*2.5. X-ray Fluorescence (XRF) Analysis Using Omnian Analysis Software*

The samples were analysed using a 4 kW wavelength dispersive XRF machine (Axios Max, Panalytical, Almelo, Holland, The Netherlands). This machine has a maximum voltage of up to 60 kV and 160 microamps (µA), and is equipped with several crystals such as, PE002, LiF200, PX1 and Ge 111. A representative portion of each sample was ground into 50 µm grain size using a motorised grinding machine, and was further ground manually to a finer grain size of 20 µm, suitable for XRF analysis. The specimen for the XRF analysis was made by igniting approximately 0.5 g of sample and 5.0 g of spectroflux, at approximately 1100 ◦C (for a duration of 20 min), before it was casted into a glass disc with 32 mm in diameter. The specimen was analysed for 10 major elements using a fully automated XRF spectrometer (Axios Max, Panalytical, Almelo, Holland, The Netherlands), with a standard elemental setup. The calibration technique was employed. The 10 element curves were constructed using 30 high quality international standard reference materials, comparable in composition to the unknown samples. For minor and trace elements, each sample was formed into pressed-powder pellet, using approximately 1 g of sample baked with approximately 6.0 g of Boric acid (in a 32 mm diameter round shaped disc, with the sample placed at the center, and Boric acid as a binder around it). The samples were pressed with a hydraulic press machine at 15 tonnes for 2 min. The XRF analysis was done by scanning for the presence of elemental peaks using the Omnian software. The weight percentages of the compound presented in the samples were recorded in the results upon the completion of the analysis.

#### *2.6. Internal Bonding Analysis*

Internal bond analysis was performed for the fabricated *Rhizophora* spp. bonded with different percentages of binders [8]. Metal blocks with the dimension of approximately (5.0 <sup>×</sup> 5.0) cm<sup>2</sup> were utilised in this analysis, and a hot melt glue was used to bind the samples to the metal blocks. Internal bond strength was determined by using a mechanical testing machine (Model UTM-5582; Instron, Norwood, MA, USA) with a load capacity of approximately 1000 kg, adapted from a previous study by Zuber et al. [8].

#### **3. Results and Discussion**

#### *3.1. Evaluation of Functional Group using FTIR Analysis*

The analyses for the *Rhizophora* spp. particleboard bonded with different percentages of binders were recorded between wavenumbers of 4000 cm−<sup>1</sup> and 400 cm−<sup>1</sup> , with a resolution of 4 cm−<sup>1</sup> . Different relative transmittance values were determined by FTIR spectral analysis, as shown in Figures 1–3. Based on the overview of the FTIR values for cellulose, hemicellulose, saccharides and lignin, the broad peak at 3385.07 cm−<sup>1</sup> in Figure 1 may represent cellulose and hemicellulose related to O-H stretching, in relation to hydrogen bond of hydroxyl groups [41,42]. The peak at 2904.80 cm−<sup>1</sup> may indicate cellulose in relation to C-H stretching, whereas the peak at 1735.93 cm−<sup>1</sup> may represent the softwood or hardwood with C=O stretching [41]. All the samples displayed almost the same spectra, as indicated by all the peaks presented in the Figures 1–3. Compared to the spectrum of binderless *Rhizophora* spp. sample, the spectra of samples incorporated with lignin and soy flour as binders showed a small increase in the peak intensity, as shown in

C-H stretching, whereas the peak at 1735.93 cm−1 may represent the softwood or hardwood with C=O stretching [41]. All the samples displayed almost the same spectra, as

C-H stretching, whereas the peak at 1735.93 cm−1 may represent the softwood or hard-

indicated by all the peaks presented in the Figures 1–3. Compared to the spectrum of binderless *Rhizophora* spp. sample, the spectra of samples incorporated with lignin and

soy flour as binders showed a small increase in the peak intensity, as shown in peak

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peak 3402.43 cm−<sup>1</sup> and 2809.80 cm−<sup>1</sup> . However, no new peak appeared in the spectrum of *Rhizophora* spp. samples bonded with the binders, indicating that the introduction of lignin and soy flour did not change the functional groups according to the FTIR spectrum. 3402.43 cm−1 and 2809.80 cm−1. However, no new peak appeared in the spectrum of *Rhizophora* spp. samples bonded with the binders, indicating that the introduction of lignin and soy flour did not change the functional groups according to the FTIR spectrum. soy flour as binders showed a small increase in the peak intensity, as shown in peak 3402.43 cm−1 and 2809.80 cm−1. However, no new peak appeared in the spectrum of *Rhizophora* spp. samples bonded with the binders, indicating that the introduction of lignin and soy flour did not change the functional groups according to the FTIR spectrum.

**Figure 1.** Transmittance spectrum of *Rhizophora* spp. particleboard bonded with 0% binder. **Figure 1.** Transmittance spectrum of *Rhizophora* spp. particleboard bonded with 0% binder. **Figure 1.** Transmittance spectrum of *Rhizophora* spp. particleboard bonded with 0% binder.

**Figure 2.** Transmittance spectrum of *Rhizophora* spp. particleboard bonded with 6% binders. **Figure 2. Figure 2.** Transmittance spectrum of Transmittance spectrum of *Rhizophora Rhizophora* spp. particleboard bonded with 6% binders spp. particleboard bonded with 6% binders. .

**Figure 3.** Transmittance spectrum of *Rhizophora* spp. particleboard bonded with 12% binders. Reprinted with permission from ref. [7]. **Figure 3.** Transmittance spectrum of *Rhizophora* spp. particleboard bonded with 12% binders. Reprinted with permission from ref. [7].

#### *3.2. Evaluation of XRD Spectrum 3.2. Evaluation of XRD Spectrum*

XRD spectrums of the *Rhizophora* spp. bonded with different percentages of binders are depicted in Figures 4–6. A peak intensity at around 2θ = 22° was observed in the diffractogram of the sample bonded with 0% binders, whereas for samples with 6% and 12% binders, the peak intensity can be seen at 2θ = 22.20°, which corresponds to the crystalline properties of the composites [43]. The addition of lignin and soy flour led to a small increment of peak intensity at approximately 2θ = 22.20°, suggesting that the introduction of binders may indicate the changes in the structural order of the molecules, which may also influence the crystalline structure of the composites [44]. This may be due to the forces from molecular chain entanglements between the *Rhizophora* spp. wood particles and the binders, as they diffuse across the joint interface. Cross-linking may occur during the hot pressing, while interfacial diffusion during bonding is enhanced by chain scission [45]. However, no apparent disparity can be observed in all the spectrums, indicating that no major structural transformation can be clearly stipulated in this analysis. To conclude, in-XRD spectrums of the *Rhizophora* spp. bonded with different percentages of binders are depicted in Figures 4–6. A peak intensity at around 2θ = 22◦ was observed in the diffractogram of the sample bonded with 0% binders, whereas for samples with 6% and 12% binders, the peak intensity can be seen at 2θ = 22.20◦ , which corresponds to the crystalline properties of the composites [43]. The addition of lignin and soy flour led to a small increment of peak intensity at approximately 2θ = 22.20◦ , suggesting that the introduction of binders may indicate the changes in the structural order of the molecules, which may also influence the crystalline structure of the composites [44]. This may be due to the forces from molecular chain entanglements between the *Rhizophora* spp. wood particles and the binders, as they diffuse across the joint interface. Cross-linking may occur during the hot pressing, while interfacial diffusion during bonding is enhanced by chain scission [45]. However, no apparent disparity can be observed in all the spectrums, indicating that no major structural transformation can be clearly stipulated in this analysis. To conclude, incorporation of lignin and soy flour in the fabrication of *Rhizophora* spp. particleboard did not reveal apparent crystalline and amorphous transformations within the composites.

#### corporation of lignin and soy flour in the fabrication of *Rhizophora* spp. particleboard did *3.3. Evaluation of SEM Analysis*

not reveal apparent crystalline and amorphous transformations within the composites. The micrographs of the particleboards with different percentages of binders are depicted in Figures 7–9. For the sample with 6% binders, the void spaces between the molecules were reduced when compared to the binderless sample, demonstrating a much smoother appearance which can be attributed to the hot pressing that allows the auto-condensation process between the wood particles and the binders, binding them together [7]. For the *Rhizophora* spp. sample with 12% binders as shown in Figure 9, the appearance of the molecules revealed a less smooth appearance, which may be due to insufficient bonding between the wood particles and the binders, causing an increase in lumen and gaps. Increased lignin content can affect the composites' compactness as cross-linking of lignin with the cell wall components may occur, which in return, reduce

cellulose and hemicellulose accessibility to microbial enzymes, resulting in lower biomass digestibility [46]. Specks of bright appearance in all the figures may be due to the charging effect of electron or ion irradiations, which usually happens for a non-conductive specimen being examined [47]. The interactions between *Rhizophora* spp. and the binders were also influenced by the intermolecular forces that mediate the interaction, and the forces of attraction and repulsion between the molecules [25,48–50]. These attractive forces may allow better adhesion between the binders in their molecular forms, together with the raw wood particles. The natural properties of the raw wood itself are an advantage due to its cell cavities, which may allow the binders to infiltrates and provide better bonding with the help of thermal pressing. Data adapted from Zuber et al. revealed the thermogravimetric analysis (TGA) results of *Rhizophora* spp., soy flour and lignin. The mass degradation of *Rhizophora* spp., soy flour and lignin occurred at 303.35 ◦C, 236.11 ◦C and 279.64 ◦C, respectively, where the particles started to decompose at these temperatures. The mass degradation temperature should not be exceeded in the compression process of the particleboard, in order to improve the efficiency between the fibre chain of the particles and the binders [7]. The process of auto-condensation during the hot pressing had bound the molecules of the binders together with the *Rhizophora* spp. fibre; however, depending on the formulation of the composites, the microscopic appearances of the samples may slightly differ. In this study, the distribution of the *Rhizophora* spp. particles and binders was influenced by shrinking and compounding of the composites to a specified thickness by hot pressing at approximately 200 ◦C, leading to good interfacial bonding. Although there is no apparent disparity between the micrographs shown, it is important to note that incorporation of lignin and soy flour in the fabrication of the particleboard allows better bonding between the molecules, which will further improve the physical and mechanical strength of the particleboard [8]. *Polymers* **2021**, *13*, x FOR PEER REVIEW 7 of 16

**Figure 4.** X-ray diffraction spectrum for *Rhizophora* spp. particleboard bonded with 0% binder. **Figure 4.** X-ray diffraction spectrum for *Rhizophora* spp. particleboard bonded with 0% binder.

**Figure 5.** X-ray diffraction spectrum for *Rhizophora* spp. particleboard bonded with 6% binders.

**Figure 4.** X-ray diffraction spectrum for *Rhizophora* spp. particleboard bonded with 0% binder.

**Figure 5.** X-ray diffraction spectrum for *Rhizophora* spp. particleboard bonded with 6% binders. **Figure 5.** X-ray diffraction spectrum for *Rhizophora* spp. particleboard bonded with 6% binders.

**Figure 6.** X-ray diffraction spectrum for *Rhizophora* spp. particleboard bonded with 12% binders Reprinted with permission from ref. [7]. **Figure 6.** X-ray diffraction spectrum for *Rhizophora* spp. particleboard bonded with 12% binders Reprinted with permission from ref. [7].

The micrographs of the particleboards with different percentages of binders are depicted in Figures 7–9. For the sample with 6% binders, the void spaces between the molecules were reduced when compared to the binderless sample, demonstrating a much smoother appearance which can be attributed to the hot pressing that allows the autocondensation process between the wood particles and the binders, binding them together [7]. For the *Rhizophora* spp. sample with 12% binders as shown in Figure 9, the appearance

gaps. Increased lignin content can affect the composites' compactness as cross-linking of lignin with the cell wall components may occur, which in return, reduce cellulose and hemicellulose accessibility to microbial enzymes, resulting in lower biomass digestibility [46]. Specks of bright appearance in all the figures may be due to the charging effect of electron or ion irradiations, which usually happens for a non-conductive specimen being examined [47]. The interactions between *Rhizophora* spp. and the binders were also influenced by the intermolecular forces that mediate the interaction, and the forces of attraction and repulsion between the molecules [25,48–50]. These attractive forces may allow better adhesion between the binders in their molecular forms, together with the raw wood particles. The natural properties of the raw wood itself are an advantage due to its cell cavities, which may allow the binders to infiltrates and provide better bonding with the help of thermal pressing. Data adapted from Zuber et al. revealed the thermogravimetric analysis (TGA) results of *Rhizophora* spp., soy flour and lignin. The mass degradation of R*hizophora* spp., soy flour and lignin occurred at 303.35 °C, 236.11 °C and 279.64 °C, respectively, where the particles started to decompose at these temperatures. The mass degradation temperature should not be exceeded in the compression process of the particleboard, in order to improve the efficiency between the fibre chain of the particles and the binders [7]. The process of auto-condensation during the hot pressing had bound the molecules of the binders together with the *Rhizophora* spp. fibre; however, depending on the formulation of the composites, the microscopic appearances of the samples may slightly differ. In this study, the distribution of the R*hizophora* spp. particles and binders was influenced by shrinking and compounding of the composites to a specified thickness

*3.3. Evaluation of SEM Analysis*

by hot pressing at approximately 200 °C, leading to good interfacial bonding. Although there is no apparent disparity between the micrographs shown, it is important to note that incorporation of lignin and soy flour in the fabrication of the particleboard allows better bonding between the molecules, which will further improve the physical and mechanical

strength of the particleboard [8].

**Figure 7.** Micrograph of *Rhizophora* spp. particleboard bonded with 0% binder.

**Figure 8.** Micrograph of *Rhizophora* spp. particleboard bonded with 6% binders.

**Figure 8.** Micrograph of *Rhizophora* spp. particleboard bonded with 6% binders.

**Figure 9.** Micrograph of *Rhizophora* spp. particleboard bonded with 12% binders.

#### *3.4. Evaluation of EDX Analysis*

**Figure 9.** Micrograph of *Rhizophora* spp. particleboard bonded with 12% binders. *3.4. Evaluation of EDX Analysis* Figures 10–12 show EDX spectrums of the *Rhizophora* spp. particleboard at 0%, 6% and 12% binders. All the EDX spectrums of *Rhizophora* spp. particleboard bonded with different percentages of binders demonstrated visible carbon and oxygen signals, which confirmed the discernible presence of the carbon and oxygen in the composites [51,52]. Another peak in the spectrums may represent other elements such as gold, as the samples were coated with gold for better conductivity. Based on the figures, all spectrums display no discernible disparity; thus, the incorporation of binders in the fabrication of *Rhizophora*  spp. particleboard did not greatly affect the percentages of elements presented in the com-Figures 10–12 show EDX spectrums of the *Rhizophora* spp. particleboard at 0%, 6% and 12% binders. All the EDX spectrums of *Rhizophora* spp. particleboard bonded with different percentages of binders demonstrated visible carbon and oxygen signals, which confirmed the discernible presence of the carbon and oxygen in the composites [51,52]. Another peak in the spectrums may represent other elements such as gold, as the samples were coated with gold for better conductivity. Based on the figures, all spectrums display no discernible disparity; thus, the incorporation of binders in the fabrication of *Rhizophora* spp. particleboard did not greatly affect the percentages of elements presented in the composites. *Polymers* **2021**, *13*, x FOR PEER REVIEW 12 of 16

**Figure 10.** Energy dispersive X-ray spectrum for *Rhizophora* spp. particleboard bonded with 0% binders. ural composition of the particleboard. The XRF analysis of major elements is recorded in **Figure 10.** Energy dispersive X-ray spectrum for *Rhizophora* spp. particleboard bonded with 0% binders.

**Figure 11.** Energy dispersive X-ray spectrum for *Rhizophora* spp. particleboard bonded with 6% binders.

**Figure 12.** Energy dispersive X-ray spectrum for *Rhizophora* spp. particleboard bonded with 12% binders.

The XRF method is the best way to identify major and trace elements within the nat-

*3.5. Evaluation of the XRF Analysis*

**Figure 10.** Energy dispersive X-ray spectrum for *Rhizophora* spp. particleboard bonded with 0% binders.

**Figure 10.** Energy dispersive X-ray spectrum for *Rhizophora* spp. particleboard bonded with 0% binders.

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**Figure 11.** Energy dispersive X-ray spectrum for *Rhizophora* spp. particleboard bonded with 6% binders. **Figure 11.** Energy dispersive X-ray spectrum for *Rhizophora*spp. particleboard bonded with 6% binders. **Figure 11.** Energy dispersive X-ray spectrum for *Rhizophora* spp. particleboard bonded with 6% binders.

**Figure 12.** Energy dispersive X-ray spectrum for *Rhizophora* spp. particleboard bonded with 12% binders. **Figure 12.** Energy dispersive X-ray spectrum for *Rhizophora* spp. particleboard bonded with 12% binders. **Figure 12.** Energy dispersive X-ray spectrum for *Rhizophora* spp. particleboard bonded with 12% binders.

#### *3.5. Evaluation of the XRF Analysis 3.5. Evaluation of the XRF Analysis 3.5. Evaluation of the XRF Analysis*

The XRF method is the best way to identify major and trace elements within the natural composition of the particleboard. The XRF analysis of major elements is recorded in The XRF method is the best way to identify major and trace elements within the natural composition of the particleboard. The XRF analysis of major elements is recorded in The XRF method is the best way to identify major and trace elements within the natural composition of the particleboard. The XRF analysis of major elements is recorded in Table 1. Calcium oxide showed the highest weight percentage in all samples in the range of 40.319% to 52.744%. For *Rhizophora* spp. samples with the addition of lignin and soy flour, the dry weight of potassium oxide recorded the second highest concentration in the range of 14.883% to 18.399%, while chlorine percentage is between 11.623% to 12.947%. However, for the binderless particleboard (0% soy flour and lignin), chlorine recorded the second highest percentage after calcium oxide at 17.705%, followed by sodium oxide at 6.355%. The high percentage of chlorine in the wood-based sample may be due to combustion performed in a laboratory scale spectrometer. Nevertheless, the lower percentage of chlorine in the bonded samples may be advantageous to the overall particleboard outcomes, especially in maintaining the natural properties of the particleboard.

[8].


**Table 1.** X-ray fluorescence analysis of major and trace element in weight percentages using Omnian method with pressed powder pellet. **Samples Calcium Oxide (CaO) Potassium Oxide (K2O) Chlorine (Cl) Sodium Oxide (Na2O) Others**

**Table 1.** X-ray fluorescence analysis of major and trace element in weight percentages using Om-

comes, especially in maintaining the natural properties of the particleboard.

Table 1. Calcium oxide showed the highest weight percentage in all samples in the range of 40.319% to 52.744%. For *Rhizophora* spp. samples with the addition of lignin and soy flour, the dry weight of potassium oxide recorded the second highest concentration in the range of 14.883% to 18.399%, while chlorine percentage is between 11.623% to 12.947%. However, for the binderless particleboard (0% soy flour and lignin), chlorine recorded the second highest percentage after calcium oxide at 17.705%, followed by sodium oxide at 6.355%. The high percentage of chlorine in the wood-based sample may be due to combustion performed in a laboratory scale spectrometer. Nevertheless, the lower percentage of chlorine in the bonded samples may be advantageous to the overall particleboard out-

#### *3.6. Internal Bond Analysis*

nian method with pressed powder pellet.

*3.6. Internal Bond Analysis*

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The analysis of internal bonding was performed and the result is shown in Figure 13 [8]. The analysis of internal bonding was performed and the result is shown in Figure 13

**Figure 13.** Internal bonding of soy-lignin bonded *Rhizophora* spp. particleboards at different percentages. C = 0 µm to 103 µm particle sizes; 0 = 0% soy flour and lignin, 6 = 4.5% soy flour and 1.5% **Figure 13.** Internal bonding of soy-lignin bonded *Rhizophora* spp. particleboards at different percentages. C = 0 µm to 103 µm particle sizes; 0 = 0% soy flour and lignin, 6 = 4.5% soy flour and 1.5% lignin, 12 = 9% soy flour and 3% lignin.

The minimum requirement according to JIS A-5908 includes classification of Type 8 ), Type 13 (0.2 N.mm−<sup>2</sup> ) and Type 18 (0.3 N.mm−2 ) [53]. The *Rhizophora* spp. sample with binders satisfy all three classifications by JIS, whereas the sample with 0% binder did not satisfy any of the requirements. Increased internal bonding strength displayed by samples with binders indicates improved structural stability and durability of the composites [44]. Hot pressing allows the high temperature to be imparted on the formulation of *Rhizophora* spp. with binders, which may create strong bonding with reduced lumen voids between the particles in the composites. The strength of the particleboard may be achieved due to the vessel element and parenchymatous cell of the *Rhizophora* spp. The minimum requirement according to JIS A-5908 includes classification of Type 8 (0.15 N.mm−<sup>2</sup> ), Type 13 (0.2 N.mm−<sup>2</sup> ) and Type 18 (0.3 N.mm−<sup>2</sup> ) [53]. The *Rhizophora* spp. sample with binders satisfy all three classifications by JIS, whereas the sample with 0% binder did not satisfy any of the requirements. Increased internal bonding strength displayed by samples with binders indicates improved structural stability and durability of the composites [44]. Hot pressing allows the high temperature to be imparted on the formulation of *Rhizophora* spp. with binders, which may create strong bonding with reduced lumen voids between the particles in the composites. The strength of the particleboard may be achieved due to the vessel element and parenchymatous cell of the *Rhizophora* spp. that are closely attached under high pressure condition. In this study, internal bond represents the mechanical properties of the composites, and based on the result, the addition of lignin and soy flour increased the internal bond strength, which may improve its mechanical strength and structural integrity.

#### **4. Conclusions**

lignin, 12 = 9% soy flour and 3% lignin.

(0.15 N.mm−2

Evidence from the FTIR, XRD, SEM, EDX, XRF and internal bonding revealed the potential use of lignin and soy flour as binders in the fabrication of *Rhizophora* spp. particleboard, as a phantom material in radiation dosimetry applications. Different percentages of binders used in the fabrication of the particleboard did not greatly affect the properties of the particleboard as a phantom material; however, lower percentages of chlorine and

increased internal bonding strength in the sample with binders may be advantageous in maintaining the natural properties of the particleboard, and improve the mechanical strength of the samples, respectively.

**Author Contributions:** Conceptualisation, S.H.Z., N.A.A.H., M.F.M.Y., M.Z.A.A. and R.H.; methodology, S.H.Z., M.F.M.Y., M.Z.A.A. and R.H.; validation, N.A.A.H., M.F.M.Y. and R.H.; formal analysis, S.H.Z.; investigation, S.H.Z., M.F.M.Y. and M.Z.A.A.; resources, N.A.A.H., M.F.M.Y. and R.H.; data curation, S.H.Z.; writing—original draft, S.H.Z.; writing—review & editing, S.H.Z. and N.A.A.H.; visualisation, S.H.Z.; supervision, N.A.A.H., M.F.M.Y., M.Z.A.A. and R.H.; project administration, N.A.A.H.; funding acquisition, N.A.A.H., M.F.M.Y. and R.H. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Universiti Sains Malaysia Short-Term Grant (304/PFIZIK/6315322), the School of Industrial Technology Grant (1001/PTEKIND/8014083) and the Universiti Sains Malaysia Bridging Grant (304.PPSK.6316324).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Acknowledgments:** The authors thanked the School of Physics, School of Industrial Technology and Advanced Medical and Dental Institute, Universiti Sains Malaysia for allowing this research to be conducted in the respective schools/institute. The first author of this paper is financially sponsored by the UTM Academic Fellow Scheme (SLAM) (2019–2021) and the author would like to thank UTMLead and the Faculty of Science, Universiti Teknologi Malaysia, Johor for making this study possible.

**Conflicts of Interest:** The authors declare no conflict of interest.
