Hierarchical Encapsulation–Asymmetric Bridging Flocculation for Resolving the Retention–Sizing Trade-Off in Biochar-Filled Straw Fiber-Based Composites

Porous biochar is a functional filler for fiber-based composites. However, its fine particle size, weak affinity toward lignocellulosic fibers, and highly adsorptive pore structure lead to low retention and competitive AKD adsorption, creating a retention–sizing trade-off that limits composite performance. To overcome this challenge, a hierarchical encapsulation–asymmetric bridging flocculation (EABF) strategy integrating cationic starch (CS) and cationic polyacrylamide (CPAM) through sequential interfacial assembly was developed. In this strategy, CS was first adsorbed onto biochar surfaces to form a passivating encapsulation layer that partially shielded porous adsorption sites and regulated interfacial charge characteristics. Subsequently, CPAM served as an asymmetric bridging agent, preferentially linking CS-modified biochar with straw fibers to generate enlarged flocs with enhanced biochar–fiber association. The optimized assembly pathway increased biochar retention and total retention by 84.77% and 25.70%, respectively. Simultaneously, the dry tensile index, sizing degree, and water contact angle improved by 35.14%, 35.02%, and 64.13%, respectively, demonstrating concurrent enhancement of mechanical strength, surface hydrophobicity, and air barrier properties. Mechanistically, surface passivation and asymmetric bridging synergistically suppressed competitive AKD adsorption while strengthening filler–fiber connectivity, thereby coordinating particle-scale interfacial regulation with network-scale structural assembly. This work establishes a scalable interfacial engineering strategy for overcoming performance trade-offs associated with porous fillers and advances the development of sustainable high-performance fiber-based materials.