Drug release performance of straw-fiber-based carnauba wax/gelatin double-coated antibacterial mulch film

An antibacterial mulch film is required for the rapid response to two key meteorological factors (temperature and humidity) in crop diseases during agricultural production. Conventional disease prevention and control measures respond to the dynamic meteorological conditions—for instance, when a sudden rise in field temperature or a surge in humidity can trigger an outbreak of diseases, often fail to adjust prevention and control strategies in a timely manner; Prevention and control can rely mainly on the large-scale pesticide spraying. This "flood-irrigation" style of pesticide application can easily result in low pesticide utilization efficiency, i.e., substantial pesticide loss. In turn, a series of environmental issues can also cause problems, such as soil pollution and water eutrophication. Meanwhile, excessive pesticide residues in agricultural products can also pose a dual threat to the ecological environment and human health. In this study, an antibacterial mulch film with temperature and humidity responsiveness (CW-C-Gel-WFM) was successfully developed using carnauba wax, gelatin, and temperature-sensitive straw fibers as core raw materials. Each component of mulch film also shares the structural functions. Among them, carnauba wax and gelatin acted as temperature-controlled release materials. Their own structural states were adjusted at the environmental temperature, thereby regulating the pesticide release rate; While temperature-sensitive straw fibers served as carriers, due to their unique porous fiber structure. The active ingredients of pesticides were stably loaded in environmental humidity, thus synergistically achieving precise regulation of "on-demand release". The mulch film was synergistically adjusted as the timing of pesticide release using the physical barrier of carnauba wax and the temperature-sensitive responsiveness of gelatin. As such, the pesticide-controlled release was realized with the changes in field temperature and humidity. Pesticide loss caused by blind release was effectively reduced to dynamically optimize the field pesticide concentration, according to the disease risk at different stages of crop growth, significantly enhancing the controllability of the crop growth environment. Furthermore, the thickness of the mulch film's barrier layer had a significant impact on the mechanical properties (e.g., tensile strength and elongation at break) of CW-C-Gel-WFM. The tensile strength of the mulch film showed a trend of firstly decreasing, then increasing, and finally decreasing again, as the thickness of the barrier layer increased. While the elongation at break decreased slightly. The performance of the straw-fiber-based mulch film was attributed to the repeated coating with a coater and the interaction arising from carnauba wax molecules filling the gaps between gelatin molecules. The tests further indicated that the air permeability rate of the mulch film decreased significantly with the increasing thickness of the barrier layer—when the thickness of the barrier layer reached 30 μm, the air permeability rate dropped to 12.03 μm/(Pa·s). The water evaporation was effectively reduced in the area covered by the mulch film. At the same time, the stable soil humidity was maintained to avoid the excessive volatilization of pesticides caused by overly strong air permeability. In addition, the moisture absorption performance of CW-C-Gel-WFM was significantly superior to that of the control mulch film without carnauba wax (C-Gel-WFM), due to the hydrophobicity of the carnauba wax coating. The mulch film from structural collapse was also prevented under excessive moisture absorption, in order to avoid the premature degradation of pesticides caused by excessive contact with water. Pesticide release kinetics analysis was carried out to evaluate the performance of controlled-release mulch films. The results showed that the pesticide release rate slowed down significantly at constant temperature, with the increasing thickness of the carnauba wax barrier layer, and the controlled release time was extended accordingly. When the thickness of the barrier layer reached 30 μm, the maximum controlled release time reached 96, 120, and 168 h, respectively, which was 48-60 h longer than that of C-Gel-WFM. More importantly, the temperature dominated the pesticide release, with 40 ℃ as a critical threshold: Once the environmental temperature was higher than 40 ℃, pesticide release mainly followed the Fickian diffusion—at this time, the high temperature intensified the molecular movement of the carnauba wax barrier layer for the more internal pores. As such, the pesticides were mainly released from the polymer matrix through molecular diffusion; Once the temperature was lower than 40 °C, the pesticide release shifted to the non-Fickian diffusion. The release was controlled by the swelling of gelatin, the relaxation of polymer chains, and the physical diffusion of carbendazim. The mulch film can be expected to flexibly adjust the release mode according to different temperature environments, further improving the precision of pesticide use. The pot experiments and soil microbial analysis also demonstrated that CW-C-Gel-WFM shared a controlled bacteriostatic behavior to promote plant growth.