Enhanced crosslinking characteristics of blocked isocyanate-based 1 K polyurethane clearcoats via self-crosslinkable polymer networks

Abstract

One-component (1 K) polyurethane clearcoats offer significant operational advantages over two-component (2 K) systems through simplified processing and extended shelf life, yet suffer from inferior mechanical performance due to inherent limitations in their crosslinking capabilities. Conventional 1 K systems rely solely on intermolecular crosslinking between segregated polyol and blocked isocyanate components, leading to heterogeneous crosslinker distribution, microphase separation, and ultimately compromised coating properties. To address these fundamental issues, we developed a unique self-crosslinkable prepolymer (SCP) incorporating both hydroxyl and blocked isocyanate functionalities within single polymer chains, enabling to produce a self-crosslinkable polyurethane clearcoat (SC-PUP) upon heat. Our molecular architecture approach enables simultaneous intramolecular and intermolecular crosslinking mechanisms through dual reactions that minimizes crosslinking distance between functional groups and eliminates heterogeneity within the polymer matrix. The SC-PUP clearcoat containing SCP demonstrates enhanced performances, with higher storage modulus (5.08 $\times$ 10⁵ Pa), glass transition temperature of 81.39 °C, and crosslinking density (2.02 × 10⁻³ mol/g) compared to the OC-PUP and PL-PUP clearcoats via intermolecular crosslinking. Surface hardness utilizing SC-PUP increased to 272.35 MPa with superior scratch resistance, while tensile strength showed improvement over the intermolecular crosslinking PU clearcoats. It is attributed to the formation of a uniformly dense crosslinking network structure with hydroxyl conversion efficiency based on SC-PUP, substantially exceeding the other clearcoats. The self-crosslinking approach successfully bridges the performance gap between 1 K and 2 K systems while maintaining processing simplicity, establishing a new paradigm for high-performance automotive clearcoat technology.