Abstract
The design of lightweight neutrons shields has been restricted for quite some time to the use of the epoxy thermosets as the main building blocks. Meanwhile, the recent developments in the field of polymers suggest otherwise. Indeed, benzoxazine resins have taken the lead over the traditional thermosets in many exigent applications. Therefore, in a vision to introduce newer matrices with better performances and to further expand the applications of the benzoxazine resins into the nuclear field, the neutron shielding efficiency along with the thermal and thermomechanical performances of the neat benzoxazine polymer and its subsequent B4C-reinforced composites were investigated. The neutron shielding measurements were performed using an optimized experimental setup at NUR research reactor, Algiers. The neat benzoxazine polymer displayed almost similar thermal neutrons screening performances than the epoxy with a macroscopic cross-section (Σ) of a 0.724 cm−1 equivalent to a mean free path (λ) of 0.957 cm. The effect of the particle amount was also studied to maximize the shielding ability of the developed materials. For instance, the benzoxazine composite containing 20 wt.% of B4C displayed the outstanding screening ratio of about 96% for a sample thickness of 13 mm. Finally, the remarkable findings were put into context by providing multifaceted comparisons with the available shielding materials.
Introduction
Since the discovery of the nuclear energy, many researches have been carried out in order to exploit its full potential in various applications including but not limited to medicine, aerospace, military, electricity production, naval propulsion and so on.1–6 However, this highly energetic source generates subsidiary drawbacks negatively affecting human safety and materials. In fact, nuclear reactions produce energy but also other elements such as alpha, beta, gamma and neutrons. The interaction of these elements with human body may engender cancer, skin diseases, damage to vital organs and death.7,8 Aiming to overtake these shortcomings, researchers tried to utilize the existing materials (lead, concrete, water, etc.) or developed new ones to render the use of the nuclear energy safer.9–11 In fact, heavy materials and water are more suitable to be used inside nuclear reactors and to surround radioactive sources. Unfortunately, these materials cannot be used to design protective gears for human body and sensitive equipments. For this reason, hybrid polymeric-based materials were found to be a promising alternative to substitute heavy materials thanks to their improved nuclear shielding performances and relatively lower density.12–14
Neutrons are neutral particles with an atomic mass slightly greater than the mass of the proton. These particles can penetrate through matter and interact with its atoms. The interaction of the neutrons and atoms differs according to their weight. Atoms with low Z have high microscopic cross-section and may absorb neutrons or reduce their velocity. 15 However, the interactions between heavy atoms and neutrons may produce gamma rays. 16 Due to this fact, polymers, rich with lower Z atoms (hydrogen and carbon), gained the interest of researchers to develop new hybrid polymeric-based materials. Actually, these materials are the combination of a polymeric matrix reinforced with fibers and/or particles. The use of high performance thermosets, such as the traditional epoxies as polymeric matrix, produces a hybrid shielding material with proper thermo-mechanical properties. Meanwhile, the incorporation of boron rich particles (B4C, B2O3, BN, etc.) enhances the shielding performances due to the boron capacity of absorbing thermal neutrons.17–19 Considering the latest advances in the field of polymers, a new generation of thermosetting polymers is overtaking the typical epoxy systems in many areas. One of these interesting high performance thermosetting polymers is the polybenzoxazine.
Benzoxazine (BZ) resins, as one of the high performance thermosetting resins, have attracted much of the scientific and industrial attentions in the last few years. Indeed, the BZ resins, owing to their extremely simple synthesis process and to their outstanding combinations of extremely valuable properties, performed to fastest jump from academia to the industrial field. The interesting features of the BZ resins include a catalyst-free polymerization, low melt-viscosity, high thermal stability, good mechanical properties, low dielectric constant and low surface free energy.20,21 The BZ monomers can be readily synthetized via Mannich condensation reaction involving a mixture of phenols, amines and formaldehyde. The ease of synthesis and the existence of a various natural phenolic and amino derivatives may allow researchers to explore many combinations. 22
After an extensive literature review, it appeared that the majority of the developed polymer-based shields were limited to the use of the typical epoxy thermosets. Therefore, in a vision to introduce newer matrices with better performances and to further expand the applications of the BZ resins in the nuclear field, we studied the neutron shielding efficiency and the thermomechanical performances of the neat BZ polymers and its subsequent B4C reinforced composites. In parallel, the same experimentations were performed for the neat epoxy resin and its B4C-based composites, to provide a better comparison between to two systems. The neutron shielding measurements were performed using an optimized experimental setup at NUR research reactor, Algiers. The effect of the particle amount was also studied to maximize the shielding ability of the developed materials. Finally, the obtained results were put into context by providing a proper comparison with the already available shielding materials.
Experimental
Materials
All chemicals used in this work were reagent-grade and were directly used without any further purification. 2,2-bis(4-hydroxyphenyl)propane (>99.0%) (bisphenol-A), cycloaliphatic amine hardener and boron carbide particles B4C (black powder having a density of 2.52 g/cm3 with an average diameter of particles of 10 µm) were obtained from Shanghai Aladdin Reagents (China), aniline was purchased from Honeywell Riedel-de-Haen (Germany) and paraformaldehyde purchased from Merck (Germany). The typical BZ monomers were synthesized in our laboratory following a procedure described in the next section.
Composites preparation
BZ monomers were prepared by mixing bisphenol-A, aniline and paraformaldehyde, with a ratio of 1:2:4, respectively, following a two steps process. First, bisphenol-A was mixed with aniline, for 20 min at 100°C. Next, the temperature was elevated to 130°C, and paraformaldehyde was added to the mixture by small amounts and reacted for an additional 20 min. After that, B4C particles were mixed, at different amounts (0, 10 and 20 wt.% of the matrix) with the obtained BZ monomers and cured at 180°C for 3 h to obtain void-free Polybenzoxazine/B4C composites. Due to the stress of the oxazine ring, the BZ molecule has a tendency to ring-opening polymerization. The oxazine ring can be opened to produce a Mannich base and the propagation can be done on the benzene ring which is very reactive with the Mannich base to proceed with the thermal polymerization giving the polybenzoxazine.
On the other hand, epoxy-based composites were elaborated by mixing 78 wt.% of bisphenol-A-based epoxy resin (previously mixed with B4C, with the quantities already mentioned above) and 22 wt.% cycloaliphatic amine hardener at room temperature. Epoxy monomers polymerize through step-growth and chain-growth processes. The epoxy–amine reaction generates OH groups which catalyze the reaction through the formation of a trimolecular complex that favors the nucleophilic attack of the amino group.
Hereafter, the cured hybrids were labeled as BZ/X B4C and Epoxy/X B4C, where X represents the amount of the particles. Further details regarding the chemical composition of the prepared hybrids are presented in Table 1.
Theoretical weight percentage composition of the prepared composites.
Characterization
Differential scanning calorimetry (DSC) was conducted study the curing behavior of the BZ monomers with and without the presence of the reinforcing phase. Thermal Gravimetric Analysis (TGA) was realized to study the effect of the dispersed ceramic particles on the thermal stability of the polybenzoxazine matrix. The thermal tests were performed under nitrogen atmosphere using an SDT Q600 V20 9 Build 20 Thermal Gravimetric, supplied by TA Instruments at a heating rate of 10°C/min from room temperature to 350°C, for DSC, and to 800°C, for TGA. The Dynamical Mechanical Analysis (DMA) was used to provide information about the stiffness of the materials, and to assess the effects of the B4C particles on the physical characteristics of the BZ resin. The experimentations were performed on a NETZSCH DMA 242 using rectangular samples (50 mm × 10 mm × 3 mm) under a constant flow of nitrogen. The specimens were heated from 25°C to 225°C at a constant heating rate of 2°C/min with a frequency of 2 Hz.
Neutron shielding tests
Neutrons interact with matter, generally, by absorption or scattering away, and can be expressed as follows:
or
where I0 refers to the neutron beam flux without the shielding material, I represents the neutron beam flux through a thickness x (cm) of shielding material, Σ is the neutron attenuation coefficient of a specific material for neutron shielding.
The screening or shielding ratio (S) is defined as follows,
A mean free path (λ) is defined for the necessary thickness of the shielding material to stop the half of the original flux.
The thermal neutron shielding tests were performed within a Uranium-235 (235U) nuclear reactor, shielded with concrete and treated water. The fission reaction of the uranium atoms generates fast neutrons with an average energy of 2 MeV, and a filter was used to transform the polychromatic fast neutron beam into a monochromatic one with an energy of 3.7 meV. In order to determine the different nuclear shielding parameters (Σ, λ and S), the number of transmitted neutrons through various samples were measured by pulse counting. The samples were placed between the thermal neutrons flux and at a distance of 15 cm from the counter (3He proportional counter tube). The sample size was adjusted according to the opening in the reactor tunnel (100 mm × 50 mm × variable thicknesses). Figure 1 further illustrates the nuclear shielding experimental setup.
Experimental neutron shielding setup.
Results and discussion
Thermal and thermo-mechanical properties
As previously mentioned, the curing behavior of the neat BZ monomer and its related B4C-based composites was studied by DSC. As shown in Figure 2, all the samples presented a unique exothermic peak at around 235°C which is characteristic of the self-curing process of the BZ monomers. In fact, the autocatalytic curing of the BZ monomers is one of the many interesting features of this class of high performance thermosets. To better quantify the results, the polymerization enthalpies and temperatures are gathered in Table 2. The obtained data confirmed that the addition of the B4C particles did not affect the curing process, however, the polymerization enthalpy decreased by increasing the particles amounts. This result can be explained by the ceramic particles ability to absorb the heat generated during the ring-opening of the BZ monomers. Additionally, the presence of such high amounts of particles may also delay the crosslinking and impact the polymeric network stiffness. To further investigate this point, thermomechanical analyses were performed and discussed in the next section.
DSC curves of BZ monomer and its mixture with B4C.
DSC properties of the BZ-based monomers.
On the other hand, the thermal stability of the BZ-based composites is presented in Figure 3. The starting decomposition temperatures at 5 and 10 wt.% (T5 and T10) of neat BZ are 313.7°C and 330°C, respectively, with a char yield of 28.2%. The incorporation of B4C particles into the BZ matrix significantly improved its thermal stability including the char yield which gradually increased from 40.8% to 52.2%, respectively. This improvement in the thermal stability of these nanocomposites is due to the effect of the well-dispersed B4C in the polybenzoxazine that acts as a thermal insulator protecting the BZ matrix. Also, this result could be attributed to the better adhesion between the polymeric matrix and the inorganic fillers via the construction of hydrogen bonding between the polar groups from the outer surfaces of the B4C particles and the free –OH groups of the polybenzoxazine matrix.
TGA thermogramms of BZ resin and its mixture with B4C.
The thermomechanical properties were investigated to measure the changes in the storage modulus (G’) and the glass transition temperature (Tg) of the BZ-based composites after the incorporation of the B4C particles. Figure 4 presents the storage modulus and loss tangent (Tan delta) of BZ/B4C hybrids. The decomposition temperatures T5, and T10 as well as the char yield (Yc), the storage modulus at 50°C and the Tg values are all gathered in Table 3.
Evolution of storage modulus (a) and Tan delta (b) of the cured BZ-based resins.
Thermal and thermo-mechanical properties of the Polybenzoxazine-based composites.
Figure 4(a) shows that the storage modulus in the glassy state was increased from 2.8 GPa for the pure BZ resin to 3.4 GPa at 10 wt.% B4C loading to reach 3.8 GPa by increasing the fillers amount to 20 wt.%. Furthermore, by investigating the data from Figure 4(b), it appears that the Tan delta curves tend to be wider and their height increased when adding 10 and 20 wt.% of the ceramics fillers. The Tg value was observed at 186°C for the pure BZ resin and it increased by 11°C and 13°C after adding 10 and 20 wt.% of the B4C microfillers. First, by adding and increasing the B4C loading, the higher rigidity of the ceramic particles reduced the flexibility of the polymeric network resulting in higher Tg values. Overall, the DMA results corroborated the DSC findings especially clarifying the relation between the particles amount and the thermal and thermomechanical performances.
Nuclear results
The values of neutron beam flux (I) for different samples thicknesses were plotted according to the equation (2) and plotted in Figure 5. These values were then used to estimate the neutron attenuation coefficient (Σ), as shown in Figure 6.
ln(I/I0) as a function of the thickness of the studied materials: (a) BZ and (b) epoxy.
Comparative histogram of Σ values between neat BZ, Epoxy and their respective composites.
Furthermore, equations (3) and (4) were used to calculate the S and λ values, respectively. Figure 7 illustrates the variation of the screening ratio as a function of the samples thicknesses for the different values of the fillers loading.
Screening ratio variations as a function of the samples thicknesses of neat BZ, Epoxy and their respective composites for: (a) 0% B4C, (b) 10% B4C and (c) 20% B4C.
The obtained results, grouped in Table 4, showed that the pure BZ sample had a quite similar shielding behavior than that of the epoxy. This is due to the atomic composition of these two systems mainly dominated by the presence of low Z atoms (hydrogen, carbon, oxygen and nitrogen). Albeit, for scientific rigor, hydrogen has the greatest potential to capture thermal neutrons than other low Z atoms, and thus control the thermal neutron density around the sample. 23 Meanwhile, adding and increasing the B4C amount enhanced both thermosets shielding parameters, yet, the composites based on the BZ matrix showed the best results, as seen in Figure 8. For instance, BZ/X B4C composites exhibited S and Σ values ranging from 29% to 96% and from 0.724 to 2.328 cm−1, respectively. While, Epoxy/X B4C composites exhibited S and Σ values ranging from 42% to 94% and from 0.843 to 1.964 cm−1, respectively. These results can be explained by the boron’s ability of absorbing thermal neutrons thanks to its high microscopic cross-section, which is about 3840 barns (for 0.025 eV thermal neutron). Besides, the λ values were reduced by increasing the particles amount within the hybrids to reach the remarkable values of 0.298 and 0.353 cm for BZ and epoxy, respectively, at the 20 wt.% B4C loading.
Neutron shielding data (S, Σ and λ) of the studied materials.a
a S3, S4, S6, S9 and S13 are the screening ratio for the sample’s thickness of 3, 4, 6, 9 and 13 mm. respectively.
Schematic description of neutron-matter interaction: (a) BZ and (b) epoxy.
Furthermore, a comparison between the Σ values of this study and those from previous works is presented in Table 5. Globally, the elaborated hybrids showed similar or higher shielding performances compared to those elaborated by other researchers. Moreover, composites based on boron rich compound, namely, boron carbide, boric acid and boron trioxide exhibited high macroscopic cross-section.24–27 It’s important to mention that Talyani developed a similar performant shielding material based on samarium oxide. 28 Meanwhile, Adeli et al. found that the neutron attenuation coefficient reached 0.347 cm−1, for B4C/epoxy, at 5 wt.% B4C loading, which is lower than that obtained with the neat epoxy in this study. 29 This result can be explained by the different nature of the two used epoxies. Overall, the developed hybrids showed better performances than the majority of the available polymeric-based shields.30–34
Neutron attenuation coefficient (Σ) data from this study as well as those from previous works.
Conclusions
In this study, benzoxazine resins as one of the leading high performance thermosets were introduced to the nuclear field as promising building blocks for the design of advanced neutrons shields. The benzoxazine polymer and its subsequent B4C based composites displayed remarkable neutrons screening performances. The obtained results were step by step compared with their epoxy counterparts. Overall, this work provide a solid bridge between the field of polymers chemistry and the science of nuclear shielding, and deeper investigations are ongoing to further asses the benefits of these remarkable materials.
Footnotes
Acknowledgements
The authors are deeply indebted to Dr. Mahmoud Izerrouken and all the CRND reactor staff for the neutron shielding tests.
Declaration of conflicting interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) received no financial support for the research, authorship, and/or publication of this article.
