Aging effects of hindered amine versus triazine-based anti-aging agents on PBAT mulch film under different humidity levels

Poly(butylene adipate-co-terephthalate) (PBAT)-based biodegradable mulch films have gained significant attention in recent years, due to their environmental friendliness. However, their service life has been limited by rapid aging under complex field conditions. This study aims to clarify the molecular mechanisms of photo-humidity synergistic aging in PBAT. A solid theoretical model was then established to select efficient light stabilizers suitable for different humidity environments. PBAT-based films were prepared via melt extrusion and film blowing. Two types of light stabilizers were incorporated: A triazine-based ultraviolet absorber (1164-PBAT) and a hindered amine light stabilizer (944-PBAT) at an addition level of 0.8%. Accelerated aging experiments were conducted under two relative humidity conditions using a xenon lamp weathering tester. The irradiance was maintained constant at 0.55 W/(m²·nm) at 340 nm. The aging behavior was systematically characterized using multiple techniques. Mechanical properties, including tensile strength and elongation at break, were measured using a universal testing machine. Chemical structure was monitored by Fourier transform infrared spectroscopy, with particular attention to carbonyl index evolution. Molecular weight and its distribution were analyzed by gel permeation chromatography. Thermal behavior was also investigated by differential scanning calorimetry. Surface morphology was observed using scanning electron microscopy. High humidity partially inhibited the photo oxidation, thereby delaying the deterioration of mechanical properties. The hindered amine light stabilizer 944-PBAT exhibited excellent and long-lasting stabilization under humidity conditions. In PBAT films with 944-PBAT, the retention of elongation at break remained at 48% after 6 days of aging at 50% RH, and high toughness was maintained after 9 days of aging at 80% RH. Gel permeation chromatography analysis revealed that 944-PBAT effectively suppressed random chain scission and uncontrolled crosslinking, resulting in a consistently narrow molecular weight distribution in the aging period. The radical scavenging and regenerative cycling of 944-PBAT effectively interrupted the chain photo-oxidation reaction, providing sustained protection regardless of humidity level. In contrast, the triazine-based ultraviolet absorber UV-1164 exhibited pronounced humidity-dependent performance. Stabilizing efficiency was severely limited under low humidity, due to the low dispersion and molecular aggregation. PBAT films with 1164-PBAT showed a rapid decline in elongation at break to 10% of the initial value after 6 days of aging at 50% RH, indicating the sharp increase in carbonyl index. Molecular mobility and dispersion of 1164-PBAT within the PBAT matrix significantly increased the ultraviolet shielding effectiveness under high humidity. The retention period of elongation at break was extended to 9 days. Fourier transform infrared spectroscopy showed a much slower increase in carbonyl index. Differential scanning calorimetry analysis further confirmed that 1164-PBAT formed an effective protective layer under high humidity. Scanning electron microscopy observations revealed that 944-PBAT films maintained smooth surface morphology under high/low humidity, while 1164-PBAT films exhibited pronounced surface cracking and aggregation under low humidity. Multi-scale analyses, including mechanical testing, Fourier transform infrared spectroscopy, gel permeation chromatography, differential scanning calorimetry, and scanning electron microscopy, collectively revealed the molecular patterns of PBAT under photo-humidity synergistic aging. Consequently, the humidity can be expected to determine the effectiveness of different light stabilizers. 944-PBAT can provide humidity-independent long-term stabilization after radical scavenging and regenerative cycling, while 1164-PBAT efficiency can be strongly dependent on humidity-controlled dispersion. These findings can offer theoretical and technical support to optimize the environmental adaptability for the long service life of PBAT-based biodegradable mulch films under diverse climatic conditions.