In-situ construction of solvent-free polyurethane foam coating based on carbon peak strategy

To mitigate harmful volatile organic compounds (VOCs) emissions and adhere to environmental policies such as the carbon peak strategy, the development of solvent-free polyurethane (SFPU) coating materials has emerged as a crucial endeavor. In this study, a type of SFPU system, involving blocked polyurethane prepolymers (PUPs), thermal expandable microspheres (TEMs) foaming agent and diamine curing agent, was developed to construct foamed synthetic leather coating. Notably, the blocking strategy endowed the system with desirable storage stability and pot-life. The deblocking temperature range of both dimethyl and methyl ethyl ketoxime blocked PUP were 70 degrees C-120 degrees C, which was lower than that of the foaming temperature range of the prepared TEMs (118 degrees C-122 degrees C). Due to the lower deblocking temperature and curing activation energy, the curing process of SFPU system was prior to foaming in application. Further study indicates that 2 wt% TEMs loading could effectively enhance the softness and resilience of coating and increase its water vapor permeability greatly, while higher amount of TEMs decreased resilience and peel strength of coatings. Based on the compatibility between curing and foaming process, an abbreviated "two-coating and three-baking" solvent-free polyurethane synthetic leather-making process was constructed by integrating the blocking strategy and TEM foaming technology. This work provides a guidance for the synthesis of SFPU with high storage stability and long pot-life, and the sustainable solutions for the synthetic leather manufacture in line with the carbon peak strategy.