PSI - Issue 81

Olena Mikulich et al. / Procedia Structural Integrity 81 (2026) 251–254

252

Nomenclature PU polyurethane foam UV ultraviolet G Shear modulus E Young’s modulus normal stresses  tangential stresses radial stresses 

normalized time parameter

Among the main disadvantages of foam materials, it should be noted that they change their chemical and physical properties under the influence of external factors, such as solar radiation (Shi et al., 2022). The main cause of PU degradation under the influence of sunlight is ultraviolet radiation and, to a lesser extent, infrared radiation. UV light initiates chemical reactions that destroy the polymer matrix containing chemical bonds sensitive to oxidation (Alradha, et al., 2018). Absorption of ultraviolet rays leads to bond breakage and the formation of free radicals, which quickly react with oxygen, creating permanent structural changes in the polymer, which leads to the destruction of the primary polymer matrix (Barszczewska-Rybarek et al., 2022). Degradation of the polymer matrix under the influence of ultraviolet light is manifested both visually and in the deterioration of key physical and mechanical properties. The most noticeable sign of UV degradation is the yellowing of the surface layer of the foam. This effect is the result of the formation of chromophoric groups that are formed during the oxidation of aromatic isocyanates. Alradha, et al., 2018 conducted studies studying the kinetics of yellowing of PU foam under the influence of UV radiation. It was found that the degree of yellowing directly correlates with the irradiation time and is an indicator of the degradation of the surface layer. In addition, photodegradation leads to a decrease in mechanical strength due to the destruction of the polymer network, which causes brittleness, loss of elasticity and a decrease in strength, especially on the surface of the foam. In some cases, the degraded surface layer may collapse and crumble. Photodegradation also leads to an increase in thermal conductivity due to the opening of cells (Barszczewska-Rybarek et al., 2022). Fig. 1 presents laboratory samples, which were exposed to solar radiation for 6 months in natural conditions. Analysis of the results, Fig. 1, a and Fig. 1, b, shows that the effect of solar radiation on the chemical change of the material is the same for untreated samples and for samples subjected to mechanical stress. Sections of the material structure (Fig. 1, c) show the nonlinear influence of this effect.

Fig. 1. View of PU foam (a) and sample (b) under the influence of solar radiation, and range of this influence (c)

To reduce the negative effects of sunlight, photostabilizers or protective coatings are used. For optimising their properties and expanding their functionality, they are modified by adding various types of additives, such as mineral, organic, and polymeric, to the polyurethane foam (Mikulich et al., 2024; Chmiel et al., 20). In Mikulich et al., 2024 to modify the structure of polyurethane foams and increase their protection against solar exposure, various types of modifiers were used. Although this approach allowed to provide better protection of the material, when modifiers were added, the internal structure of the material changed due to the presence of additional cavities (Fig. 2). In this case, the number of cavities depends on the proportion of the modifier in the foam.

Fig. 2. View of modified polyurethane foam internal structures with different part of additives Therefore, the purpose of the study is to develop a methodology for analyzing the influence of the heterogeneity of the material's microstructure on its mechanical behaviour.