Effects of Environmental Factors on the Mechanical Properties of Palm Leaf Manuscripts: Natural Aging, Temperature, Relative Humidity, and Light Radiation

The mechanical properties of palm leaf manuscripts, a unique organic cultural heritage material, are strongly influenced by environmental conditions that directly determine their physical stability and long-term preservation. This study systematically examined the effects of natural and accelerated aging under different temperature, humidity, and light radiation conditions on the mechanical and chemical properties of palm leaf samples. Flexural strength and flexural modulus were measured to assess mechanical degradation, while FT-IR was employed to evaluate chemical structure changes. The results revealed that temperature had a pronounced effect on mechanical performance. At -20 °C, a temporary increase in flexural strength and modulus was observed due to the structural stabilization caused by frozen moisture, followed by a gradual decline attributed to ice crystal rupture and fiber damage. At 25 °C, degradation progressed steadily, while at 100 °C, the material underwent severe shrinkage and deformation, resulting in significant cracking and a subsequent sharp decline in its mechanical properties. Relative humidity also played a critical role: excessive dryness (10% RH) led to shrinkage and cracking, whereas high humidity (90% RH) caused microbial degradation and hydrolysis, both resulting in sharp declines in strength and stiffness. Samples aged at moderate humidity (50% RH) maintained superior mechanical stability. Light radiation further accelerated deterioration of mechanical properties, with UV exposure inducing the most significant loss due to photochemical reactions that disrupted lignin structures. FT-IR analysis confirmed that the degradation of cellulose and hemicelluloses was a significant cause of mechanical weakening. Overall, extreme environmental conditions accelerated both physical and chemical deterioration. Conversely, moderate and stable environments (around 25 °C, 50% RH, and limited light exposure) were found to be optimal for preserving the mechanical and structural stability of palm leaf manuscripts. These findings provide valuable guidance for the long-term conservation and environmental control of ancient organic manuscripts.