Polymer Composites and Nanocomposites for X-Rays Shielding

Radiation (X-ray/gamma-ray) shielding is an age-old problem and has been studied in greater details up till now, due to their interactions being most prominent in many applications such as in medical fields (i.e. diagnostic imaging and therapy). X-rays/gamma-rays are the most penetrating of ionizing radiation that is known to be harmful to human health and heredity. X-ray photons are produced by the interaction of energetic electrons with matter at the atomic level. As an X-ray beam passes through an object, (i) it can penetrate the section of the matter without any interaction (i.e. Thompson scattering); (ii) it can interact with the matter and totally be absorbed by the atoms of the matter (i.e. photoelectric effect) or (iii) it can interact partially with the matter and be scattered from its original direction in all directions (i.e. Compton scattering). Hence, there is no doubt that X-ray shielding requirements have become more stringent as standards for exposure of personnel (patient and radiographer worker) and the general public during the diagnosis examination is carried out.

Various composite samples were prepared by mixing lead oxide, bismuth oxide or tungsten oxide of either nano-sized or micro-sized with epoxy resin, acrylic, PLA and PVA polymers using three different methods i.e melt-mixing, ion-implantation and electrospinning. Meanwhile, there were various samples characterization tests were done such as X-ray attenuation measurement, density measurement and morphology studies in order to characterize the properties of the prepared samples so that they were applicable and suitable to be the X-ray shielding material candidate.

Composite epoxy samples filled with PbO and Pb₃O₄ were fabricated to investigate the mass attenuation characteristics of the composites to X-rays in the diagnostic imaging energy range. The effect of density on the attenuation ability of the composites for radiation shielding purposes was studied using a calibrated X-ray machine. Characterization of the microstructure properties of the synthesized composites was performed using synchrotron radiation diffraction, optical microscopy, and scanning electron micros-copy. The results indicate that the attenuation ability of the composites increased with an increase in density. The particle size of WO₃ fillers has a negligible effect on the value of mass attenuation coefficient. Microstructural analyses have confirmed the existence of uniform dispersion of fillers within the matrix of epoxy matrix with the average particle size of 1–5 μm for composites with filler loading of ≤30 wt% and 5–15 μm for composites with filler loading of ≥50 wt%.

Lead chloride, bismuth oxide and tungsten oxide filled epoxy composites with different weight fractions were fabricated to investigate their X-ray transmission characteristics in the X-ray diagnostic imaging energy range (40–127 kV) by using a conventional laboratory X-ray machine. Characterizations of the microstructure-properties of the synthesized composites were performed using synchrotron radiation diffraction, backscattered electron imaging microscopy, three-point bend test and Rockwell hardness test. As expected, the X-ray transmission was decreased by the increment of the filler loading. Meanwhile, the flexural modulus and hardness of the composites were increased through an increase in filler loading. However, the flexural strength showed a marked decrease with the increment of filler loading (≥30 wt%). Some agglomerations were observed for the composites having ≥50 wt% of filler.

The effect of particle size, filler loadings and X-ray energy on the transmitted X-ray beam intensity by WO₃-epoxy composites has been investigated using the mammography unit and a general radiography unit. Results indicate that nano-sized WO₃ has a better ability to attenuate X-ray produced by lower X-ray tube voltages (22–35 kV) when compared to micro-sized WO₃ of the same filler loading. However, the role of particle size on transmitted X-ray beam intensity was negligible at the higher X-ray tube voltage range (40–120 kV).

Characteristics of X-ray transmissions were investigated for epoxy composites filled with 2–10 vol.% WO₃ loadings using synchrotron X-ray Absorption Spectroscopy (XAS) at 10–40 keV. The results obtained were used to determine the equivalent X-ray energies for the operating X-ray tube voltages of mammography and radiology machines. The results confirmed the superior attenuation ability of nano-sized WO₃-epoxy composites in the energy range of 10–25 keV when compared to their micro-sized counterparts. However, at higher synchrotron radiation energies (i.e., 30–40 keV), the X-ray transmission characteristics were similar with no apparent size effect for both nano-sized and micro-sized WO₃-epoxy composites. The equivalent X-ray energies for the operating X-ray tube voltages of the mammography unit (25–49 kV) were in the range of 15–25 keV. Similarly, for a radiology unit operating at 40–60 kV, the equivalent energy range was 25–40 keV, and for operating voltages greater than 60 kV (i.e., 70–100 kV), the equivalent energy was in excess of 40 keV. The mechanical properties of epoxy composites increased initially with an increase in the filler loading but a further increase in the WO₃ loading resulted in deterioration of flexural strength, modulus and hardness.

The epoxy samples were implanted with heavy ions such as tungsten (W), gold (Au) and lead (Pb) to investigate the attenuation characteristics of these composites. Near-surface composition depth profiling of ion-implanted epoxy systems was studied using Rutherford Backscattering Spectroscopy (RBS). The effect of implanted ions on the X-ray attenuation was studied with a general diagnostic X-ray machine with X-ray tube voltages from 40 to 100 kV at constant exposure 10 mAs. Results show that the threshold of implanted ions above which X-ray mass attenuation coefficient, μ_m of the ion-implanted epoxy composite is distinguishably higher than the μ_m of the pure epoxy sample is different for W, Au and Pb.

Samples of acrylic and glass were implanted with tungsten (W) and lead (Pb) to investigate their X-ray attenuation characteristics. The near-surface composition depth profiles of ion-implanted acrylic and glass samples were studied using ion-beam analysis (Rutherford backscattering spectroscopy—RBS). The effect of implanted ions on the X-ray attenuation ability was studied using a conventional laboratory X-ray machine with X-ray tube voltages ranging from 40 to 100 kV at constant exposure 10 mAs. The results were compared with previous work on ion-implanted epoxy. As predicted, the RBS results and X-ray attenuation for both ion-implanted acrylic and glass increase with the type of implanted ions when compared to the controls. However, since the glass is denser than epoxy or acrylic, it has provided the higher X-ray attenuation property and higher RBS ion concentration implanted with a shorter range of the ion depth profile when compared to epoxy and acrylic. A prolonged time is necessary for implanting acrylic with a very high nominal dose to minimize a high possibility of acrylic to melt during the process.

The characteristics of X-ray transmission in electrospun nano(n)- and micro(m)-Bi₂O₃/poly lactic acid (PLA) nanofibre mats with different Bi₂O₃ loadings were compared using mammography (22–49 kV) and X-ray absorption spectroscopy (XAS) (7–20 keV). Results indicate that X-ray transmissions by electrospun m-Bi₂O₃/PLA nanofibre mats are distinctly higher than those of n-Bi₂O₃/PLA nanofibre mats at all energies investigated. In addition, with increasing the filler loading (n-Bi₂O₃ or m-Bi₂O₃), the porosity of electrospun Bi₂O₃/PLA nanofibre mats decreased thus decreasing the X-ray transmission except for the nanofibre mat containing 38 wt% of Bi₂O₃ (the highest loading in the present study). The latter showed higher porosity with some beads formed thus resulting in a sudden increase in X-ray transmission.

The effect of Bi₂O₃ particle sizes filled PVA composites on X-ray transmission for X-ray shielding purpose had been successfully fabricated and analyzed by using X-ray fluorescent spectroscopy (XRF) and mammography units with various low X-ray energy ranges. Besides, a preliminary investigation was carried out by using XRF unit to obtain the effect of starch addition into the composite on the X-ray transmissions by both particle sizes of Bi₂O₃–PVA composites. The results showed that the ability of the composite to attenuate the initial X-ray beam was augmented with the increased Bi₂O₃ weight percentage (wt%). The density of both particle sizes of Bi₂O₃–PVA composites was compared with the addition of 1 and 3 wt% starch, while a fluctuation of density occurred for the composites without starch. Moreover, the nanosized Bi₂O₃–PVA composite without starch did not exemplify better X-ray attenuation capability compared to its micro-sized counterpart even though their density was higher than the micro-sized Bi₂O₃–PVA composite. However, the nano-sized Bi₂O₃–PVA composite with starch offered better particle size effect for X-ray shielding ability than its micro-sized counterpart compared to the Bi₂O₃–PVA composites without starch.

The use of viable processing methodologies for designing new materials with advanced nanotechnology to enhance radiation shielding purposes in order to meet the safety requirements for use in medical X-ray imaging facilities has been achieved. However, very limited or little information is still available in this emerging research field. This leaves a wide scope for future investigators to make further advances in new materials design and processing.