Calendering-mediated TOCNF molecular slippage and h-BN aggregation on compactness and thermal conductivity of h-BN/TOCNF composites

Pore structures and interface issues have long impeded significant improvements in the thermal conductivity of biphasic composites. This work introduced a calendering technique, adapted from the traditional papermaking industry, to address these challenges in hexagonal boron nitride/tempo-oxidized cellulose nano-fiber (h-BN/TOCNF) composites. Calendering effectively reduced porosity, enhanced compactness, and improved the interface between the two phases. The results indicated that without calendering, the thermal conductivity of the composites increased with higher h-BN loading. At 50 wt% h-BN content, the through-plane thermal conductivity reached 0.82 W/m·K, which was 116% higher than that of a pure TOCNF film (0.38 W/mK). Contrary to expectations, higher calendering pressure did not yield better performance. The optimal balance was achieved with the composites containing 30 wt% h-BN processed at 2 MPa (h-BN30/TOCNF-2 composites), achieving a high thermal conductivity of 0.71 W/m·K coupled with the highest mechanical strength (40.21 MPa). This synergistic enhancement was attributed to the molecular slippage of TOCNF and preferential orientation of h-BN induced by appropriate calendering pressure, leading to tighter bonding and the formation of a “brick-mortar-brick” structure conductive to phonon transfer and mechanical reinforcement. This work demonstrates that calendering, a simple and scalable post-processing technique, can effectively engineer the microstructure of biphasic composites, leading to simultaneous enhancement in thermal and mechanical properties.