Synthesis and application of sizing agents for improved mechanical performance of CF/BMI composites

The interfacial performance between carbon fibers (CF) and heat-resistant bismaleimide (BMI) matrices is a key factor governing the mechanical behavior of advanced composites. In this study, three HDI-trimer polyisocyanate-terminated sizing agents with different chemical linkages, namely urea (UH-ISO), amide (AH-ISO), and melamine-based (MH-ISO), were designed to tailor the fiber–matrix interface. The trifunctional NCO groups of the HDI-trimer enable chemical bonding and crosslinking with the BMI matrix. Mechanical results showed that UH-ISO-treated composites exhibited the highest performance, reaching 667 MPa flexural strength and 47.9 MPa interlaminar shear strength, corresponding to an increase of approximately +110% compared to DDS/NS composites. MH-ISO systems showed moderate improvements (up to +57%), while AH-ISO demonstrated limited enhancement, particularly in MXDA-based systems (+1%). Surface and chemical analyses (SEM, optical profilometry, and XPS) indicated that interfacial performance is mainly governed by coating uniformity and functional group distribution. UH-ISO formed a uniform interphase that allowed effective resin infiltration and stress transfer. In contrast, AH-ISO caused local accumulation and restricted matrix penetration, leading to defect formation. MH-ISO exhibited a relatively homogeneous interphase, where localized surface irregularities were observed, likely related to its multi-functional structure.