A dynamic approach for investigating castor oil-based polyurethane adding macadamia nutshell residue and its effects on natural weathering

Abstract

Polyurethane foams are highly versatile materials and are extensively used in outdoor applications, including construction, repair, reinforcement, and the protection of civil engineering infrastructures. Furthermore, reinforcing polyurethane (PU) materials with agro-industrial waste can have both economic and environmental benefits. This study focuses on the development of castor oil-based polyurethane biocomposites incorporating macadamia nutshell residue (PU/MR) in various concentrations (0%, 5%, 10%, 15%, and 20%). It investigates the effects of natural exposure on these samples. Morphological, chemical, thermal, and mechanical properties, along with the sol fraction, were investigated to gain insights into the degradation behavior of PU/MR foams. The incorporation of MR led to the formation of a porous structure with small pores, which enhanced the thermal stability and mechanical properties of the foams. After 5 days of aging, all samples exhibited an initial weight increase due to moisture absorption, followed by a gradual weight loss over the course of the year-long testing period. Modifications in the molecular structure of both pure PU and the biocomposites were observed. Several oxidative reactions, hydrolysis, and photochemical cleavage induced by weathering conditions (such as sunlight, rain, and temperature fluctuations) were confirmed through UV-vis spectrometry. The results demonstrated that biocomposites with MR contents above 5% experienced less photochemical degradation, attributed to the protective effect of MR, which delays bond rupture and prevents significant structural integrity of the PU foams. These characteristics indicate that PU/MR biocomposites are promising for civil construction applications, offering an environmentally friendly alternative to traditional PU foams.