Enhanced Dispersion and Structural Homogeneity in LDPE/Bi2O3/B4C Composites via Cryogenic Ball Milling with Binary-Sized Grinding Media

The study investigated the effect of a dense two-fraction ball load on the efficiency of cryogenic dispersion and the structural uniformity of polymer composite materials based on low-density polyethylene (LDPE) filled with bismuth oxide (Bi2O3) and boron carbide (B4C). A comparative analysis of two grinding modes—mono-fraction and binary (dense) loading of grinding media—showed that using binary loading reduces the modal diameter of LDPE particles from 19.1 to 13.4 μm, and the number of Bi2O3 particles smaller than 5 μm increases by 1.25 times. Using XRF and IR spectroscopy, it was established that cryogenic grinding is accompanied by mechanochemical degradation of polyethylene, resulting in the formation of hydroxyl and carbonyl groups, which improve the interfacial interaction between the polymer and inorganic phases of the composite. At the same time, during the subsequent thermopressing of the product, thermal relaxation of the polymer structure occurs and unstable oxidized fragments are eliminated. Energy-dispersive X-ray mapping confirmed a more uniform filler distribution with binary loading: the proportion of element localization regions exceeding 500 μm2 decreased for B (0.46 → 0.24), C (0.85 → 0.74), and Bi (0.36 → 0.31). Ultrasonic testing showed a ~3.5% increase in sound velocity (from 2038 to 2110 m/s) and an increase in elastic modulus from 2.36 to 3.34 MPa, indicating improved structural homogeneity and elastic properties. This is the first application of binary-sized grinding media to cryogenic milling of polymer composites. The results demonstrate the potential of cryogenic mechanical activation with a dense two-fraction ball charge for producing highly homogeneous polymer composite materials without organic solvents.