Waterborne polyurethane composite coatings as UV-light converter and directional infrared light barrier for light and heat management in greenhouse

Light and heat management in greenhouses are important for crops yield and quality. Here, we created waterborne polyurethane (WPU) composite coatings for greenhouse films, which can act both as ultraviolet (UV) light down-conversion and directional infrared light barrier for the light and temperature conditions in pepper greenhouses. Rare-earth organic complexes (REOC) that can convert UV light to red light were mixed with WPU by ball-milling. The REOC nanoparticles presented good dispersion in WPU coatings, which were determined by atomic force microscopy, and the coating with 1 wt% REOC still remained comparably high transparency. The luminescence properties were increased with REOC loadings. The coatings with REOC showed lower infrared (IR) light transmittance and higher IR light reflectance than those of the pristine film and WPU coatings. The preferable location of the composite coatings on inner side of films acted as IR light barrier and facilitated the maintenance of heat inside the greenhouses, increasing the temperature. Simulation was performed to testify the temperature increase using IR barrier covered layer. Small and big greenhouses with different films were built and the temperatures were recorded. The results show that the film containing REOC increase the temperature in the greenhouses by >2 °C compared to traditional agricultural films. Finally, due to light conversion and IR light barrier, the yields of green peppers can be increased by 35 %. The mass of single fruits can also be increased by 8 g.