Abstract
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.