KH550-Modified PVOH Sizing Agent for Tibetan Paper: Performance Enhancement and Antibacterial Function of Bio-Based Systems

Abstract

A composite sizing system was developed based on polyvinyl alcohol (PVOH) and the silane coupling agent KH550, with the innovative incorporation of root extract from Stellera chamaejasme L.—a characteristic raw material for traditional Tibetan paper. A novel bio-based surface sizing agent with integrated strength enhancement, water resistance, and natural antibacterial properties was successfully prepared. Investigation of KH550 dosage revealed that it acts as a “chemical bridge” between PVOH and paper fibers via a cross‑linking reaction, in which it condenses with hydroxyl groups on cellulose and PVOH to form covalent Si–O–C bonds. This enhanced interfacial bonding strength and film density, whereas excessive addition led to molecular aggregation. The optimal paper properties were obtained at a KH550 dosage of 1 g, with a sizing weight of 34.1%, tensile index of 50.1 N·m/g, sizing degree of 33 s (43.5% higher than that with PVOH alone), tear index of 219.5 mN·m²/g (163% higher than that of the unsized control), and water contact angle of 106°. Performance indicators were significantly improved compared with the control group without KH550. Notably, the Stellera chamaejasme root extract derived from traditional Tibetan papermaking raw materials effectively endows the paper coating with excellent antibacterial properties.