A new way for lead-boron resin composite modification: SiO₂ coated lead powders by a sol-gel method

In order to improve the composition distribution and flame retardancy of composites, the effects of silicon dioxide (SiO₂) coatings with different contents on physical properties of lead (Pb) powders and composites were investigated in this research. SiO₂ coated Pb powders (SiO₂@Pb) with contents of 0, 0.237, 0.486, 0.683, 0.967 wt% were synthesized by a sol-gel method, then mixed with boron carbide (B₄C) powders and boron phenolic resins (BPRs) to prepare SiO₂@Pb/B₄C/BPRs composites by molding. SiO₂ coating on the surface of Pb powders in flakes or islands increases the specific surface area and oxidation temperature of the SiO₂@Pb powders. For the SiO₂@Pb/B₄C/BPRs composites, the composition uniformity of composites is improved due to the reduction of the true density difference value between fillers (Pb, B₄C), which is beneficial for the physical properties of the composites. Furthermore, the mechanical properties and thermal conductivity increase with the addition of SiO₂ content, achieving a maximum value at 0.237 wt%, and then decrease gradually with a further increase of SiO₂ content. Moreover, SiO₂ coatings improve the limit oxygen index (LOI) of the composites and reduce the cracks of composites after burning. Composites with the SiO₂ content of 0.486 wt% have optimal comprehensive physical properties, where the tensile strength, bending strength, impact toughness are 42.5 MPa, 72.4 MPa, 6.5 kJ m^-2, respectively and the LOI is 41.8%.