Valorization of oil palm frond ash via alkaline activation as reinforcement in polypropylene composites

The tensile properties of PP, PP/OPFA and PP/AAOPFA composites were obtained from the tensile test by using Shimadzu’s 50KN AG-X plus series universal tester equipped with Trapezium X software. The tensile strength, Young’s modulus and elongation at break of the composites were obtained. The tensile test was conducted according to ASTM D-638 with a crosshead speed of 50 mm/min.

The water absorption test on PP, PP/OPFA and PP/AAOPFA composites were cut into a dimension of 20 × 10 × 1 mm and were weighed to measure the initial weight according to ASTM D570. Five samples of each composition were immersed in distilled water for 48 h. Then, the samples were weighed to measure the final weight. The following equation was used to measure the percentage of water absorption for each sample.

W_f = Initial weight of the sample.

W_i = Final weight of the sample.

Scanning electron microscope model JOEL JSM-6010LV was used to analyze the morphology of OPFA and AAOPFA fillers, and the tensile fracture surface of the PP, PP/OPFA composites and PP/AAOPFA composites at 15 kV accelerating voltage. The samples were coated with gold layer of 20 nm by using Auto Fine Coater machine before the analysis was carried out to prevent electrostatic charges during examination.

Perkin-Elmer Spectrum 400 Series instrument was used to measure the functional groups of PP, PP/OPFA and PP/AAOPFA composites by using attenuated total reflectance (ATR) technique. The spectrum resolution and the scanning range were set at 4 and 400–4000 cm^− 1, respectively.

Four-point probe method was used to measure the conductance of the OPFA, GOPFA, PP/OPFA and PP/AAOPFA composites. This method was setup based on EECS 143 instructional lab which consists of four equally spaced tungsten metal tips with a tip spacing of 1 mm and average sample thickness of 1 mm. A direct current of 5 V supplied from Keithley 2400 sourced meter was used. The composite specimens had a thickness comparable to the probe spacing (t/s = 1), indicating bulk-type behaviour. According to Smits [46], a small thickness correction factor f (t/s) = 0.93 was applied. As the ratio in this study was close to 1, the correction factor was approximately unity, and its effect on the calculated resistance and conductivity was minimal. Therefore, the following equations were used to determine the conductance of the samples:

R = resistance, Ohm (Ω).

V = voltage, Volt (V).

I = current, Ampere (A).

C = conductance, Siemens (S).

Thermogravimetry analysis (TGA) of PP, PP/OPFA and PP/AAOPFA composites were conducted using a PerkinElmer Pyris 6 TGA analyser. 10 mg of samples were scanned from 30 to 800 °C in alumina pan with a heating rate of 10 °C/min under a constant nitrogen gas flow of 50 ml/min to prevent thermal oxidation. Temperature of 50% weight loss (T _− 50%wt), final decomposition temperature and residual mass were recorded.

The oxide compositions of OPFA and AAOPFA were determined using X-ray fluorescence (XRF, MiniPAL 4, PANalytical), and the results are summarized in Table 2. As can be seen in Table 2, the primary elements of OPFA and AAOPFA were SiO_2, K₂O, ClO₂, CaO, MgO and Al₂O₃, respectively. Minor elements such as P₂O₅, SO₃, MnO and FeO were found in OPFA and AAOPFA. However, the percentage of SiO₂ in AAOPFA increased 330.64% compared with OPFA was due to the combination of SiO₂ from OPFA with SiO₂ molecules from the alkaline activators to form more Si-O-Si network, therefore amount of SiO₂ was increased in AAOPFA [47]. The SiO₂ influences the formation of the filler interaction, yielding better Al₂O₃- SiO₂ bonds that result in better mechanical properties [48]. The increment of SiO₂ also capable to improve the thermal stability of the fillers, it remains stable and decomposes only at high temperature [49]. Klapiszewsk et al. [50] discussed that the presence of SiO₂ in the polypropylene matrix promotes the formation of an interlayer zone at the filler surface thus the immobilization of polymeric chains on the filler surface. This interfacial interaction enhances the thermal stability of the polymeric phase. Besides, it can be observed that LOI for AAOPFA was lower than OPFA, this was attributed to the removal of organic and carbonaceous residues after the alkali activation, thus enhanced the fillers surface reactivity and promotes interfacial properties with the PP matrix [51].

Based on Fig. 3(a), OPFA particles were irregular in shape. SEM microstructure of OPFA also revealed that the small size particles tended to attach to each other as shown in Fig. 3(b). On the other hand, Fig. 3(c) displays a rougher surface after the alkaline activation process, whereas 3(d) indicates AAOPFA tended to form needle-like structure under 4000 times magnification. Rougher surface of fillers indicates larger effective area and more active sites on the fillers’ surfaces, which provides more contact sites for the interaction between fillers and matrix [52]. Furthermore, the rougher surface of fillers improves the interfacial adhesion and enhances mechanical interlocking effect, where resists fillers pull-out from matrix under stress, therefore improves the stress transfer efficiency and tensile strength of the composites [53, 54]. According to Edoziuno et al. [55], they reported that water absorption of composite is not directly proportional to the amount of fillers that added but it depends on interfacial properties between fillers and matrix. Rough surface promotes good interfacial adhesion among fillers-matrix, therefore reduces interfacial gaps and limits the water molecules penetrate the composite.

Figure 4 shows the FTIR spectra of OPFA and AAOPFA that recorded from 400 cm¹ to 4000 cm⁻¹ and the functional groups are summarized in Table 3. It can be observed that both fillers exhibited almost similar spectra pattern but have different intensity. OPFA has six main peaks at wavelength of 538.91 cm⁻¹, 742.45 cm⁻¹, 1259.18 cm⁻¹, 1704.72 cm⁻¹, 2508.74 cm⁻¹, and 3237.61 cm⁻¹. The peaks at wavelength of 538.91 cm⁻¹ and 742.45 cm⁻¹ were assigned to S-S stretching and C-H bending, respectively. The peaks observed at 1259.18 cm⁻¹ was attributed to C-O stretching vibration of aromatic ester [56]. There was also C = O stretching of conjugated acid in OPFA that can be observed at 1704.72 cm⁻¹. Besides, the sharp peak at 2508.74 cm⁻¹ and the broad peak at 3237.61 cm⁻¹ were referred to the O-H stretching vibration [57]. Meanwhile, AAOPFA has eight major peaks. The peaks were at 528.49 cm⁻¹, 742.23 cm⁻¹, 976.18 cm^>−1, 1259.38 cm⁻¹, 1708.99 cm⁻¹, 2317.59 cm⁻¹, 2426.13 cm⁻¹, and 3229.69 cm⁻¹. The peak at 976.18 cm⁻¹ was attributed to the C = C bending vibration. Besides, there was a new peak appeared at 2317.59 cm⁻¹ which was assigned to O = C = O stretching vibration [58]. The peaks at 2426.13 cm^>−1 and 3229.69 cm⁻¹ were referred to O-H group where they displayed a lower transmittance percentage than the similar peaks appeared in OPFA. The appearance of more intense spectrum of AAOPFA compared to OPFA was attributed to the improvement of surface reactivity of the fillers as the activation process produced more reactive surface sites for the interaction with the polypropylene matrix, thus it helps to improve the interfacial properties of the composites [59].

As can be seen from Fig. 5, the conductance for OPFA and AAOPFA are 2.48\(\:\times\:\)10⁻⁷ S and 6.78\(\:\times\:\)10⁻⁷ S, respectively. OPFA demonstrated a lower conductivity value due to its carbon content. Carbon is a material with excellent conductivity, the higher the carbon content in the ash, the darker its resulting colour [60, 61]. The colour of AAOPFA (Fig. 2) displayed darker colour due to higher carbon content, whereas OPFA (Fig. 1) showed light grey, indicating lower carbon content.

Geopolymerization process plays a crucial role in enhancing electrical conductivity of AAOPFA. Fundamentally, the mechanism of conductivity in alkali-activated materials is through Na⁺ ions hopping between the cation’s sites [62]. Geopolymerization process through alkali-activators transform solid aluminosilicate (AAOPFA) into an alkali aluminosilicate inorganic polymer, produces an amorphous material consisting of [SiO₄]⁴⁻ and [AlO₄]⁵⁻ tetrahedra framework [63]. A part of the Na⁺ ions from the alkali solution present in the framework cavities to balance the negatively charged aluminosilicate structure, while the remaining ions become mobile and able to move freely and contribute to the ionic conductivity of the system [64]. Therefore, the conductivity mechanism in powder form of AAOPFA is mainly contributed by ion mobility, which creating a continuous ionic pathway within the activated fillers network [65].

Based on the Fig. 6, PP/OPFA composites with filler loading of 15 phr had the highest tensile strength, indicating the optimum filler content for PP/OPFA composites. Tensile strength can be defined as the ability of a composites’ interface to transfer shear strength [66]. For PP/OPFA composites with 5phr and 10phr filler loading, the tensile strength was lower compared to pure PP. This was due to the insufficient amount of OPFA in the matrix-filler stress transfer. The insufficient amount of OPFA reduced the interfacial adhesion within the matrix, therefore limited the efficiency of stress transfer from matrix to the fillers. As the result of poor bonding between filler and the matrix, the efficiency in transferring the stress from PP matrix to OPFA filler decreased [67]. In addition, non-uniform filler dispersion and air entrapment in the composites affected composites’ properties [68].

PP/AAOPFA composites at 10 phr exhibited highest tensile strength due to AAOPFA had a good interfacial adhesion with PP, therefore produced homogeneous composites [11]. Thus, the stress applied to the composites can be distributed evenly throughout the matrix and the filler and leads to better tensile properties [69]. However, as the fillers loading increased from 15 to 25 phr, the tensile strength of the PP/AAOPFA composites decreased due to filler-filler interactions rather than matrix-filler interactions [70]. Furthermore, increasing amount above optimum range of filler loading caused the inability for the fibres to mix homogenously with the matrix and caused agglomeration [71]. This may lead to the void creation in the composites which reduced the efficiency of stress transfer from the matrix to the fibres [72].

Figure 7 displays the elongation at break of PP, PP/OPFA and PP/AAOPFA composites with different filler loading. It was found that the elongation of break of PP/OPFA and PP/AAOPFA composites decreased as the filler loading increased. The reduction of the elongation at break indicated the mobility and ductility of the polymer chains of the matrix was hindered by the fillers [73]. The lower elongation at break of PP/AAOPFA composites compared to PP/OPFA composites was due to AAOPFA impart a greater stiffening effect to PP/AAOPFA composites [74].

The Young’s modulus of PP/OPFA and PP/AAOPFA composites increased with increasing of the filler loading as shown in Fig. 8. Generally, fillers are stiffer than matrix because they impart inherent rigidity, therefore improved the Young’s modulus of the composites [75, 76]. As the filler increased, Young’s modulus of PPPP/AAOPFA composites showed higher Young’s modulus compared to PP/OPFA composites at any filler loading was due to better interfacial adhesion among AAOPFA filler and PP matrix, thus significantly enhancing the composites stiffness [77]. Additionally, the result in Fig. 8 supports the finding of the elongation at break in Fig. 7 where higher filler loading caused the reduction in ductility of the composites.

Figure 9 shows the water absorption of PP, PP/OPFA and PP/AAOPFA composites at different filler loading. Water absorption is used to determine the porosity, interfacial voids, and polymer free volumes of composites [78]. The water is absorbed by the pores or cracks occurred within the matrix, diffusion inside the capillarity, or through the fibre-matrix interfaces [79]. As can be seen from Fig. 9, PP has the lowest water absorption compared to PP/OPFA and PP/AAOPFA composites due to its hydrophobic properties. Moreover, it can be observed that the water absorption decreased from 5 to 10 phr followed by an increment from 15 to 25 phr. The increment of water absorption was due to the agglomeration of the filler tend to form at the interfacial voids between filler and matrix where the filler-filler interaction rather than filler-matrix interaction promotes agglomeration at high filler loading [80]. Besides, PP/AAOPFA composites demonstrate lower water absorption compared to PP/OPFA composites in all filler loading variations. This was due to good interfacial adhesion formed between AAOPFA fillers and PP matrix, therefore there were less voids and gaps between fillers and matrix that limit the penetration of water molecules to diffuse into composites [81]. This finding also supports the tensile results where PP/OPFA composites had lower tensile strength compared to PP/AAOPFA composites due to poor interface adhesion between fiber and matrix. The presence of higher intensity of OH groups in FTIR revealed more surface-active OH groups in PP/AAOPFA composites than PP/OPFA composites. This suggests more surface-active OH groups presence after the alkali activation, which promotes more interfacial interaction of AAOPFA with PP matrix [82]. This is consistent with the SEM observations in Fig. 10, where better interfacial adhesion among filler-matrix in PP/AAOPFA composites, minimizes the filler pill-out and reduces voids formation, therefore restricts the penetration of water molecules, as evidenced by SEM morphology analysis.

Figure 10 exhibits the tensile fracture surface of PP, PP/OPFA and PP/AAOPFA composites after performing tensile test. The tear lines can be observed in Fig. 10 (a) of PP indicated the ductility of the thermoplastic where polymer chains were stretched and deformed under stress. As more fillers were added into the composites particularly at 25phr of PP/OPFA and PP/AAOPFA composites, more voids were observed due to the detachment of fillers. The presence of voids was due to the filles were pulled out during the tensile test [83]. This indicates that poor interfacial adhesion between filler and matrix, resulting in less efficient stress transfer from the matrix to the fillers at high filler loading. Besides, fillers tend to agglomerate at high filler loading due to filler-filler interaction rather than filler-matrix interaction. The PP/AAOPFA composites showed better dispersion of AAOPFA fillers in the PP matrix compared to PP/OPFA composites at the same filler loading (at 5, 15, and 25phr). The surface reactivity of AAOPFA was improved, it provides more reactive sites to interact with PP matrix, therefore promote stronger filler-matrix interactions and better interfacial adhesion among filler-matrix. As a result, AAOPFA reduces filler pull-out and void formation, leading to a more efficiency of stress transfer from matrix to fillers. This observation is consistent with the tensile strength results, where PP/AAOPFA composites exhibited higher tensile strength compared to PP//OPFA composites at any filler loading.

Figure 11 shows the FTIR spectra of PP, PP/OPFA composites and PP/AAOPFA composites that recorded from 400 cm^{− 1} to 4000 cm^{− 1}. Table 4 displays the summary of functional groups and the wavenumbers of all three samples. In PP, the peaks at 449.14 cm^{− 1} and 474.89 cm^{− 1} indicate the S-S stretching, while peaks at 718.10 cm^{− 1} and 971.55 cm^{− 1} indicate the C = C bending of alkene. Besides, the spectrum of PP also has peaks at 1162.93 cm^{− 1}, 1375.02 cm^{− 1}, 1455.84 cm^{− 1}, and 1643.97 cm^{− 1} which are assigned to C-O stretching of tertiary alcohol, O-H banding of phenol, C-H bending of alkane, and C = C stretching of alkene, respectively [84]. The peak at 2050.00 cm^{− 1} is due to C = C = N stretching of ketenimine. The O = C = O stretching of carbon dioxide is presented at vibration band of 2393.77 cm^{− 1}. Finally, vibration bands at 2841.38 cm^{− 1} and 2916.52 cm^{− 1} are referring to C-H stretching [85]. The presence of OH groups can be observed in PP/OPFA and PP/AAOPFA composites, whereas no OH group is detected in PP. This absence of OH group in PP can be attributed to hydrophobic properties [86]. The peaks appeared in the spectra of PP/OPFA and PP/AAOPFA composites were almost the same; however, the intensity of the OH band at 3646.20 cm^{− 1} was more pronounced in PP/AAOPFA composites, indicating a higher concentration of hydroxyl groups due to the alkaline activation process. This can be attributed to the formation of silanol groups (Si–OH) on the filler surface resulting from the partial dissolution of silica during the alkaline activation process, as well as the possible presence of residual unreacted NaOH from the activator [87].

Figure 12 shows the conductance of PP/OPFA and PP/AAOPFA composites at different filler loading. As can be seen from Fig. 12, PP/OPFA composites with 5phr of OPFA loading has the lowest conductance. Low loading of OPFA in the PP/OPFA composites cause small number of conductive pathways that could exist in the composite [60]. By increasing the filler loading at 15phr, the conductance of PP/OPFA composites increased due to the increasing of the carbon content and provided more conductive paths in the composite. However, at high filler loading particularly at 25phr, PP/OPFA composites at showed a decrement in conductance due to the void formation between the OPFA filler and PP matrix produced high resistance to electrical current flow [88]. Besides, agglomeration at high filler loading produced a non-uniform filler dispersion and reduce the conductive paths in the composite [89, 90].

PP/AAOPFA composites had better electrical conductivity compared to PP/OPFA composites at different filler loading. The conductivity of AAOPFA filler was enhanced by the presence of Na + ions as the ionic conductor [64]. Moreover, PP/AAOPFA composites had better interfacial adhesion among filler and matrix compared to PP/OPFA composites. The good matrix-filler interactions promote better conductive pathways throughout the composite [88]. Furthermore, better dispersion of AAOPFA filler in matrix lower the resistance and allow more electric currents to be transmitted through the composite [62]. The conductivity mechanism in the composites is different compared to filler powder. In the composites form, PP matrix is electrically insulating, therefore the electrical conductivity mainly depends on the filler dispersion and interaction in the matrix [91]. When AAOPFA was added, its rougher surface and higher ionic activity improved the interfacial adhesion among fillers and matrix. The AAOPFA enhanced the electrical performance of the PP matrix by bridging the insulating polymer regions and establish electron transport pathways [92]. However, the overall conductivity of PP/AAOPFA composites were still lower than AAOPFA in powder form, as the polymer matrix limits the ionic pathways.

Figure 13 illustrates the thermogravimetric curves of PP, PP/OPFA and PP/AAOPFA composites at varying filler loading. Table 5 shows the temperature of 50% weight loss, residual mass and final decomposition temperature for PP/OPFA and PP/AAOPFA composites at different amount of filler. PP/AAOPFA composites gave the highest temperature of 50% weight loss, final decomposition temperature and residual mass compared to AA/OPFA composites at 5-25phr of filler loading, whereas PP showed the lowest reading among all. It can be observed that addition of fillers (OPFA and AAOPFA) improved the thermal stability of the composites by improving T_{− 50%wt}, residual mass and final decomposition temperature. The presence of Si (from previous elemental analysis) in OPFA and AAOPFA significantly inhibited the thermal decomposition of PP [93]. PP/AAOPFA composites displayed better thermal stability in comparison to PP/OPFA composites at the same filler loading, this was attributed to the higher Si content in AAOPFA compared to OPFA as evidenced in XRF analysis. Besides, it is suggested that the good interfacial adhesion of AAOPFA and PP where AAOPFA formed a protective barrier that restricts volatile gas released during degradation, therefore provided better thermal stability to PP/AAOPFA composites [94].