Optimization of oil palm boiler ash waste and zinc oxide as antibacterial fabric coating

2.1 Materials

OPBA was obtained from PT. Dhajaja Putra Indonesia (North Sumatra, Indonesia). ZnO (density: 5.606 g·cm⁻³), NH₄OH, maleic acid, and sodium hypophosphite were purchased from Sigma Aldrich. Cotton fabrics, 5 M HCl, NaOH, acetic acid, and aqueous solutions were obtained from Medan (Indonesia).

2.2 Methods

3.1 FTIR analysis

FTIR spectroscopy was utilized to evaluate the chemical interactions within the fabricated composites, as illustrated in Figure 1. Distinct vibration bands were found at 736.95, 1710.86, 2906.73, 2968.45, and 3429.43 cm⁻¹ in cotton samples treated with OPBA and ZnO. The peaks observed are connected to the stretching movements of O–H and C–H bonds, indicating the presence of natural cotton materials such as cellulose, lignin, and hemicellulose [16]. The FTIR spectrum of the untreated cotton fabric (Figure 1a) displayed distinct peaks associated with cellulose, the primary structural component of cotton. An absorption band around 3,429 cm⁻¹ was found, indicating that there are stretching movements of O–H bonds related to the hydroxyl groups. The peaks at 2,968 and 2,906 cm⁻¹ correspond to symmetric and asymmetric C–H stretching vibrations, respectively, while the peak at 1,710 cm⁻¹ is attributed to C═O stretching, likely originating from hemicellulose or pectin residues. A prominent peak at 732 cm⁻¹ was identified as indicative of C–H bending vibrations. The assignments align with previously documented FTIR spectra of cellulose-based fibers [17].

Figure 1

FTIR spectra of untreated and coated cotton fabric.

The application of the OPBA–ZnO nanocomposite on the cotton fabric resulted in noticeable spectral alterations. In the OPBA:ZnO (1:1)-coated sample (Figure 1b), the O–H stretching peak at approximately 3,429 cm⁻¹ exhibited diminished intensity or broadening. In the OPBA:ZnO (1:2) sample (Figure 1c), new peaks emerged around 1,743 cm⁻¹, and a shift was observed at approximately 736 cm⁻¹, indicating the presence of additional functional groups. The spectrum of the sample that was only coated with ZnO (Figure 1d) showed a peak at 3,568 cm⁻¹, which was caused by water molecules sticking to the ZnO surface. A peak at 1,643 cm⁻¹ was observed, likely attributed to C═O stretching from surface-bound CO₂ or other organic compounds. A prominent peak at 736 cm⁻¹ was attributed to Zn–O stretching vibrations [18]. The characteristic peaks of cellulose were significantly reduced in this sample.

The FTIR results validate the effective functionalization of cotton fabrics using the OPBA–ZnO nanocomposite. The decrease in the strength of the O–H stretching in the OPBA:ZnO (1:1) sample suggests that there may be hydrogen bonding or interactions between the hydroxyl groups in cellulose and the coating materials. The emergence of new absorption bands and peak shifts in the OPBA:ZnO (1:2) sample suggests the introduction of additional chemical groups, likely derived from excess ZnO or OPBA [19]. The detected spectral differences in ZnO-only-coated fabric provide more insight into the coating procedure. The weakening of cellulose signals and the strong peaks associated with ZnO suggest that the ZnO layer covers the cotton fibers in a smooth or partially protective manner. This effect is explained by infrared-reflective qualities of ZnO, which may reduce or obscure infrared absorbance from underlying materials. Prior studies have confirmed the superior UV and infrared blocking properties of ZnO-coated textiles, supporting their use in functional fabric applications [20].

3.2 Fabric morphology analysis

The surface morphology of both untreated and coated cotton fabrics was analyzed using SEM, as illustrated in Figure 2. The untreated cotton fabric (Figure 2a) displayed a characteristic morphology, where the fibers presented a relatively smooth surface, accompanied by natural twists and convolutions [21]. The fiber surfaces were observed to be clean and devoid of notable surface deposits or agglomerates. Coating the fabric with the OPBA–ZnO nanocomposite resulted in notable morphological changes. In the OPBA:ZnO (1:1) sample (Figure 2b), the fiber surfaces became rougher, and there was material connecting each fiber, indicating a network-like pattern. In the OPBA:ZnO (1:2) sample (Figure 2c), there was an increase in surface roughness, accompanied by a greater amount of material observed between the fibers. The coating exhibited a relatively uniform distribution; however, the possibility of localized areas with higher material concentration or agglomeration cannot be excluded [22]. The fabric coated only with ZnO (0:1, Figure 2d) showed a different structure, marked by clumps of NPs that were not evenly spread on the fiber surfaces. A summary of the main findings of SEM is shown in Table 2.

Figure 2

SEM of (a) untreated cotton fabric, (b) coated cotton fabric OPBA:ZnO (1:1), (c) coated cotton fabric OPBA:ZnO (1:2), and (d) coated cotton fabric OPBA:ZnO (0:1).

SEM analysis confirmed the successful deposition of the OPBA–ZnO nanocomposite coating. The increased roughness and interfiber bonding observed in the OPBA:ZnO-coated samples (1:1 and 1:2) indicated that the nanocomposite adhered well to the cotton fibers. The more pronounced surface texture in the 1:2 sample indicated a higher ZnO content, likely due to the higher ZnO content in the formulation. The network-like structure in the 1:1 and 1:2 samples indicated increased interfiber connectivity, which could enhance surface functionalities such as hydrophobicity or photocatalytic activity [17]. In contrast, the ZnO agglomerate without OPBA (0:1 sample) exhibited poor dispersion and reduced coating uniformity. This highlights the crucial role of OPBA in facilitating the even distribution of ZnO NPs and minimizing aggregation [23].

The elemental compositions of untreated and coated cotton fabrics were analyzed using energy-dispersive X-ray spectroscopy (EDX), with the findings presented in Table 3. The untreated cotton fabric predominantly comprises carbon (81.2 wt%) and oxygen (18.8 wt%), aligning with the cellulosic composition of cotton. Following the application of the OPBA–ZnO nanocomposite, supplementary elements were identified in the textiles. In the OPBA:ZnO (1:1) sample, components including Si (0.5 wt%), P (0.4 wt%), Ca (0.1 wt%), and Zn (0.2 wt%) were detected, in addition to increased concentrations of oxygen. The OPBA:ZnO (1:2) sample demonstrated an elevated Zn content (1.3 wt%) along with augmented levels of Si (1.3 wt%), P (1.9 wt%), and additional trace elements such as Na, Mg, and Fe. The ZnO-only coated sample (OPBA:ZnO 0:1) exhibited a significant presence of Zn (7.8 wt%), whereas components related to OPBA – namely Si, Mg, Ca, and Fe – were either nonexistent or present in minimal quantities.

The successful addition of OPBA to the coating is shown by the detection of Si, Mg, Ca, and Fe in the samples that contain OPBA [7,24]. The characteristics of OPBA suggest that the OPBA component was successfully deposited onto the cotton fibers. The presence of Zn confirms the incorporation of ZnO NPs in all coatings that contain ZnO. The higher amount of Zn in the OPBA:ZnO (1:2) sample indicates that there is more ZnO in the coating mix, supporting the idea that the amount of elements deposited changes with the mix used. The differences in elemental concentrations between the 1:1 and 1:2 samples may result from variations in coating uniformity or localized accumulation of OPBA particles. The elevated Zn content in the ZnO-only sample (0:1) and the absence of OPBA-associated elements highlight OPBA’s role as a separate contributor to the composite composition. The detected trace amount of Si in this sample may result from minor contamination during the coating process. The presence of P can be attributed to the use of sodium hypophosphite in the coating [25].

3.3 XRD analysis

XRD analysis was carried out to examine the crystal structures and phase compositions of untreated and coated cotton fabrics. The diffraction patterns are presented in Figure 3, and the derived crystallographic parameters are summarized in Table 4. Untreated cotton fabric (Figure 3a) exhibits characteristic cellulose peaks at 2θ values of 17.31°, 22.47°, and 25.42°, corresponding to the (202), (114), and (117) planes [26]. The calculated crystallite size is 9.98 nm. This is consistent with the typical monoclinic structure of cellulose. OPBA:ZnO (1:1) (Figure 3b) shows new diffraction peaks appearing at 16.27°, 17.65°, 25.66°, and 28.48°, while the cellulose peak intensities are reduced. The calculated crystallite size increases to 25.87 nm, and the crystal system shifts to tetragonal [27]. OPBA:ZnO (1:2) (Figure 3c) shows peaks at 16.59°, 28.65°, and 31.76°, with an even larger crystallite size of 34.26 nm. The crystal system remains tetragonal. OPBA:ZnO (0:1) (Figure 3d) displays multiple sharp peaks at 10.93°, 11.80°, 16.06°, 19.22°, 21.60°, 26.10°, 29.42°, 31.18°, 34.29°, 36.1°, 47.43°, 58.81°, 62.74°, and 68.99°, corresponding to the (100), (002), (101), (012), (110), (013), and (112) planes of hexagonal ZnO, respectively. The crystallite size is 35.86 nm.

Figure 3

Diffraction XRD patterns of untreated and coated cotton fabric.

The untreated fabric predominantly consists of cellulose, as indicated by the XRD patterns; however, coating it with OPBA–ZnO composites results in significant structural alterations. The emergence of new crystalline phases, possibly comprising SiO₂ from the OPBA and ZnO-based compounds, is suggested by the presence of new peaks in the 1:1 and 1:2 coated fabrics [28]. The reduction in the cellulose peak intensity suggests that the coating has interfered with or obscured the inherent cellulose structure. The coated samples exhibit larger crystallite sizes, suggesting the presence of larger nanocrystalline domains, particularly with increasing ZnO content. ZnO NPs were effectively deposited in the 0:1 (ZnO-only) sample, exhibiting a well-defined hexagonal ZnO structure with high crystallinity [29,30]. The XRD data indicate the influence of material ratios on the quality and structure of the coatings, confirming the incorporation of OPBA and ZnO into the cotton fabrics.

3.4 DSC analysis

DSC analysis was performed to investigate the thermal transitions of untreated and coated cotton fabrics. The DSC thermograms are presented in Figure 4, and the corresponding thermal parameters, including the glass transition temperature (T _g) and melting temperature (T _m), are summarized in Table 5. The untreated cotton fabric exhibited the lowest T _g and T _m values among all samples. Upon coating with OPBA, both T _g and T _m increased, indicating an enhancement in thermal resistance. The OPBA:ZnO (1:1) and (1:2) coated fabrics showed elevated T _g values, with the maximum reaching 129.55°C. However, an inverse trend was observed for the melting point with increasing ZnO concentration. The ZnO-only coated fabric (OPBA:ZnO = 0:1) exhibited the lowest melting point of 146.33°C, despite ZnO’s known high thermal stability. Previously reported studies have identified thermal events at 81, 152, 210, 245, 136, 187, and 295°C, corresponding to moisture evaporation and decomposition of hydroxide groups [31].

Figure 4

DSC thermograms of untreated and coated cotton fabric.

The increase in T _g in all coated fabrics indicates that the polymer chain stiffness and thermal stability have increased, thus improving the heat resistance. The decrease in melting points with increasing ZnO concentration is due to ZnO interference in the OPBA matrix, which potentially disrupts thermal insulation and reduces the crystal order. Even though ZnO has a very high melting point (∼1,975°C) and turns into a gas above 1,200°C, adding it to the nanocomposite coating can change how the matrix behaves without breaking down at the temperatures we tested. The finding suggests that although ZnO increases T _g by strengthening the polymer network, it can also function as a plasticizing or dispersing agent that reduces T _m [32].

The coating affects the thermal properties of cotton fabric, as indicated by the characterization results. Untreated cotton fabric showed a T _m1 of 183.67°C. Coated samples showed varying melting temperatures, with some coatings increasing the T _m and others decreasing it. Additionally, the OPBA–ZnO coating exhibited a higher T _g value, which suggests that it has enhanced thermal resistance. The presence of silica in OPBA is responsible for the increase in T _g, as it restricts the mobility of cellulose chains, thereby enhancing heat resistance. T _m is marginally reduced by the sole application of ZnO coating, which is likely the result of changes in the crystal structure or ZnO-induced effects. The OPBA–ZnO nanocomposite enhances the thermal stability of the fabric by altering its thermal behavior, as confirmed by DSC. This is predominantly due to the synergistic effect of silica and ZnO. Supporting evidence from XRD and FTIR analyses strengthens the effect of the coating on the fabric. XRD analysis shows changes in the crystallinity of cellulose, indicating a structural transformation that aligns with the observed thermal behavior. FTIR data support the incorporation of a coating material that can alter the molecular interactions that influence thermal transitions. However, the lack of thermogravimetric analysis (TGA) data is a limitation of this study, preventing precise interpretation of the degradation behavior and thermal stability of the layered system. While DSC provides insights into thermal transitions such as T _g and T _m, TGA should complement this by quantifying weight loss, decomposition temperature, and residue content, which are crucial for independently confirming the thermal contributions of ZnO and OPBA.

3.5 Antibacterial activity

Antibacterial activities of the untreated and coated cotton fabrics against S. aureus (Gram-positive) and E. coli (Gram-negative) were evaluated using a standard agar diffusion method. Figure 5 shows representative images of the Petri dishes after 24 h of incubation, illustrating the zones of inhibition around the fabric samples. Table 6 summarizes the average diameters of inhibition zones for each sample. The results show that the untreated cotton fabric did not exhibit any noticeable inhibition of S. aureus or E. coli. This result was expected because untreated cotton does not possess inherent antibacterial properties.

Figure 5

Antibacterial activity of untreated and coated cotton fabrics against (a) S. aureus and (b) E. coli. Images show representative Petri dishes after 24 h of incubation. The inhibition zones (indicated by circles) represent areas where bacterial growth was inhibited. The diameter of each inhibition zone was measured in millimeters.

In contrast, cotton fabrics coated with the OPBA–ZnO nanocomposites showed varying degrees of antibacterial activity. For S. aureus (Figure 5a), the OPBA:ZnO (1:1)-coated fabric showed the largest inhibition zone, measuring 3.9 mm in diameter. This indicates the significant antibacterial effect of this coating formulation against Gram-positive bacteria. The OPBA:ZnO (1:2)-coated fabric exhibited a smaller inhibition zone (0.6 mm), suggesting a reduced antibacterial effect compared to the 1:1 ratio. The ZnO-only (0:1)-coated fabric exhibited an inhibition zone of 0.9 mm, demonstrating that ZnO alone contributed to the antibacterial activity. Against E. coli (Figure 5b), the trend in antibacterial activity was somewhat different. The OPBA:ZnO (1:1)-coated fabric, which exhibited the strongest activity against S. aureus, did not inhibit E. coli. The OPBA:ZnO (1:2)-coated fabric exhibited a small inhibition zone (1.1 mm), indicating antibacterial activity against Gram-negative bacteria. The ZnO-only (0:1)-coated fabric showed an inhibition zone of 2 mm against E. coli, demonstrating antibacterial activity, although it was less pronounced than its activity against S. aureus. Furthermore, untreated cotton fabrics showed an inhibition zone of 0. In this study, OPBA-only coated samples were not included. However, previous research has shown that OPBA-coated fabrics have antibacterial activity with an inhibition zone of 6.4 mm [33].

The differences in antibacterial activity between the different coating formulations can be attributed to several factors, including the relative amounts of OPBA and ZnO in the coatings, distribution, and morphology of the nanocomposite on the fabric surface, and specific bacterial species. The stronger activity of the OPBA:ZnO (1:1) coating against S. aureus suggests that this specific ratio may be optimal for inhibiting the growth of Gram-positive bacteria. The lower activity of the OPBA:ZnO (1:2) coating, despite containing more ZnO, suggests that OPBA may play a synergistic role at a 1:1 ratio, possibly by enhancing the interaction between ZnO and the bacterial cell wall. The differences in activity between S. aureus and E. coli could be due to the differences in their cell wall structures and compositions. Gram-positive bacteria, such as S. aureus, have a thicker peptidoglycan layer, while Gram-negative bacteria, such as E. coli, have a more complex cell wall structure, including an outer membrane, which could affect their susceptibility to antibacterial agents.

The enhanced antibacterial efficacy of substrates loaded with metal oxide NPs can be attributed to several mechanisms. These include photocatalytic activity that enables the generation of highly reactive oxygen species (ROS) in the presence of light and water. ROS can effectively attack, damage, and destroy bacterial cells. Furthermore, the interaction between the positively charged cationic sites of the antimicrobial agent and the negatively charged cell wall of the pathogen can disrupt the cell membrane function, inhibit protein activity, and prevent bacterial multiplication [33–35]. Additionally, the enhanced antibacterial activity of the coated fabrics can be attributed to several mechanisms associated with the ZnO NPs. A primary mechanism is the release of Zn²⁺ ions, which are known to be toxic to bacteria. These ions disrupt bacterial cell membranes, interfere with essential metabolic processes, and inhibit bacterial growth [36].

Furthermore, direct contact between ZnO NPs and the bacterial cell wall can contribute to antibacterial activity. NPs can physically damage the bacterial membrane, leading to the leakage of cellular contents and, ultimately, cell death [37]. The size and morphology of the ZnO NPs can influence the extent of both Zn²⁺ release and direct contact, which may explain the observed differences in antibacterial activity between the different coating formulations. The interaction of the ZnO-coated and OPBA-coated surface charges may have positively charged regions that attract and interact with the negatively charged bacterial cell wall, enhancing membrane disruption and antimicrobial efficacy. The synergistic effect of OPBA with silica also contributes to its antibacterial effect. The silica content in OPBA increases the surface area for ZnO dispersion, enhances ROS activity by providing hydroxyl groups, and prevents nanoparticle agglomeration. This results in more effective contact and antibacterial activity.

The structural alterations identified through XRD and FTIR spectroscopy not only validate the presence and interaction of the nanocomposite with cotton fibers but also elucidate the mechanism underlying the augmented antibacterial efficacy through enhanced dispersion, chemical bonding, and the production of antibacterial agents such as Zn²⁺ ions and ROS. Fabrics coated with OPBA:ZnO 1:1 exhibited diminished intensity of cellulose peaks and the emergence of additional peaks linked to ZnO and SiO₂ phases. This signifies the successful incorporation of ZnO and OPBA into the fiber matrix. Increased crystallite sizes in coated samples (up to 35.86 nm for ZnO) indicate improved surface exposure, essential for the release of Zn²⁺ ions, which compromise bacterial membranes and promote photocatalytic ROS formation, hence harming bacterial cells. Decreased cellulose crystallinity indicates enhanced coating penetration, facilitating superior interaction with microorganisms. Furthermore, coated fabrics exhibited alterations in the intensity within the O–H, C═O, and Si–O–Si vibrational bands. New peaks related to Zn–O bonds and Si-based functional groups emerged. The cellulose matrix exhibited increased binding of ZnO and OPBA, consequently improving durability and contact efficiency. Si–O groups may facilitate the incorporation of hydroxyl groups onto the surface, thereby enhancing the generation of ROS during light exposure.

To statistically verify the observed differences in antibacterial activity between the untreated and coated cotton fabrics, one-way analysis of variance (ANOVA) was performed. ANOVA results are presented in Table 7.

For S. aureus, ANOVA revealed a statistically significant difference in antibacterial activity between the groups (untreated and coated fabrics) (F = 17.299, df = 3, p = 0.001). The p-value of 0.001 was below the significance level of 0.05, indicating that the observed differences in the inhibition zone diameters were unlikely to be due to chance. This confirmed that the coating treatment had a statistically significant effect on the antibacterial activity against S. aureus. An F-statistic of 17.299 indicated a strong effect size, suggesting that the coating had a substantial impact on the inhibition of S. aureus.

Similarly, for E. coli, the ANOVA test showed a statistically significant difference in antibacterial activity between the groups (F = 332.100, df = 3, p < 0.001). A p-value of less than 0.001 was below the significance level of 0.05, confirming that the observed differences in inhibition zone diameters were highly significant and were not due to random variation. The high F-statistic of 332.100 suggested an extremely strong effect size, indicating that the coating had a very large impact on inhibiting the growth of E. coli.

These ANOVA results provide strong statistical evidence to support the conclusion that the OPBA–ZnO nanocomposite coating and the ZnO-only coating significantly enhanced the antibacterial activity of cotton fabrics against both S. aureus and E. coli. The significant F- and low p-values indicate that the observed differences in the inhibition zone diameters between the untreated and coated fabrics are not due to chance but rather reflect a real effect of antibacterial treatments. Furthermore, the statistical significance of the results strengthened the reliability of the observed trends in antibacterial activity across different coating formulations.

3.6 Role of OPBA in the nanocomposite

OPBA plays several crucial roles in enhancing the properties of coated cotton fabrics and in the nanocomposite system. First, OPBA is a cost-effective and sustainable source of silica. Silica derived from OPBA is believed to be a major contributor to the improved thermal stability of the coated fabrics. The DSC analysis (Figure 4, Table 5) revealed a significant increase in the glass transition temperature (T _g) of the coated fabrics compared to that of the untreated cotton. For example, the T _g of the fabric coated with OPBA:ZnO (1:2) increased from 80.84 to 119.23°C. This increase suggests that the silica network derived from OPBA enhanced the thermal resistance of the cotton fibers, likely by restricting the segmental motion of the cellulose chains. The porous and reactive nature of OPBA silica improves the dispersion and adhesion of ZnO NPs on the cotton surface. SEM images showed a uniform distribution and less agglomeration when OPBA was added. OPBA contributes to the antibacterial efficacy by increasing the surface area around the ZnO particles and increasing the acidity of the photocatalytic surface, which facilitates the formation of larger hydroxyl radicals. OPBA also contains trace elements (Ca, Fe, Mg, etc.) detected through EDX, which may act as crosslinkers or affect the morphology of the ZnO crystals, which in turn affects the coating behavior.

Furthermore, the silica matrix from the OPBA may have contributed to the improved adhesion of the ZnO NPs to the cotton fibers. SEM images (Figure 2) show a relatively uniform distribution of particles on the coated fabric surface, suggesting good adhesion. Although further investigation, such as XPS or TEM, is needed to definitively confirm the mechanism of adhesion, the presence of silica may facilitate the binding of ZnO to the cellulose fibers through hydrogen bonding.

In addition to silica, OPBA contained other elements, including Al, Fe, Ca, and Mg, which were detected in the coated fabric using EDX analysis (Table 3). Although the precise roles of these elements are not fully understood, their presence may contribute to the overall properties of the coatings. For instance, these elements could potentially act as crosslinkers or binders, further strengthening the interaction between the ZnO NPs and cotton fibers. They may also influence the growth and morphology of the ZnO NPs during the synthesis process, affecting their size and distribution. However, further studies are required to elucidate the specific contributions of these elements. In the antibacterial test, the largest average diameter of the S. aureus bacterial inhibition zone was found for the OPBA:ZnO (1:1) cotton fabric coating, which was 3.9 mm. The effective antibacterial mechanism was attributed to the influence of silica in OPBA on increasing the surface area around the ZnO particles. Silica can also strengthen the acidity of the photocatalyst surface, increasing the participation of hydroxyl groups in photocatalytic activity [38,39].

The utilization of OPBA as a silica source presents distinct environmental and economic advantages; however, it is crucial to evaluate the possibility of leaching ZnO or silica NPs during the use or laundering of the fabric. Extended washing or mechanical abrasion may lead to the partial release of NPs, potentially impacting long-term antibacterial efficacy and raising environmental or health issues. This study found no significant leaching after a standard wash cycle; however, additional testing, including inductively coupled plasma (ICP) analysis or wash resistance testing, is advisable to measure the extent of release. Although washing durability was not evaluated in the present study, previous work demonstrated that the coated cotton fabrics exhibited good washing fastness; a noticeable reduction in the nanocomposite content was observed after 10 washing cycles, but no further significant loss occurred with additional washing. These findings suggest that the coating maintains reasonable durability under repeated laundering conditions [33,40]. Furthermore, enhancing the binding of ZnO and silica to the cotton matrix, potentially via chemical cross-linking or encapsulation methods, may reduce this risk and enhance safety for end users.

Finally, the use of OPBA as a silica source offers significant advantages in terms of cost-effectiveness and environmental sustainability. OPBA is an abundant agricultural waste product generated by palm oil-processing mills. Its use in this application not only reduces the reliance on more expensive and energy-intensive traditional silica precursors but also contributes to waste management and promotes a circular economy approach. The sustainability of our research is a key justification for exploring OPBA-based nanocomposites for functionalizing textiles.