect of Ball Milling
Ball milling is low cost and environmental friendly, and is used worldwide in industrial scale. Planetary ball milling is a powerful and reliable method for reducing the size of fillers to the nanoscale. This mechanism is largely based on centrifugal accelerating force rather than gravitational accelerating force, resulting in a short time requirement for the process [39]. Moreover, several others parameters such as time of each cycle, total number of cycles, pause time between each cycles, milling balls size speed (RPM), material to ball weight ratio, nature of the fillers etc. can affect the final properties of the fillers. Therefore, understanding overall procedure of ball milling is complex and challenging in order to get desired shape and size of the fillers. This conditions (15 min' cycles-15 min' pause time between each cycle-total 99 cycles and 300 RPM) have been chosen after extensive trials of the experiment (data not shown).

Morphological, Elemental and Chemical Composition Analysis
The morphology of nanofillers is illustrated in Fig. 3 by TEM examination. Those nanoparticles revealed in a heterogeneous mix of quadrilateral and irregular shapes. This evidenced the ball milling treatment had effectively disintegrated the micro-fibers into varying nano-sized feature. Apparently, the pack-like structure of quadrilateral-shaped particle may impart them with highly preserved rigidity for advantageous application in reinforcing composite materials. In size measurement from TEM image, they presented a ranging size of about 30-110 nm in both width and length dimensions. It was similar to the findings from Saba et al. [39], who had obtained a nanofiller size up to 100 nm from oil palm empty fruit bunch fibers in a previous study, by treating the ground fiber material with bromine water and tin chloride solution, followed by cryocrushing and ball milling processes. As from Zetasizer analysis, those particles exhibited relatively consistent size at around 300 nm. Despite this size is far larger than the size shown in TEM image, the results is still considerably valid for Zetasizer analysis since this equipment is more accurately used in measuring round shape particles rather than nanofillers particles with diverse shapes. Another explanation was likely due to the poor dispersion of nanofillers in the aqueous solution and ultimately resulted in the agglomeration into larger particles. This was in agreement with the Nanoplus analysis results, where the particles showed three distinct sizes at 81.3 nm, 159.0 nm, and 850.2 nm, respectively, indicating the fillers As viewed by SEM microscopy (Fig. 4), the microfibers are presented as scattering particles, as a result of the deposition on carbon tape through ethanol solvent evaporation process. From surface morphology, the particles revealed a smooth texture, which promoted by the extensive impact and shear forces generated from mechanical ball milling. Meanwhile, the microfibers also showed random shape feature that could aid in promoting its fire retarding behavior as filler in composite materials due to non-uniform transmission/ conduction of heat at each point in time [39,40]. Moreover, a few microns in diameter and while tens of micrometer in length was displayed by the microfibers, suggesting the fiber is suitable acting as a micro-filler in composite matrix fabrication. For elemental analysis (Fig. 5), microfibers revealed carbon and oxygen as their major compositional elements, implying the fiber is mainly composed of hydrocarbon compounds. In addition, the traces for magnesium, silica, chloride, calcium and iron, indicating the typical residual elements for a plant biomass fiber. From chemical composition analysis (Table 1), nanofiller sample showed slightly increased α-cellulose, whilst with slightly declined hemicellulose and lignin contents comparing to microfiber. This was likely contributed by the effect of ball milling that somehow disintegrated the outer hemicellulose and lignin components on the surface layer [40,41].

Thermal Analysis
The thermo-stability of the samples is evaluated by TGA and DSC curves as depicted in Figs. 6 and 7, respectively. From TGA curve (Fig. 6), four distinct phases were revealed by both samples, i.e. moisture evaporation, hemicellulose decarboxylation, cellulose decomposition, and lignin pyrolysis. At beginning, both samples showed weight loss in 70-130 °C temperature regions, in corresponding to the evaporated moisture content. However, the microfibers lost more weight at this region when comparing to nanofillers. It was probably caused by the greater free volume in microfibers that had higher affinity in entrapping water molecules. In subsequent weight loss stage at 250-350 °C, it involved the decomposition process for hemicellulose and cellulose. This was attributing to their closely similar structural feature built-up with adjacent intermolecular linkage via physically twisting and hydrogen bonding [42]. Additionally, the microfibers exhibited slightly higher decomposition temperature at 263.2 °C, when compared to nanofillers at 260.8 °C temperature. It was possibly contributed by the lower crystallinity as well as the smaller nanoscale size of nanofillers that induced it for earlier thermal degradation. Between 350 and 500 °C, the liquefaction and gasification process happened for lignin since this compound had strong fire retardant behavior. At final phase, microfibers sample showed constant weight beyond 550 °C, whereas nanofillers continued to lose weight and only presented constant weight after 620 °C. This evidenced the thermal degradation is relatively consistent in nanofillers, which may endow it with tunable properties for high temperature fabrication process [41].
As for DSC analysis (Fig. 7), both samples showed broad endotherms extending from 70 to 140 °C temperature, which basically correlated to the water vaporization process [43]. The enthalpy heat gained by microfibers at this temperature range was 65.09 J/g, which remarkably larger than the nanofillers sample with 14.82 J/g. This showcased the microfibers required higher heat energy for evaporating larger water content that in line with the weight loss shown in TGA curve. Besides this, the nanofillers revealed second wide endotherm at around 230 °C with 25.33 J/g enthalpy heat, in correlating to the energy required to decompose cellulose-based compounds. But for microfibers, it was presented as horizontal curve rather than in endothermic band. This was possibly    crystallography [44]. It was in response to the distinctive lignocellulosic material with Iβ native cellulose structure. Meanwhile, the peak at 22.6° was observed broadening for nanofillers and had somehow overlapping the peak at 16.8° when comparing to microfibers sample. This indicated the crystals domain in nanofillers sample was decreased and the fiber bonding structure became homogeneous possibly resulted by the amorphization effect of ball milling [39,45]. From crystallinity measurement, the microfibers gave a crystallinity index of 71.8%, which slightly higher than the crystallinity of nanofillers with 68.9%. Nonetheless, the obtained crystallinity in this work is considerably high for nanofillers fiber (68.9%), evidencing its highly crystalline structure is still preserved although undergoing harsh mechanical grinding process. It would endow it with great rigidity for acting as load-bearing agent in material reinforcement application.

Conclusions
The major findings of present work revealed the nanofillers were successfully produced from date palm based microfibers by only using mechanical ball milling process without involving any chemical reagents and/or other  Besides this, the crystallinity analysis revealed the rigidity of nanofillers had decreased as compared to microfibers, showing the ball milling treatment could aid in improving fibers amorphous domains. Also, the thermal analysis showed that the microfibers had better stability of heat resistance comparing to nanofillers fiber, which affected by the reduced crystallinity and nano-sized structure. Nonetheless, when in view of the overall results, it suggests that the prepared nanofillers in this work can be utilized for reinforcing polymer composites in the future.