Sustainable bio-nanocomposite from lignocellulose nanofibers and HDPE for knee biomechanics: A tribological and mechanical properties study

2.1 Materials

In this work, a variety of materials, including HDPE powder, sodium stearate, polyethylene graft maleic anhydride (MAPE), and lignocellulose nanofiber CNF-L, were used. HDPE powder is a thermoplastic polymer with high density and good wear resistance and is widely used in the plastics industry. This powder was provided by Sigma-Aldrich, France. Sodium stearate (CH₃(CH₂)₁₆COONa, purity ≥92%) is an organic compound and the sodium salt of stearic acid. It is commonly used as a surfactant or lubricant. This material was supplied by Roth Company in France. MAPE is an anhydride-modified polymer used as a crosslinking agent to improve the properties of polymers with a viscosity of 500 cP at 140°C (liters). CNF-L was developed and obtained from rice straw fiber. Rice straw was collected from Giza ranches in Egypt. To remove soil, the straw was rinsed with drinking water and allowed to dry in the air. Chemical analysis and processing included the use of glacial acetic acid, sodium sulfite, sodium chlorite (80% purity), and sodium carbonate. Supplies were provided by Fisher Scientific, UK [30].

2.2 Nanocomposite preparation

Three mechanical processes – a one-step injection molding technique, a two-phase twin-screw extrusion process, and an efficient operation – were used to produce nanocomposites as previously documented in the scientific literature [31,32]. All these preparation steps synthesize compounds and dilute the extruded nanocomposite masterbatch. A masterbatch consisting of HDPE was processed in a DSM-Xplore15cc twin-screw micro-extruder (Xplore Instruments BV – The Netherlands). Extrusion was performed under a temperature gradient of 200–230°C to produce nanocomposites containing freeze-dried CNF-L as nanofiller, MAPE as compatibilizer, and sodium stearate as plasticizer. A recirculating loop extruder was used to extract the nanocomposites have different contents from the barrel over the course of 10 min at a constant screw speed of 50 rpm. Fresh polymer matrix (and MAPE for additive compositions) was used to additionally dilute HDPE/CNF-L composite to 0.5, 1.0, 1.5, and 2.0 wt% CNF-L from the masterbatch. The extrusion process was performed with 2.0 wt% sodium stearate. The names of the samples and the contents of fillers and additives from this experiment are shown in Table 1. The originally extruded material was cut into small pieces, melted at 200°C (heater mold temperatures), and then injected using a Haake Minijet II injection molding machine (Thermo Fisher Scientific, Karlsruhe, Germany) for 15 s at a pressure of 550 bar. The traction profile and the 25 mm × 1.5 mm disks were measured.

3.1 X-ray diffraction

The reflectance data of the extruded nanocomposite samples were collected at room temperature using an XRD instrument (model PANalytical – Netherlands) with a CuK anode (λ = 0.154 nm) set at 45 kV and 30 mA. The patterns were recorded between 4° and 80° using the fixed pattern technique, which is time-consuming and has a phase separation of 0.02°. The amorphous part of any material is usually visualized as a halo. The amorphous halo and crystallographic peaks were analyzed using the X’Pert High Score Plus software [33] and calculated using the equation:

3.2 DSC

DSC was performed on the samples studied using thermal phase evaluation equipment (model PerkinElmer – USA). The evaluation can provide valuable insight into melting temperature, crystallization, and thermal stability. All samples underwent a heating–cooling–heating cycle in an N₂ environment, within the allowable range of −100 to 250°C, at a rate of 10°C/min.

3.3 WCA

A fully computerized optical system (model OCA 40 Micro-Japan) was used to determine the contact angles of the injection molded nanocomposites using 2 ml of distilled water per drop. The contact angles of each water droplet were measured at 22°C and 45% relative humidity, and an animation was recorded at 72 fps. Such a technique allows precise monitoring of even the smallest changes on the droplet surface, resulting in accurate contact angle estimates down to 0.1°.

3.4 Mechanical characterization

The mechanical parameters of the dumbbell-injected specimens, particularly yield strength, elongation at break, and modulus of elasticity, were evaluated using standard testing equipment (model Instron 5965, USA) in accordance with ASTM D638. The samples were annealed for 48 h at 25°C and 50% humidity prior to testing. The initial distance between the two air jaws was set at 10 mm with a crosshead speed of 2 mm/s, and tests were performed on five specimens.

Shore D durometer is used to determine the hardness of HDPE samples based on ASTM D2240, which applies a standardized force to evaluate the resistance of the surface to indentation. Five locations were performed to evaluate the depth of indentation on each sample to calculate the hardness of the injection molded nanocomposite samples, and then, each test was calculated as an average.

3.5 Tribological characterization

An ASTM G99 pin-on-disk tribometer was used to determine the COF and wear of specimens of nanocomposite materials in a dry state. In the apparatus, a 180 mm diameter, 3 mm thick stainless-steel plate serves as a mating disk. A surface roughness tester is used to accurately measure the surface roughness of a backing plate: R _z = 0.179 μm and R _a = 0.023 μm. Disk-shaped nanocomposite specimens were used instead of a stationary pin in the experiment. The tribological characteristics of each specimen were investigated by adjusting the normal load measurement variables (2, 4, 6, 8, and 10 N) while maintaining a constant sliding speed of 0.4 m/s. The COF result of each test was the average of the collected readings. Wear loss tests were performed every 2 min. An electronic balance with an accuracy of 0.0001 g was used to measure the initial and final weights of the specimens. The weights and losses of the specimens were reported. In addition, the worn surfaces were examined using a Jeol’s table SEM (model SEM JSM-6000 – Japan) and an optical microscope (model Olympus BX53M – Japan).

4.1 Characterizations of CNF-L

Optical images of the pre-grinding fibrillation process of unbleached rice straw, in which microscopic particles and partial nanofibers appear, are shown in Figure 1. For unbleached pulp, the refining process starts early and lacks extensive fiber degradation. The article also explores the properties of nano paper made from the isolated nanofibers. In the final refining step, unbleached rice straw showed no non-combined fibers [34]. The final CNF-L ring was detected by SEM, as shown in Figure 2(a). A slight disappearance of microfibrils was observed in the CNF-L, simultaneously indicating the presence of nanofibrils. The CNF-L measurements were further refined using atomic force microscopy (AFM) of air-dried, highly diluted (0.001%, w/w) suspensions deposited on freshly bonded mica surfaces, as shown in Figure 2(b). The AFM images showed highly homogeneous nanofibrils, irrespective of the initial fiber shapes used before grinding [35].

Figure 1

Optical microscope images of (a) unbleached rice straw before fibrillation and (b) unbleached during the grinding process.

Figure 2

(a) AFM and (b) SEM scan lignocellulose nanofibers of CNF-L.

4.2 XRD analysis of HDPE/CNF-L nanocomposites

Figure 3 shows X-ray diffractograms illustrating the influence of various CNF-L-based HDPE samples on crystallinity. The refraction planes of the orthorhombic unit cell (110) and (200) show two separate peaks at angles of 23.8 and 21.46°, respectively [36]. The presence of both crystalline and amorphous regions in the cellulose chains is confirmed by the semi-crystalline pattern exhibited by the CNF-L peaks [37]. The gradient amount of CNF-L filler causes little change in the (110) and (200) planes – all concentrations – 0.5, 1.0, 1.5, and 2.0%. The diffraction pattern for each sample showed a slight shift in the diffraction peaks as the nanocellulose fiber content was increased. These results show that the orthorhombic crystal structure of HDPE was not affected by the addition of CNF-L. The overlapping peak of HDPE and lignocellulose nanofibers at approximately 22° in 2θ was previously published research using CNF-L filler in the HDPE matrix [38,39].

Figure 3

X-ray diffraction patterns of HDPE/CNF-L nanocomposites.

4.3 DSC thermograms

The DSC thermograms of the prepared nanocomposite samples are highlighted in Figure 4. It was shown that the melting and crystallization temperatures of the composites were almost the same as those of the free matrix. It was found that the thermal stability of the samples was maintained. While DSC is a relative method and not as accurate as XRD for identifying the level of crystallinity of the polymers. It is evident that an increase in the degree of crystalline was detected in the composites, as described in the XRD part.

Figure 4

DSC of HDPE/CNF-L nanocomposites.

4.4 WCA

The WCA results measured as an average value for the native HDPE, 0.5, 1.0, 1.5, and 2.0 wt% CNF-L composites are shown in Figure 5. The incorporation of lignocellulosic CNF-L greatly increased the hydrophilicity, as indicated by the WCA values of the CNF-L nanocomposite samples, which ranged from 67° to 83°. This is probably a consequence of the hydrophilic identity of cellulose and the presence of hydroxyl groups [40]. Thus, the nanocomposites are more hydrophilic than HDPE at all concentrations. The increased hydrophilicity of these nanocomposites is supposed also would aid in cell adhesion and culture [41,42].

Figure 5

WCA of HDPE/CNF-L nanocomposites.

4.5 Mechanical properties of HDPE/CNF-L nanocomposites

The mechanical parameters of the derived HDPE nanocomposites are shown in Table 2, and the stress-strain graph is shown in Figure 6(a), respectively. According to these experiments, the yield strength showed that the samples containing CNF-L exhibited excellent elongation, Young’s modulus, and average tensile strength. The results showed that the properties of CNF-L applied to HDPE increased steadily between 0.5 and 2% by weight. For CNF-L loading contents of 0.5, 1.0, 1.5, and 2.0%, the tensile stresses of these nanocomposites increased by 1.82, 18.44, 22.6, and 23.4%, respectively. In addition, significant increases in Young’s modulus and elongation at break were observed in the samples compared to the neat HDPE samples. The perceived increase in strength may be associated with the use of a coupling agent, maleic anhydride grafted polyethylene (MAPE) [43], as well as with the implementation of a twin-screw extrusion and injection molding processes utilizing a masterbatch. These approaches have been shown to be effective in facilitating the dispersion of cellulose nanofibers within HDPE [44,45]. The importance of composites is expressed by their respective degrees of hardness, which serve as indicators of their ability to withstand compression. This property may be more important than composite tensile strength when used them for prosthetic joints manufacturing [46].

Figure 6

Mechanical experiments of HDPE/CNF-L nanocomposites: (a) stress–strain curve and (b) hardness value.

Figure 6(b) shows a comparison of the Shore D hardness values between HDPE and nanocomposites, as indicated by the graphical representation. The observed performance of the 2 wt% HDPE/CNF-L composite can be attributed to its increased hardness value combined with a satisfactory tensile strength value, resulting in an improvement of 18.5%. In addition, except for CNF-L at a concentration of 0.5%, which has a slightly lower hardness value than HDPE, the hardness values of all samples show a direct correlation with their respective mechanical properties. It is possible that the results will clarify why fillers improve the mechanical properties of nanocomposites.

4.6 HDPE/CNF-L nanocomposites’ tribological performance

To evaluate the friction performance, tribological experiments were performed on a tribometer at different normal loads at the same sliding speed. Figure 7(a) shows the COF, which is a fundamental measure of how fast two moving surfaces can slip against each other. An important measure of the lifetime of friction materials is their wear resistance (volume loss) [47]. The wear loss values of the nanocomposites are shown in Figure 7(b), considering the wear loss of the unmodified HDPE. In general, both wear loss and COF tend to increase with the average applied force. As can be seen, the application of CNF-L in this study reduced the wear loss and the COF of the nanocomposites. As shown in Table 3, the incorporation of CNF-L into the HDPE resulted in a significant improvement in both wear resistance and frictional property. This fact can be attributed to several things, the main of which is the addition of lignocellulose to the HDPE matrix. This admixture greatly improves the mechanical and tribological properties of the material due to the increased strength of the fibers and their ability to tolerate flexural stress, as reported in the literature [48]. In addition, the increased surface area-to-volume ratio of nanocellulose fibers improves particle bonding in composites. Tribological performance can be improved through stronger bonding by enhancing properties such as hardness and crystalline [49]. The inherent property of hydrophilicity helps to reduce the COF [50]. Table 3 shows a remarkable improvement in COF and weight loss observed in the samples of our HDPE/CNF-L nanocomposite. The rice straw fibers have a remarkable silica concentration, which has the potential to improve the tribological characterization [51,52].

Figure 7

(a) COF and (b) weight loss of HDPE/CNF-L nanocomposites.

Through visual inspection of the worn surfaces shown in Figures 8 and 9, it is evident that the surfaces of the nanocomposites are smoother than the surfaces of the native HDPE. Compared to other materials, the composites appear to have less wear due to their flat surface. Regarding the wear volume loss of the nanocomposites, the previous statement is supported by the 3D photographs of the worn surfaces, as shown in Figure 8. Compared to the native HDPE without nanofibers, the worn surfaces generally showed less wear after the addition of the lignocellulose CNF-L. As shown in Figure 9, SEM images were taken to evaluate the worn surfaces in more detail. The worn surface of the native sample appears extremely degraded due to areas of plowing and plastic morphology. The SEM images show that the interface surfaces of the nanocomposite samples are not as damaged as those of the free HDPE sample. It is evident that the use of CNF-L filler serves to improve the mechanical properties of the composites, especially durability and shear resistance. The surface of the sample loaded with 1.5 wt% filler shows a favorable and uniform texture, suggesting that a specific substance referred to as CNF-L is the most effective filler content.

Figure 8

2-D and 3-D scans of the contact surfaces of HDPE/CNF-L nanocomposites: (a) native HDPE, (b) HDPE/0.5 wt% of CNF-L, (c) HDPE/1 wt% of CNF-L, (d) HDPE/1.5 wt% of CNF-L, and (e) HDPE/2 wt% of CNF-L.

Figure 9

SEM scans of the contact surfaces of HDPE/CNF-L nanocomposites: (a) native HDPE, (b) HDPE/0.5 wt% of CNF-L, (c) HDPE/1 wt% of CNF-L, (d) HDPE/1.5 wt% of CNF-L, and (e) HDPE/2 wt% of CNF-L.