Objective Round bamboo and bamboo-based composites have been widely used as outdoor building structural materials. The fundamental difference in their weathering resistance lies in the presence or absence of the green outer layer of bamboo. However, current research mainly focuses on the bamboo with the green outer layer removed, leaving the real function and mechanisms of the green outer layer of bamboo in natural degradation unclear. This study aims to explore the role and mechanism of the green outer layer of bamboo in the processes of photodegradation, mildew and decay.

Method Two types of bamboo (with/without the green outer layer) were selected as test materials. In the laboratory, both types were subjected to artificial accelerated UV photodegradation, mildew and fungal decay to simulate natural weathering. Surface performance during the deterioration process was quantified through colorimeter, gloss meter, and contact angle measuring instrument. Microstructure and chemical changes were characterized using SEM and FTIR. These data were then used to clarify the real role and mechanism of green outer layer during degradation.

Result After photodegradation, the samples with the green outer layer showed reductions in total color difference (ΔE) and contact angle of 60.18% and 51%, respectively, relative to the samples without green outer layer. After benzene-alcohol extraction, the surface acidity of samples without green outer layer increased, while the samples with green outer layer declined slightly. The samples without green outer layer exhibited severe surface cracking and cell-wall disintegration with delamination. In contrast, the samples with green outer layer maintained structural integrity with only fine micro-cracks observed on the surface. Chemical analyses confirmed a lower degree of surface photodegradation in the latter, indicating certain UV resistance conferred by green outer layer. During mildew and decay trials, mass losses for the latter were 4.89% and 16.3%, respectively, both significantly lower than 6.33% and 27.11% recorded for the former, demonstrating additional resistance to microbial deterioration.

Conclusion Consequently, the green outer layer protects bamboo against photodegradation, mildew, and decay. Its waxy and siliceous films act as a physical barrier that attenuates UV radiation and moisture ingress while blocking fungal spores and hyphae, thereby isolating internal cells from external degradative agents and enhancing overall weathering resistance of bamboo.