Optimization of the Composite Formulation for Rotational Molding Based on Recycled Polyethylene and Antioxidants

In this work, the content of recycled polyethylene (rPE) and pigment (Cp), the antioxidant content (CAO) and ratio (RAO), and the peak internal air temperature (PIAT) of the rotational-molding cycle were optimized. Their combined influence on the mechanical, thermal, acoustic, and surface properties of the composites was studied by the probabilistic–deterministic planning method, which reduced the number of experiments from 3125 to 25. Internal stresses were monitored non-destructively through the third-harmonic amplitude (β) of the ultrasonic signal (mirror-shadow method), supported by mechanical testing, thermogravimetric analysis, infrared spectroscopy, and porosity characterization; weather resistance was assessed by accelerated ultraviolet (UV)-B aging for 333 and 666 h, treated as conditional 5- and 10-year service horizons for Northern Kazakhstan rather than as a literal calendar equivalence. The third-harmonic β was shown to be a sensitive correlational marker of internal stresses (R = 0.81 for the mean failure energy (MFE), Fisher-criterion validated), correlating with the elastic modulus, the impact strength, and the environmental stress cracking resistance (ESCR), and tracking the transition between incomplete (ICS), normal (NS), and thermo-oxidative degradation (TDS) sintering; the correlational nature of this evidence is discussed. For products without UV exposure, the optimal formulation (rPE = 25%, PIAT ≈ 200 °C, Cp = 0.1–0.2%, CAO = 0.25–0.5%, RAO = 0.5–1.0) gave a minimum β (0.076–0.09), MFE ≈ 2.6 J/mm, and 2.5–3% porosity; for sun-exposed products, rPE = 25%, Cp = 0.5%, CAO = 0.5–0.75%, and RAO = 1.0 are recommended. An rPE content of up to 50% did not significantly impair thermal stability or surface hydrophobicity, and a nomogram CAO = f(β,rPE) was developed for rapid formulation selection and quality control.