X-ray Shielding, Mechanical, Physical, and Water Absorption Properties of Wood/PVC Composites Containing Bismuth Oxide

The potential utilization of wood/polyvinyl chloride (WPVC) composites containing an X-ray protective filler, namely bismuth oxide (Bi2O3) particles, was investigated as novel, safe, and environmentally friendly X-ray shielding materials. The wood and Bi2O3 contents used in this work varied from 20 to 40 parts per hundred parts of PVC by weight (pph) and from 0 to 25, 50, 75, and 100 pph, respectively. The study considered X-ray shielding, mechanical, density, water absorption, and morphological properties. The results showed that the overall X-ray shielding parameters, namely the linear attenuation coefficient (µ), mass attenuation coefficient (µm), and lead equivalent thickness (Pbeq), of the WPVC composites increased with increasing Bi2O3 contents but slightly decreased at higher wood contents (40 pph). Furthermore, comparative Pbeq values between the wood/PVC composites and similar commercial X-ray shielding boards indicated that the recommended Bi2O3 contents for the 20 pph (40 ph) wood/PVC composites were 35, 85, and 40 pph (40, 100, and 45 pph) for the attenuation of 60, 100, and 150-kV X-rays, respectively. In addition, the increased Bi2O3 contents in the WPVC composites enhanced the Izod impact strength, hardness (Shore D), and density, but reduced water absorption. On the other hand, the increased wood contents increased the impact strength, hardness (Shore D), and water absorption but lowered the density of the composites. The overall results suggested that the developed WPVC composites had great potential to be used as effective X-ray shielding materials with Bi2O3 acting as a suitable X-ray protective filler.


Introduction
X-rays are ionizing radiation with energies of 100 eV-100 keV and frequencies of 10 16 -10 20 Hz and are currently utilized in various applications, including X-ray imaging for the diagnosis of brain and lung cancers [1], low-dose X-ray radiotherapy [2], X-ray fluorescence (XRF) and X-ray diffraction (XRD) for material and archeological characterizations [3][4][5], and X-ray irradiation of economic plants to accelerate breeding and mutations [6]. Despite its great benefit and potential, excessive exposure to X-rays could harmfully affect both the radiation users and the general public, whose symptoms may vary from mild conditions (nausea, vomiting, diarrhea, fever, loss of appetite, skin burn, and hair loss) to severe conditions (cognitive impairment, seizures, electrolyte disturbance, cancers, and death) depending on the X-ray energy, exposure dose and rate, and the organ response to exposure [7,8]. Consequently, to reduce and/or prevent risks from As aforementioned, the current work aimed to expand the limited information/data on WPC, especially on WPVC composites, in X-ray shielding applications by determining the X-ray attenuation, mechanical, morphological, density, and water absorption properties of WPVC composites containing Bi 2 O 3 particles. The contents of wood particles were varied from 20 to 40 parts per hundred parts (pph) of PVC by weight and the Bi 2 O 3 contents were varied from 0 to, 25,50,75, and 100 pph. The X-rays used for the measurement of shielding properties were generated from an X-ray tube with supplied voltages of 60, 100, and 150 kV, and the following X-ray shielding properties were determined and reported: µ, the mass attenuation coefficient (µ m ), and the Pb equivalent thickness (Pb eq ). In addition, recommended Bi 2 O 3 contents for each X-ray energy were determined. The outcomes of this work should not only present novel, effective, and safe X-ray shielding materials from Bi 2 O 3 /WPVC composites but also broaden the data availability of WPVC-based materials for future development as improved radiation shielding materials.

Materials and Chemicals
Suspension PVC powder (trade name SIAMVIC-258RB) with a K value of 58 was used as the main matrix for this work. Other chemicals, with their functions, contents, and suppliers, for the production of WPVC composites are given in Table 1. Optical and micrograph images of wood particles and Bi 2 O 3 taken using a scanning electron microscope are shown in Figure 1. It should be noted that their mean (±standard deviation) particle sizes were 0.5 ± 0.1 mm and 27.4 ± 8.2 µm, respectively, determined using their respective micrograph images in the Image J software (version 1.50i).

Preparation of WPVC Composites
The wood particles were chemically surface-treated with a silane coupling agent (KBM603) solution using a high-speed mixer at 1000 rpm for 5 min and dried in a hot-air oven (GT-7017-L, Gotech Testing Machine, Taichung City, Taiwan) at 80 • C for 72 h until constant weight was achieved. It should be noted that the treatment of the wood particles followed procedures optimized by our previous works [34]. Then, the surface-treated wood particles were mixed with the suspension PVC and other chemicals (Table 1) using a highspeed mixer at 1000 rpm for 5 min. The mixtures were then melt-blended using a twin-screw extruder (CTW 100 QC, HAAKE™ Rheomex, Kahlsruhe, Germany) with a screw speed of 40 rpm and temperature settings of 140, 150, 160, and 160 • C for the feed zone, plastification zone, mixing zone, and die zone, respectively. The extrudate was then pelletized into granules and dried in a hot-air oven at 80 • C overnight to completely remove moisture. The dried granules were then molded into specimens using hot compression molding (LP-20M; Labtech Engineering Co., Ltd., Bangkok, Thailand) at 170 • C and pressure of 150 kg/cm 2 for 8 min. It should be noted that the molds used in this work were 15 cm × 15 cm with two different thicknesses (3 mm and 10 mm).
based materials for future development as improved radiation shielding materials.

Materials and Chemicals
Suspension PVC powder (trade name SIAMVIC-258RB) with a K value of 58 used as the main matrix for this work. Other chemicals, with their functions, contents suppliers, for the production of WPVC composites are given in Table 1. Optical and crograph images of wood particles and Bi2O3 taken using a scanning electron micros are shown in Figure 1. It should be noted that their mean (±standard deviation) pa sizes were 0.5 ± 0.1 mm and 27.4 ± 8.2 µ m, respectively, determined using their respe micrograph images in the Image J software (version 1.50i).

X-ray Shielding Measurement
The X-ray shielding properties of the WPVC composites were characterized by determining the values of the X-ray transmission ratio (I/I 0 ), the linear attenuation coefficient (µ), the mass attenuation coefficient (µ m ), and the lead (Pb) equivalent thickness (Pb eq ) [12].
The tests were performed at the Secondary Standard Dosimetry Laboratory (SSDL), the Office of Atoms for Peace (OAP), Bangkok, Thailand, using the experimental setup shown in Figure 2.
To perform the X-ray shielding measurement, X-rays, generated using an X-ray tube with supplied voltages of 60, 100, and 150 kV, and collimated using a 1 mm Pb pinhole, were directed at the center of 10 mm-thick WPVC samples. The transmitted X-rays were then detected and counted using a free air ionization chamber (Korea Research Institute of Standards and Science, KRISS; Daejeon, Korea), which was installed on the calibration bench in the setup. It should be noted that the detector was powered by a high voltage power supply (Keithley 247, Cleveland, OH, USA) and connected to an electrometer (Keithley 6517B, Cleveland, OH, USA) to complete the detection system. The X-ray source used in this work was controlled by X-ray systems (YXLON MGC41, Hudson, NY, USA) and their energies were selected based on the standard method recommended by ISO4037-1:2019.
where I, I0, x, ρ, µ Pb, and µ WPVC are the intensity of transmitted X-rays, the intensity of the incident X-rays, the thickness of the WPVC samples, the density of the WPVC samples, the linear attenuation coefficient of Pb, and the linear attenuation coefficient of the WPVC samples, respectively. For comparative purposes, a pure Pb sheet was also tested using the same testing procedure and setup, and its I/I0 and µ Pb were calculated and reported.

Mechanical Properties
The flexural strength of the WPVC composites was investigated using a universal testing machine (The Starrett FMS5000; Lynchburg, VA, USA), following the ASTM D790-10 standard testing. The Izod impact strength of the WPVC composites was measured according to ASTM D256-10, using a pendulum impact testing machine (United Test JB-300B; Beijing United Test Co., Ltd., Beijing, China). For hardness (Shore D) measurement, all samples were tested according to ASTM D2240-05 standard testing using a hardness durometer (Shore D) (Teclock GS-720N, Nagano, Japan). It should be noted that all mechanical measurements were conducted with at least three specimens for each formulation.

Morphology and Density Measurement
The morphology and dispersion of the Bi2O3 and wood particles in the WPVC composites were determined using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) (Quanta 450 FEI: JSM-6610LV, Eindhoven, The Netherlands) at a 10-kV accelerating voltage. Prior to the SEM-EDX images being taken, all specimens were coated with gold using a sputter coater (Quorum SC7620: Mini Sputter Coater/Glow Discharge System, Laughton, UK) at a power voltage of 10 kV and a current of 10 mA for 120 s.
The density (ρ) of each sample was determined by finding the ratio of the mass (m) to the volume (V) of the specimen [11] and the results are shown in Table 2, which indicates that the densities tended to increase with increasing Bi2O3 contents but slightly de- To determine I/I 0 , µ, µ m , and Pb eq , three independent 5 min tests were performed and their values were calculated using Equations (1)-(3): where I, I 0 , x, ρ, µ Pb , and µ WPVC are the intensity of transmitted X-rays, the intensity of the incident X-rays, the thickness of the WPVC samples, the density of the WPVC samples, the linear attenuation coefficient of Pb, and the linear attenuation coefficient of the WPVC samples, respectively. For comparative purposes, a pure Pb sheet was also tested using the same testing procedure and setup, and its I/I 0 and µ Pb were calculated and reported.

Mechanical Properties
The flexural strength of the WPVC composites was investigated using a universal testing machine (The Starrett FMS5000; Lynchburg, VA, USA), following the ASTM D790-10 standard testing. The Izod impact strength of the WPVC composites was measured according to ASTM D256-10, using a pendulum impact testing machine (United Test JB-300B; Beijing United Test Co., Ltd., Beijing, China). For hardness (Shore D) measurement, all samples were tested according to ASTM D2240-05 standard testing using a hardness durometer (Shore D) (Teclock GS-720N, Nagano, Japan). It should be noted that all mechanical measurements were conducted with at least three specimens for each formulation.

Morphology and Density Measurement
The morphology and dispersion of the Bi 2 O 3 and wood particles in the WPVC composites were determined using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) (Quanta 450 FEI: JSM-6610LV, Eindhoven, The Netherlands) at a 10-kV accelerating voltage. Prior to the SEM-EDX images being taken, all specimens were coated with gold using a sputter coater (Quorum SC7620: Mini Sputter Coater/Glow Discharge System, Laughton, UK) at a power voltage of 10 kV and a current of 10 mA for 120 s.
The density (ρ) of each sample was determined by finding the ratio of the mass (m) to the volume (V) of the specimen [11] and the results are shown in Table 2, which indicates that the densities tended to increase with increasing Bi 2 O 3 contents but slightly decreased with increasing wood contents. This could have been due to the much higher density of Bi 2 O 3 (ρ Bi2O3 = 8.9 g/cm 3 ) [12,19,23] compared to PVC (ρ PVC = 1.43 g/cm 3 ) [35] that resulted in the higher overall densities of the WPVC composites after the addition of Bi 2 O 3 into the composites. In contrast to Bi 2 O 3 , the addition of wood particles slightly reduced the overall densities of the composites as the wood particles had much lower densities than Bi 2 O 3 and PVC (ρ wood = 0.48-0.65 g/cm 3 ) [36], resulting in an approximately 2% decrease in the densities of the WPVC composites containing 40 pph wood particles compared to those with 20 pph wood particles.

Water Absorption Measurement
The measurement of water absorption for all WPVC composites was performed following ASTM D570-98 (2018) standard testing, with at least three specimens for each formulation tested. The WPVC specimens were dried in a hot-air oven (GT-7017-L, Gotech Testing Machine, Taiwan) at 50 • C for 24 h to achieve a constant weight and then immersed in a deionized water bath for 24 h. The specimens were taken out of the water, wiped with tissue paper to remove surface water, and immediately reweighed on a balance with precision of 0.0001 g. The percentage of water absorption was then calculated by finding the ratio of the weight difference between the sample submerged in water (W s ) and the dried sample (W d ) to the weight of the dried sample (W d ) as shown in Equation (4):

Mechanical Properties
Mechanical properties, namely the flexural strength, the Izod impact strength, and the hardness (Shore D), of the WPVC composites containing varying contents of Bi 2 O 3 are shown in Figure 3. The results indicated that values of flexural strength for all samples were not statistically different (the values fluctuated within their standard deviations), with the values being in the range 40.92-45.49 MPa. This behavior was observed due to the wood particles, for which their surfaces were chemically treated using a silane coupling agent, exhibiting improved compatibility with the PVC matrix, as well as the added Bi 2 O 3 being uniformly dispersed throughout the matrix by the high shear stress of a twin-screw extruder. In addition, the results suggested that the values of the Izod impact strength and hardness (Shore D) were enhanced with the addition of Bi 2 O 3 , except for the WPVC composites containing 100 pph Bi 2 O 3 and 40 pph wood particles that had noticeably lower Izod impact strength than those with 75 pph Bi 2 O 3 and 40 pph wood particles (approximately 30% decrease). Similar to Bi 2 O 3 , the samples having a higher wood content (40 pph) generally had higher overall mechanical strength at the same Bi 2 O 3 content than those with 20 pph wood particles, implying the role of wood particles in the WPVC composites as a reinforcing filler.  The increases in the Izod impact strength with the addition of Bi 2 O 3 and wood particles, which was as high as 217.5 ± 1.3 J/m in the sample with 75 pph Bi 2 O 3 and 40 pph wood particles, could have been due to both fillers in the matrix limiting chain segmental motions and consequently reducing the flexibility of the matrix reins that required higher energy to fracture the WPVC composites [40,41]. However, for the composites with very high filler contents, such as with 100 pph Bi 2 O 3 and 40 pph wood particles, the Izod impact strength was considerably lower than those having less filler due to increases in the agglomeration of fillers in the PVC matrix that resulted in increased numbers of defects and voids. To illustrate the effects of a high filler content on the morphology of a composite, SEM images showing filler distribution as well as particle agglomeration in different composites are shown in Figure 4, which indicates that the composites with 100 pph Bi 2 O 3 (Figure 4e,f) clearly had more voids in the matrix than the neat WPVC composites (Figure 4a,b) and those containing 50 pph of Bi 2 O 3 (Figure 4c,d), resulting in a substantially reduced Izod impact strength. It was notable that the observed behaviors were in agreement with previous reports of SiO 2 /epoxy and graphene oxide/epoxy composites for which the impact strength decreased at high filler contents caused by the agglomeration of the fillers [40,41]. In addition to the flexural and Izod impact strengths, Figure 3c suggested that the hardness (Shore D) of the WPVC composites increased with increasing Bi 2 O 3 and wood contents. This behavior was observed due to the high rigidity of the Bi 2 O 3 and wood particles that enhanced the overall rigidity and, subsequently, surface hardness of the WPVC composites [19,42]. The increases in the Izod impact strength with the addition of Bi2O3 and wood particles, which was as high as 217.5 ± 1.3 J/m in the sample with 75 pph Bi2O3 and 40 pph wood particles, could have been due to both fillers in the matrix limiting chain segmental motions and consequently reducing the flexibility of the matrix reins that required higher energy to fracture the WPVC composites [40,41]. However, for the composites with very high filler contents, such as with 100 pph Bi2O3 and 40 pph wood particles, the Izod impact strength was considerably lower than those having less filler due to increases in the agglomeration of fillers in the PVC matrix that resulted in increased numbers of defects and voids. To illustrate the effects of a high filler content on the morphology of a composite, SEM images showing filler distribution as well as particle agglomeration in different composites are shown in Figure 4, which indicates that the composites with 100 pph Bi2O3 (Figure 4e,f) clearly had more voids in the matrix than the neat WPVC composites (Figure  4a,b) and those containing 50 pph of Bi2O3 (Figure 4c,d), resulting in a substantially reduced Izod impact strength. It was notable that the observed behaviors were in agreement with previous reports of SiO2/epoxy and graphene oxide/epoxy composites for which the impact strength decreased at high filler contents caused by the agglomeration of the fillers [40,41]. In addition to the flexural and Izod impact strengths, Figure 3c suggested that the hardness (Shore D) of the WPVC composites increased with increasing Bi2O3 and wood contents. This behavior was observed due to the high rigidity of the Bi2O3 and wood particles that enhanced the overall rigidity and, subsequently, surface hardness of the WPVC composites [19,42].   that the values of flexural strength of WPVC composites were lower than the neat PVC, which could be due to incompatibility between the PVC matrix and the wood particles as well as differences in shrinkage of the matrix and wood particles during cooling, which generated defects and voids within the composites [37]. On the other hand, the Izod impact strength and hardness (Shore D) of the WPVC composites were noticeably higher than the neat PVC, mainly due to the reduced chain segmental motions caused by the added fillers and the high rigidity of the fillers, respectively.

Water Absorption
The results for the determination of water absorption in the WPVC composites containing varying contents of Bi 2 O 3 and wood particles are shown in Figure 5, which revealed that the percentage of water absorption decreased (increased) with increasing Bi 2 O 3 (wood) contents. For example, samples containing 40 pph wood particles had approximately two times higher water absorption than those containing 20 pph wood particles at the same Bi 2 O 3 content, due to the increased numbers of hydroxyl groups in the wood fiber structure that enhanced the overall hydrophilicity and, subsequently, the water absorption of the former composites [31]. In contrast, increases in the Bi 2 O 3 contents resulted in lower water absorption of the WPVC composites. This could have been due to the dilution effects of Bi 2 O 3 , which had lower hydrophilicity than the wood particles as seen by greater water contact angle of~107 • in Bi 2 O 3 [43] than those of~65 • in wood particles [44], which resulted in a lower weight fraction of wood particles in the composites and, consequently, reduced the overall hydrophilicity and water absorption of the composites [42].
Polymers 2021, 13, x FOR PEER REVIEW 9 of 17 Figure 3 also shows mechanical properties of a neat PVC (dashed lines), indicating that the values of flexural strength of WPVC composites were lower than the neat PVC, which could be due to incompatibility between the PVC matrix and the wood particles as well as differences in shrinkage of the matrix and wood particles during cooling, which generated defects and voids within the composites [37]. On the other hand, the Izod impact strength and hardness (Shore D) of the WPVC composites were noticeably higher than the neat PVC, mainly due to the reduced chain segmental motions caused by the added fillers and the high rigidity of the fillers, respectively.

Water Absorption
The results for the determination of water absorption in the WPVC composites containing varying contents of Bi2O3 and wood particles are shown in Figure 5, which revealed that the percentage of water absorption decreased (increased) with increasing Bi2O3 (wood) contents. For example, samples containing 40 pph wood particles had approximately two times higher water absorption than those containing 20 pph wood particles at the same Bi2O3 content, due to the increased numbers of hydroxyl groups in the wood fiber structure that enhanced the overall hydrophilicity and, subsequently, the water absorption of the former composites [31]. In contrast, increases in the Bi2O3 contents resulted in lower water absorption of the WPVC composites. This could have been due to the dilution effects of Bi2O3, which had lower hydrophilicity than the wood particles as seen by greater water contact angle of ~107° in Bi2O3 [43] than those of ~65° in wood particles [44], which resulted in a lower weight fraction of wood particles in the composites and, consequently, reduced the overall hydrophilicity and water absorption of the composites [42].  Table 3 shows the values of µ and µ m (representing the fraction of attenuated incident X-rays in a monoenergetic beam per unit thickness and unit mass, respectively) in the WPVC composites containing 0-100 pph of Bi2O3 and 20-40 pph of wood particles. The results indicated that increases in the Bi2O3 content led to the overall enhancement of Xray shielding properties as seen by the notable increases in the values of µ and µ m at higher Bi2O3 contents. The most pronounced increases in µ and µ m were observed when Bi2O3  Table 3 shows the values of µ and µ m (representing the fraction of attenuated incident X-rays in a monoenergetic beam per unit thickness and unit mass, respectively) in the WPVC composites containing 0-100 pph of Bi 2 O 3 and 20-40 pph of wood particles. The results indicated that increases in the Bi 2 O 3 content led to the overall enhancement of X-ray shielding properties as seen by the notable increases in the values of µ and µ m at higher Bi 2 O 3 contents. The most pronounced increases in µ and µ m were observed when Bi 2 O 3 was initially added to neat WPVC composites (from 0 to 25 pph), resulting in approximately a two-times improvement in the µ and µ m values. The positive dependency of the X-ray shielding abilities on the Bi 2 O 3 contents was mainly due to the effective role of Bi 2 O 3 in the enhancement of X-ray attenuation, with Bi 2 O 3 acting as an X-ray attenuator that increased the interaction probabilities between the incident X-rays and the materials [22,24]. To show the distribution of Bi elements in the WPVC composites, especially for those having high Bi 2 O 3 contents, SEM-EDX images of all samples containing Bi 2 O 3 are shown in Figure 6, which revealed that there were clearly more Bi atoms (shown as red areas) in the samples having higher Bi 2 O 3 contents, especially in Figure 6g,h. Thus, there was a higher X-ray shielding capability because there were more available Bi atoms to interact with X-rays. Table 3. X-ray shielding properties, including linear attenuation coefficients (µ) and mass attenuation coefficient (µ m ) of WPVC composites containing 0-100 pph of Bi 2 O 3 and 20-40 pph of wood particles, Pb sheet, and a neat PVC, determined for X-ray levels of 60, 100, and 150-kV. Results are shown as mean ± standard deviation.  Table 3. X-ray shielding properties, including linear attenuation coefficients (µ ) and mass attenuation coefficient (µ m) of WPVC composites containing 0-100 pph of Bi2O3 and 20-40 pph of wood particles, Pb sheet, and a neat PVC, determined for X-ray levels of 60, 100, and 150-kV. Results are shown as mean ± standard deviation.  The Pb equivalent thickness (Pbeq), which represents the thickness of material of concern affording the same X-ray attenuation as a Pb sheet with a certain thickness (under the same specified conditions such as X-ray energies and beam sizes) at 60, 100, and 150-kV X-rays for all WPVC composites are shown in Figure 7. The results revealed that the dependence of Pbeq on Bi2O3 was similar to those of µ and µ m (Table 3) as the Pbeq values increased with increasing Bi2O3 content but decreased with increasing wood content/Xray energy. For example, the Pbeq values of the 10 mm-thick samples were as high as 0.8 mmPb in those containing 100 pph Bi2O3, which were considerably greater than for the neat WPVC composites (0 pph Bi2O3) that only had approximately 0.1 mm Pb.

Wood
In order to determine recommended Bi2O3 contents in the WPVC composites for actual use, especially in medical applications, Pbeq values obtained from the datasheets of three commercial X-ray shielding products based on a gypsum board (XRoc board) [48], On the other hand, Table 3 showed that increasing the wood content from 20 pph to 40 pph resulted in slightly lower values of µ and µ m , although the reduced values were less than 10% (determined for the same Bi 2 O 3 content). This lower X-ray shielding property for samples with more wood particles added to the composite was due to the much lower X-ray interaction probability for the wood particles (mostly comprised of C, H, and O) compared to Bi 2 O 3 (µ m-wood = 0.162 cm 2 /g and µ m-Bi2O3 = 5.162 cm 2 /g for 100 keV X-rays [45]), resulting in suppressed effects of Bi 2 O 3 in X-ray attenuation and thus, less X-ray shielding capability for those samples containing 40 pph wood particles [20]. Additionally, notable from Table 3 was that the values of µ and µ m decreased at higher X-ray energies (determined at the same Bi 2 O 3 and wood contents). This could have been due to higher-energy X-rays being less likely to interact with materials through dominant and effective photoelectric absorption, which rapidly decreased with increasing X-ray energies/frequencies, as depicted in Equation (5) [20,46]: where σ pe is the photoelectric cross section, Z is the atomic number of the element, h is Planck's constant, and ν is the frequency of the X-rays that is directly related to X-ray energy through Equation (6): It is also notable that the values of µ and µ m for a neat PVC, calculated using a webbased software (XCOM) [20,47], were similar to WPVC composites without the addition of Bi 2 O 3 , due to the low σ pe of C, H, and Cl in PVC, leading to low interaction probabilities between the X-rays and the neat PVC.
The Pb equivalent thickness (Pb eq ), which represents the thickness of material of concern affording the same X-ray attenuation as a Pb sheet with a certain thickness (under the same specified conditions such as X-ray energies and beam sizes) at 60, 100, and 150-kV X-rays for all WPVC composites are shown in Figure 7. The results revealed that the dependence of Pb eq on Bi 2 O 3 was similar to those of µ and µ m (Table 3) as the Pb eq values increased with increasing Bi 2 O 3 content but decreased with increasing wood content/X-ray energy. For example, the Pb eq values of the 10 mm-thick samples were as high as 0.8 mm Pb in those containing 100 pph Bi 2 O 3 , which were considerably greater than for the neat WPVC composites (0 pph Bi 2 O 3 ) that only had approximately 0.1 mm Pb.
In order to determine recommended Bi 2 O 3 contents in the WPVC composites for actual use, especially in medical applications, Pb eq values obtained from the datasheets of three commercial X-ray shielding products based on a gypsum board (XRoc board) [48], plasterboard (GIB X-Block board) [49] and Knauf Safe board [50], for which the average Pb eq values of the commercial products were 0.45, 0.70, and 0.40 mm Pb for the 60, 100, and 150-kV X-rays, respectively, were compared with those from the current work. It should be noted that the Pb eq values of the commercial products for 100 kV X-rays were higher than other X-ray energies due to the sharp increase in X-ray interaction probabilities at the K-edge absorption of barium (Ba), which was used as the main X-ray protective filler for the commercial products, that occurs at 37.4 keV [20]. Hence, the 100 kV X-rays, with their average energy around 40-60 keV depending on the type and setup of the X-ray machine, were just above the binding energy of the electron K shells inside the Ba atoms, leading to immensely enhanced probabilities of X-ray interaction through photoelectric absorption and, subsequently, higher Pb eq values at these particular energies [51].
As shown in Figure 7, the Pb eq values from the commercial products (represented as red dotted lines) intercepted with the lines of Pb eq values from the current work at different Bi 2 O 3 contents depending on the X-ray energy and wood content. These interception points between the two lines implied the least Bi 2 O 3 content that could produce the same X-ray attenuation ability as from commercial products, which could be regarded as the recommended Bi 2 O 3 contents for actual production and use. The results from the determination of the recommended Bi 2 O 3 contents for all X-ray energies are shown in Table 4, which indicated that the 60 kV and 150 kV X-rays required similar Bi 2 O 3 contents of 35-40 pph and 40-45 pph, while the 100 kV X-rays required the highest Bi 2 O 3 contents of 85 pph and 100 pph, for the WPVC composites containing 20 pph and 40 pph wood particles, respectively. Notably, the recommended Bi 2 O 3 contents for the 100 kV X-rays were higher than for the other X-ray energies due to the higher Pb eq values of the commercial products used as references. Notably, the recommended Bi 2 O 3 contents for WPVC composites containing 40 pph wood particles were higher than those containing 20 pph wood particles due to the suppressed effects of Bi 2 O 3 particles in attenuating the incident X-rays due to the greater number of wood particles in the composites.    40 45 Lastly, the comparative X-ray shielding properties of the WPVC composites and previously reported composites containing Bi 2 O 3 with similar filler contents and X-ray energies are shown in Table 5. The results indicated that the WPVC composites in the current work could attenuate X-rays with higher efficiencies than silicone rubber (SR) containing 50 wt.% of Bi 2 O 3 but with slightly lower efficiencies than 50 wt.%-Bi 2 O 3 /natural rubber latex (NRL) and 35 wt/%-Bi 2 O 3 /epoxy composites. The lower µ m values in the WPVC composites could have been due to the WPVC composites being investigated at higher X-ray energies as well as having less Bi atoms dispersed inside the matrix than those in the NRL and epoxy composites, which was possibly caused by fewer filler contents, the type of the main matrix, and the processing method. Nonetheless, despite their lower X-ray shielding properties, the WPVC composites were more rigid and stronger than the NRL and SR composites, making the former suitable in movable partition walls and transportation casks. Furthermore, the WPVC composites also promoted the use of natural products, which can help in reducing agricultural and industrial wastes.

Conclusions
This work developed X-ray shielding materials from WPVC composites containing Bi 2 O 3 , with varying contents of wood particles from 20 to 40 pph and of Bi 2 O 3 from 0 to 100 pph in 25 pph increments. The results suggested that an increased Bi 2 O 3 content led to non-statistically differences in flexural strength (the values fluctuated in the range 40.92-45.49 MPa), increased X-ray attenuation, Izod impact strength, hardness (Shore D), and density, but decreased water absorption. Furthermore, the results showed that an increased wood content tended to increase the Izod impact strength, hardness (Shore D), and water absorption, but to slightly decrease the X-ray attenuation and density of the WPVC composites. A comparison of the Pb eq values obtained from the current work with similar commercial X-ray shielding products for 60, 100, and 150-kV X-rays indicated that the 60 kV and 150 kV X-rays required Bi 2 O 3 contents of 35-45 pph, while the 100 kV X-rays required Bi 2 O 3 contents of 85-100 pph for these WPVC composites to attenuate X-rays with the same levels of efficiency as the referenced products. Lastly, a comparison of X-ray shielding properties between the WPVC composites in the current work and other composites containing Bi 2 O 3 with similar filler contents and X-ray energies revealed that the former could offer comparable or better X-ray attenuation than the latter for the same range of X-ray energy, indicating that the current work was successful in developing WPVC composites that had both sufficient X-ray shielding and high strength for actual production and use.