Curing Kinetics and Structure-Property Relationship of Moisture-Cured One-Component Polyurethane Adhesives

Moisture-curable polyurethane (PU) adhesives have been exploited in the automotive industry because of their usability, non-toxicity without the use of volatile organic compounds (VOC), outstanding solvent resistance, and adhesiveness. However, there has been a lack of quantitative and comprehensive analyses related to the chemical and thermomechanical properties of moisture-curable PU adhesives depending on the applied curing conditions. Herein, we systematically investigated the effects of curing conditions, including relative humidities (25, 35, 45, 55, 65, and 75%RH) and curing duration (0, 0.25, 0.5, 1, 2, 3, 4, 5, 6, and 7 days), on the structure-property relationships of moisture-cured PU adhesives. As the relative humidity increased, the chemical reaction of the PU adhesives accelerated, and they exhibited a high crosslinking density and storage modulus. However, even prolonged curing time and higher relative humidity did not lead to a continuous enhancement in the mechanical properties of the PU adhesive. After a sufficient curing time, the storage modulus of the PU adhesive reached saturation, and the PU adhesives cured above 65%RH demonstrated a comparable saturated storage modulus. This structure-property relationship of PU adhesives will provide fundamental understanding and insights for quality control and optimization of processing time in the industry.