Physical appearance of treated and untreated palm fiberThe physical changes in palm fiber before and after chemical treatment were observed visually. Figure 1 shows the varied colors of fibers after treatment. Before treatment with benzoyl chloride, the UTF exhibited a dark brown color that gradually turned to a deep black. The change in color of natural fibers following treatment is associated with the breakdown of hemicellulose and lignin components from the fibers upon NaOH treatment22,23.Fig. 1Physical appearance of treated and untreated palm fiber.FTIR analysisFourier transform infrared (FTIR) spectra are a powerful way to see how benzoic acid chloride changes the chemical structure of palm fiber. Figure 2 displays significant differences in the FTIR spectra of untreated palm fiber (UTF) and palm fiber treated with benzoyl chloride (BTF). It is important to note that a drop in the absorption band is linked to the O–H stretching of hydroxyl groups in cellulose and hemicellulose, which happens around 3422 cm−1. This decrease shows that the reaction between these hydroxyl groups and benzoyl chloride went well, which changed the surface chemistry of the fiber22,23. At the same time, lower peaks at 2920 cm−1 and 2850 cm−1, which are connected to methylene group vibrations, show that aliphatic chains have changed. These changes, along with changes in the CH2 group bending vibrations (shown by the peak at 1460 cm−1), suggest that the molecular structure of the fiber has been completely changed24. Additionally, characteristic bands at 1120 cm−1 and 1046 cm−1, indicative of the elongation vibrations of C–O–C groups, are evident25. An indicator of the benzoylation process is the increased carbonyl band centered at 1737 cm−1. This prominent feature signifies the formation of ester linkages, a direct result of the reaction between hydroxyl groups and benzoyl chloride. The emergence of this band, coupled with new peaks at 1273 cm−1 and 1312 cm−1, provides strong evidence for the successful esterification of the palm fibers.Fig. 2FTIR spectra of untreated fiber (UTF) and benzoyl chloride-treated fiber (BTF).The appearance of a peak at 1605 cm−1, due to C=C stretching in aromatic rings, further confirms the introduction of benzoyl groups. This, along with the higher intensity at 710 cm−1, which shows aromatic rings with only one substituent, shows that benzene rings are part of the fiber structure. These modifications significantly alter the fiber’s surface properties and, potentially, its interactions with other materials18. An important consequence of the benzoyl chloride treatment is a reduction in the fiber’s hydrophilicity. The decreased band, typically associated with water absorption, evidences this. The lowered hydrophilicity can enhance the fiber’s compatibility with hydrophobic matrices, making it more suitable for certain composites applications26. Figure 3 schematizes the benzoyl treatment of palm fibers.Fig. 3Benzoyl treatment of date palm fibers by benzoyl chloride.Scanning electron microscopyFigure 4’s scanning electron microscope (SEM) pictures provide a close-up view of palm fibers without treatment (Fig. 4a) and those treated with benzoyl chloride (Fig. 4b). These micrographs clearly demonstrate how the chemical treatment altered the fibers’ shape and influenced their surface properties27.Fig. 4Micromorphology of palm fibers; (a) untreated fiber, (b) treated fiber.Figure 4a, depicting the untreated palm fiber, reveals a complex and irregular surface topography. The fiber surface is characterized by pronounced roughness, with numerous protrusions and indentations creating a highly-textured landscape. A substantial accumulation of impurities and dust particles, forming a thick, heterogeneous layer across the fiber’s exterior, further accentuates this roughness. The fiber’s surface structure deeply embeds these contaminants, likely comprising various polysaccharides, extractives, and other non-cellulosic components. Such impurities not only contribute to the fiber’s irregular appearance, but also play a significant role in its physical and chemical properties. The presence of these surface contaminants and structural irregularities in the untreated fiber can have a number of negative effects. Firstly, they act as potential stress concentration points, creating weak spots in the fiber’s structure that could initiate or propagate fractures under mechanical stress. Additionally, these impurities significantly impair the fiber’s ability to form strong, coherent interfaces with polymeric matrices in composite materials. The inconsistent surface chemistry and topography hinder effective wetting and bonding, resulting in poor stress transfer between the fiber and the surrounding matrix. Unfortunately, this weak interfacial adhesion can make composites that use these untreated fibers perform badly and last a long time28,29In stark contrast, Fig. 4b showcases the remarkable transformation achieved through benzoyl chloride treatment. The treated fiber exhibits a dramatically altered surface morphology, characterized by a notably cleaner and more uniform appearance. The most striking change is the significant reduction in visible surface impurities and contaminants. The treatment process has effectively stripped away the layer of non-cellulosic materials, exposing a more homogeneous fiber surface.The benzoyl chloride treatment appears to have induced a chemical etching effect on the fiber surface. This etching process not only removes impurities but also modifies the underlying cellulose structure, potentially increasing the surface area available for bonding. The resulting surface is markedly smoother, with a more consistent texture that suggests improved uniformity in both physical structure and chemical composition. As Fiore et al.30 reported, removing lignin, wax, pectin, and hemicellulose contributes to an augmented roughness of the fiber surface. This increased roughness, in turn, improves the adhesion of polymers within composites materials. After the treatment, the observed phenomenon was a reduction in fiber size. This shrinking will help the fibers reduce moisture absorption22.Figure 5a–c presents SEM micrographs of the neat PVC and their composites. At first observation, the SEM morphology of all PVC/palm fiber composites appeared similar yet significantly different from pure polyvinyl chloride resin. Adding date palm fibers to PVC composites caused the surface to become more rough and irregular. Figure 5b illustrates that including 10 wt% UTF fillers roughens the surface compared to pure PVC resin. In addition, the SEM micrograph of PVC-UTF composites showed clumping, fiber pull-outs caused by the incompatible palm filler’s poor dispersion and minimal adherence, and the formation of voids that promote the production of fragile interfaces or critical stress intensity points, which allow cracks to begin and spread, resulting in composites failure and a diminution in the value of mechanical properties31,32.Fig. 5Micromorphology of PVC/palm fiber composites; (a) neat PVC, (b) reinforced with untreated fiber (PVC-UTF) and (c) reinforced with treated fiber (PVC-BTF).The fracture surface of the PVC-BTF composites, where the fibers were treated with benzoyl chloride (Fig. 5c), reveals a pattern of fiber breakage rather than pull-out that indicates a more effective adhesion between the surfaces of composites reinforced with treated fibers, with the PVC-BTF composites demonstrating superior performance. In contrast to other composites, the bond strength between the PVC matrix and BTF fibers was significantly higher, and no visible voids were observed at the fracture interfaces. The benzoylation process was crucial in toughening the fiber surface and strengthening the bond between the PVC matrix and the fibers. This heightened bonding was attributed to decreased hydroxyl groups, fostering improved fiber cohesion9,33.Thermal propertiesFigure 6 presents the thermal analysis (TGA) and derivative (DTG) curves for untreated (UTF) and benzoyl chloride-treated fibers (BTF). The thermal degradation behavior is illustrated through four distinct stages: initial dehydration, removal of hemicellulose and a portion of lignin, alpha-cellulose decomposition, and lignin pyrolysis34. The annotated curves clearly indicate the weight loss percentages and temperature ranges for each stage, with treated fibers showing higher thermal stability and shifted degradation temperatures.Fig. 6Thermogravimetric and derivative thermogravimetric curves of untreated fiber (UTF) and benzoyl chloride-treated fiber (BTF).During the initial Stage of decomposition, occurring between 50 and 150 °C, the fibers undergo dehydration, resulting in a 7.16% mass loss within the temperature range of 50.62 °C to 148.81 °C. The observed mass loss primarily results from the evaporation of volatile extracts within the fibers. The significant rise in moisture loss at elevated temperatures suggests robust hydrogen bonding between cellulose and water, indicating an increased abundance of hydroxyl groups OH. It underscores the inherently hydrophilic nature of natural fibers18,26. Not only that, but the temperature at which benzoyl chloride-treated fibers (BTF) break down is much higher than that of untreated palm fibers (UTF), which causes a slight drop in weight (2.54%). This weight reduction indicates a decrease in fiber moisture content after treatment, highlighting the effectiveness of the benzoyl chloride treatment process.The final three stages are associated with the deterioration of hemicellulose, lignin, and cellulose linkages as the temperature increases22. The second Stage, observed at 210 °C to 303 °C for raw fibers and 156 °C to 242 °C for treated fibers, signifies the removal of hemicelluloses and a portion of lignin35. Alpha-cellulose decomposition occurs in the temperature range of 320–430 °C for untreated fibers, while this process is observed within the 280–444 °C range for benzoyl-treated fibers. These temperature ranges indicate the thermal stability improvements achieved through benzoyl chloride treatment, as evidenced by the shifted decomposition stages and elevated degradation temperatures. During this degradation phase, the UTF exhibits a weight loss of 57.03%, whereas the BTF experiences a weight loss of 64.02%. The maximum degradation temperature of benzoyl-treated palm fibers exhibits an increase, measuring 390.72 °C, compared to the raw fibers, with maximum degradation temperature of 349.71 °C. Cellulose undergoes a slow decomposition process at lower temperatures. In contrast, cellulose molecules are rapidly disintegrated at high temperatures, resulting in the decomposition of glycosyl units and the subsequent production of levoglucosan36,37.During the fourth Stage, two minor peaks were detected at temperatures of 409 °C and 470 °C, corresponding to the stages of lignin degradation and pyrolysis, respectively38. At temperatures exceeding 500 °C, the molecules undergo decomposition, forming diverse low molecular weight substances, including CO2, CO, H2O, hydrocarbons, and hydrogen (H2)37. Untreated fibers exhibited a residue weight of approximately 28.46% at a temperature of 600 °C, whereas the benzoyl chloride-treated palm fibers showed a residue weight of 27.86%. The benzoyl fiber treatment did not increase thermal stability, resulting in a lower temperature than UTF. Fewer decomposition peaks for treated palm fiber demonstrated the elimination of specific components from the fiber during treatment22. That is due to the varied degradation tendencies of benzoyl-treated fiber molecules. The dissociation of C–C chain bonds and H-abstraction at the dissociation site39 contribute to the intricate thermal behavior observed. Providing a comprehensive summary, Table 2 offers valuable insights into the thermal degradation characteristics of treated (BTF) and untreated (UTF) palm fibers. The tabulated information encompasses critical parameters such as the initial degradation temperature, percentage weight loss, and percentage residue, elucidating the impact of benzoyl chloride treatment on the thermal stability of the fibers.Table 2 Inflection temperatures related to the maximum rate of degradation of treated and untreated palm fibers.Figure 7 displays the TGA and DTG curves for neat PVC and PVC composites reinforced with treated and untreated palm fibers. The curves demonstrate two distinct stages of thermal decomposition: the removal of Hydrogen Chloride molecules and the degradation of polyene into a carbon black residue. The annotated curves provide clear demarcations of the onset and maximum decomposition temperatures, with PVC composites showing enhanced thermal stability due to improved adhesion between the matrix and fibers. The corresponding thermal analysis data, summarized in Table 3, provides valuable insights into the distinctive thermal characteristics of PVC and its composites. The thermal decomposition of the polyvinyl chloride matrix happens in two distinct stages: the first one initiates at 257.75 °C and concludes at 367.06 °C, reaching a peak decomposition temperature of 310.75 °C. In this phase, there is a weight loss of 53.60%, attributed to the removal of Hydrogen Chloride molecules and the creation of double bonds of polyene within the PVC chains. The subsequent degradation stage, spanning from 416.55 to 559.22 °C, involves the degradation of polyene into a carbon black residue, which remains stable above 559.22 °C and contributes to 27.15% of the total weight40,41.Fig. 7Thermogravimetric and derivative thermogravimetric curves of neat PVC and composites reinforced with untreated fiber (PVC-UTF) and benzoyl chloride-treated fiber (PVC-BTF).Table 3 Decomposition temperatures and weight loss levels of neat PVC and PVC-treated and untreated palm fibers composites.Both treated and untreated PVC composites reinforced with palm fiber exhibit a marginal weight loss at approximately 100 °C, attributed to the evaporation of water present in UTF and BTF fibers. The introduction of palm fiber decreases the onset decomposition temperature, estimated at 238.36 °C and 231.76 °C for PVC-UTF and PVC-BTF, respectively. This reduction is attributed to the chain scission of composites, leading to increased release of hydrochloric acid (HCl) and the decomposition of holocellulose (cellulose and hemicellulose)42. Enhanced adhesion between the PVC matrix and cellulose fiber leads to enhanced contacts, reducing the composites’ thermal stability. This is corroborated by the degradation process of composites, in which the deterioration of one component in a composite has a multiplicative effect on the deterioration of all other components41. The second peak at approximately 454.36 °C is evident in treated and untreated composites. This phase is accompanied by a weight loss between 52.01% and 51.18%. The decomposition process involves the breakdown of lignin, the residual residue, and the production of ashes, indicating the comprehensive combustion of the composites9,43.Mechanical characterizationTensile testingFigure 8 illustrates neat PVC and composites’ tensile strength and modulus before aging. It is noteworthy that the incorporation of date palm fiber into PVC results in a reduction in tensile strength from 12.8 ± 1.12 MPa for pristine polyvinyl chloride to 11.3 ± 0.48 MPa for untreated composites, specifically PVC-UTF. The decrease in tensile strength observed in untreated composites (PVC-UTF), compared to the pristine polyvinyl chloride (PVC), can be ascribed to the compromised interfacial adhesion within the composites. Conversely, reduced adhesion between the matrix and filler at the interface results in their separation, enhancing deformability and causing an increase in deformability19,44. The tensile strength trend reveals a notable enhancement in treated PVC-BTF composites compared to untreated PVC-UTF composites. These findings suggest an improved compatibility between hydrophilic cellulose fibers and the hydrophobic PVC matrix following the application of benzoyl chloride treatment. The improvement in tensile strength properties, observed in treated composites (PVC-BTF) attributed to the modification of palm fibers surface. The benzoylation treatment reduces the presence of free OH groups on the fiber interface. Consequently, it amplifies the hydrophobic properties of the fibers, promoting improved interaction with the matrix. This, in effect, augments the mechanical properties of the composites, aligning with observations documented in prior literature13.Fig. 8Ultimate tensile strength and modulus of neat PVC and composites.The tensile modulus of the composites exhibits an increase, rising from 112.2 ± 11.06 MPa for polyvinyl chloride (PVC) to 304.5 ± 7.26 MPa for the treated composites. It is anticipated that benzoyl modification will facilitate the removal of solidifying materials, including lignin and natural oils, from the fiber’s surface45. This removal process results in enhanced packing of cellulose chains, increased fiber surface roughness, and enhanced contact area between the fiber and polyvinyl chloride (PVC)30. Therefore, this chemical modification has been found to have a beneficial effect on the mechanical properties of the composites46. An elevation in stiffness was also noted in untreated samples, attributed to the inherent rigidity of fibers compared to the polymer. This increased stiffness contributes to a stiffer composite without altering its strength. These outcomes align with findings reported in9,47.Flexural testingFigure 9 illustrates the flexural strength and modulus variations for PVC composites reinforced with treated and untreated palm fibers. The results display a similar trend to the tensile strength findings. The benzoyl-treated PVC-BTF composites exhibit the highest flexural strength at approximately 2.63 ± 0.04 MPa. The enhanced flexural strength observed in benzoyl-treated PVC-BTF composites can be ascribed to improved homogeneity, adequate wetting, and enhanced interfacial bonding between benzoyl-treated date palm fibers (BTF) and the PVC matrix under compressive loads during bending. Additionally, the fiber-matrix interface has increased transfer capacity in the reinforced composites with treated fibers47,48.Fig. 9Flexural Strength and modulus of neat PVC and composites.The flexural modulus, as depicted in Fig. 9, has experienced a positive increase due to the reinforcement with palm fibers. This increase can be attributed to the enhanced strength of the reinforcement material. The flexural modulus is primarily influenced by the properties of the matrix and the bonding between them and fibers49. It has been observed that the treatment of benzoylation significantly influences the flexural modulus. The treated composites PVC-BTF exhibited the highest flexural modulus (88.66 ± 4.10 MPa) compared to the PVC-UTF composites and pure PVC. The application of the treatment resulted in significant alterations on the surface of the palm fiber. These alterations included the elimination of alkali-soluble components and a decrease in the thickness of the palm fiber.Consequently, numerous small voids were formed on the fiber surface45. The benzene rings in the benzoyl group (C6H5O) instead of hydroxyl groups attached to the fibers enhance the hydrophobic nature of the fiber surfaces17. Thus, enhancing the adhesion between the surfaces of palm fiber and the PVC matrix contributes to improving the flexural modulus in PVC-BTF composites 50.Hydrothermal aging of compositesHumidity aging is widely recognized as a significant factor contributing to the degradation of organic materials exposed to air or in contact with water over time. Several mechanisms of aging caused by dampness have been acknowledged, including matrix plasticization, differential enlargement due to concentration gradients, and damage at the matrix/fiber interface51,52.The curves shown in Fig. 10 depict the water uptake and the trend of loss of tensile strength under exposure to hydrothermal aging versus the square root of time for the virgin PVC and composites. By comparing the mass gain plots of specimens, it can be seen that the influence of increased temperature on the diffusion characteristics is very apparent. The observed phenomena may be explained by the widely recognized impact of temperature on the thermodynamic contribution, known as the “Soret effect” of diffusion substances. Elevated temperatures expedite the process of water absorption53. Since the PVC matrix is a hydrophobic material, the rate of water uptake was less than other composites and did not exceed 0.13%54. The untreated specimens PVC-UTF show higher absorbed water, about 1.23%, after 5 h of immersion in water at 50 °C. The observed phenomenon can be attributed to the gaps and micro-voids between UTF and PVC composites, which provide the channels through which water can pass, causing plasticization and reducing the glass transition temperature55,56. The untreated palm fiber shows a significant amount of OH groups, which readily interactions with water molecules forming hydrogen bonds, leading to swelling and weakening of the fiber structure as described in Fig. 11. This interaction can also disrupt the fiber-matrix interface, leading to debonding and further mechanical degradation. Consequently, the increase in the concentration of hydroxyl groups corresponds to an elevated water absorption rate24.Fig. 10Weight uptake and loss of tensile strength after hydrothermal aging of neat PVC and composites.Fig. 11Formation of hydrogen bonds between cellulose molecules and water molecules.The benzoyl treatment significantly reduced water absorption for the PVC-BTF composites, amounting to 0.65%. Using benzoylation treatment enhanced the hydrophobic nature of PVC composites reinforced with palm fiber, decreasing mass absorption. This reduction in weight absorption is explained by the hydrophobic nature imparted by benzoyl groups on the fiber surface18,21,57. The trend of tensile strength loss (Fig. 10) is the same for PVC, PVC-UTF, and PVC-BTF composites after hydrothermal aging for 5 h at 50 °C. Observing untreated fiber-reinforced PVC composites (PVC-UTF) shows that when specimens are submerged in water, their tensile strength drops sharply, from 11.3 MPa to 8.85 MPa. This drop in tensile strength, which is around 21.68% compared to the initial tensile strength, is due to the swelling of palm fibers along water absorption, water gets into composites by diffusing into the matrix through capillarity at the fiber-matrix interface. Hydrophilic parts of the matrix then absorb the water and move along the inside fibers. Then, palm fibers caused a visible growth because the lumen physically grew and cellulose molecules mixed with water molecules. When it became big enough, matrix cracked around the swollen fibers. Over time, as the material aged, pectins, hemicelluloses, and some celluloses that weren’t fully crystallized started to leak out of the palm fibers. This caused the fiber-matrix interface to delaminate completely. Lastly, the different ways that each cell wall layer swelled caused the strengthening fibers to split and peel off, which reduced the fiber/matrix interface and caused further damage58,59. Nevertheless, treated PVC-BTF composites exhibit a slightly lower tensile strength than untreated composites. Karlsson et al.60 suggest that fibers tend to swell when water infiltrates the interior of composites materials, leading to matrix structure alterations through chain reorientation and contraction. This phenomenon results in decohesion, weakening interfacial adhesion, and ultimately diminishing mechanical properties.This drop in tensile strength, which is around 21.68% compared to the initial tensile strength, is due to the swelling of palm fibers along water absorption, which reduced the fiber/matrix interface and caused further damage58,59. Nevertheless, treated PVC-BTF composites exhibit a slightly lower tensile strength than untreated composites. Behera et al.60 suggest that fibers tend to swell when water infiltrates the interior of composite materials, leading to matrix structure alterations through chain reorientation and contraction. This phenomenon results in decohesion, weakening interfacial adhesion, and ultimately diminishing mechanical properties.
