Wet Chemical Synthesis of Benzalkonium Chloride-Hectorite Composites: Structural Regulation and Enhanced Antibacterial/Antifungal Performance for Indoor High-Humidity Decorative Materials

To mitigate health hazards from pathogenic bacteria (Escherichia coli, Staphylococcus aureus) and fungi (Aspergillus niger) as well as the coating mildew issue in high-humidity indoor environments, and to overcome the challenges of particle agglomeration and non-uniform distribution in conventional benzalkonium chloride (BAC)-clay composites, this study proposes a wet chemical strategy to prepare BAC-hectorite antimicrobial composites using synthetic hectorite as a high-performance carrier, which is superior to natural clays such as montmorillonite and kaolin in structural uniformity, ion-exchange efficiency, and dispersion stability. Characterization using X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and Brunauer–Emmett–Teller (BET) analysis confirmed the successful intercalation of BAC cations into the hectorite interlayers through ion exchange. This resulted in a significant expansion of the interlayer spacing from 1.0–1.2 nm to 1.5–1.8 nm, a marked alleviation of particle agglomeration, and an optimized pore structure. A clear structure–activity relationship between preparation conditions, microstructure regulation, and antimicrobial performance is systematically established. Antibacterial tests revealed superior efficacy against Gram-positive bacteria; the composite exhibited an inhibition zone of 13.31 mm and a minimum inhibitory concentration (MIC) of 4 μg/mL against S. aureus, compared to 11.62 mm and 32 μg/mL against E. coli. Practical application tests demonstrated that at an ultralow addition level of 0.4%, incorporating this composite into latex paint achieved an antibacterial rate exceeding 99.9% against both pathogens. When added to putty powder, it yielded Grade 0 mold resistance with no observable growth. Furthermore, compounding with polypropylene (PP) increased the elongation at break to approximately 600%, simultaneously realizing antibacterial, antifungal, and toughening functions, thereby not only conferring antibacterial functionality but also significantly enhancing toughness—resolving the typical polymer embrittlement caused by traditional inorganic antibacterial fillers. Short-term evaluations confirm that this composite offers a stable structure, high-efficiency antimicrobial properties, and improved substrate mechanics at low loading levels. These findings provide technical support and experimental guidance for the functional upgrading of indoor decorative coatings, putties, and polymer materials used in high-humidity scenarios such as kitchens and bathrooms.