Coordination Engineering of Metal-Guanazole Complexes for Flame-Retardant and Anti-Microbial Bamboo

1.Preparation of the modified bamboo The bamboo was soaked in 15% wt% sodium hydroxide solution and then rinsed with deionized water. This process was repeated three times and marked as pre-treated bamboo (PB). PB was immersed in zinc nitrate hexahydrate solution and then in guanazole solution. Finally, the bamboo was rinsed with deionized water and labelled as the BZG. The molar ratios of zinc nitrate hexahydrate/guanazole solutions are 0.4 M, 0.2 M, 0.1 M, 0.05 M and 0.025 M, respectively, labeled as BZ4G1, BZ2G1, BZ1G1, BZ1G2 and BZ1G4. the bamboo-guanazole-Zn2+ samples (BGZ) are obtained by reversing the order of zinc nitrate hexahydrate impregnation and guanazole impregnation. The molar ratios of guanazole solution/zinc nitrate hexahydrate are 0.4 M, 0.2 M, 0.1 M, 0.05 M and 0.025 M, respectively labelled as BG4Z1, BG2Z1, BG1Z1, BG1Z2 and BG1Z4. For control, PB was solely treated separately with 0.1 mol/L zinc nitrate hexahydrate and guanazole solution (BZ and BG), following the same procedure. These steps were conducted under a vacuum pressure of -0.08 MPa at room temperature. After treatment, the sample is dried in the oven at 50℃ to obtain the modified bamboo. The preparation process for BZ, BG, BZG and BGZ is detailed and prestented in Fig.1. 2.CharacterizationA TESCAN MIRA LMS scanning electron microscope (SEM) with energy dispersive X-ray spectrometer (EDS) was used to capture the morphology and spectral mapping of the sample. Fourier transform infrared spectra (FTIR) were recorded using an FTIR spectrometer with a KBr disk. The spectra were recorded in absorbance mode in the range of 4000~400 cm-1 at a resolution of 32 cm-1, and the data was analyzed with OPUS Spectra software. The crystallinity of the specimens was analyzed by using X-ray diffraction (XRD, Panalytical Empyrean, Netherlands) in reflection at 2θ ranging from 5° to 90°. Cu Kα radiation was employed during the test. X-ray photoelectron spectroscopy (XPS) was performed on a Thermo Scientific ESCALAB Xi (USA) to characterize the curves of the specimens and char residues after the cone calorimeter test. Raman spectra of the char residues after the cone calorimeter test were observed using a Laser Raman spectrometer (Horiba Lab RAM HR Evolution, Japan) in the range of 4000~50 cm-1, with an excitation wavelength of 532 nm. Ultraviolet-visible light spectrum (UV–Vis) diffused spectra were recorded by Purkinje General T9 double beam UV–Vis spectrophotometer in the wavelength ranges from 200 to 400 nm. 3.Flame-retardant properties test The thermal properties of the samples were assessed using TA DISCOVERY 5500 apparatus (TA Instruments, USA) under nitrogen atmospheres. Thermogravimetric analysis(TG) of samples was performed from room temperature to 800 ℃ at a heating rate of 20℃/min. Furthermore, the flammability of the sample was measured using the UL-94 vertical burning test (UL-94). The combustion performance of the bamboo specimen dimensions of 100× 100 × 5 mm3 was evaluated.According to the Chinese national standard (GB/T 2406.2-2009), a 5801A digital oxygen index analyzer (FTT0077, USA) was used to test the limiting oxygen index (LOI) of the sample with 80 × 10× 5 mm3. The cone calorimeter (CONE, FTT007, US) was used to evaluate the combustion behavior of bamboo samples. According to the ISO 5660-1 standard, all sides of the sample except the heating surface were wrapped with aluminum foil and placed in the sample rack. The sample was systematically studied under the thermal radiation intensity of 50 kW/m². The results of recorded data were the heat release rate (HRR), the total heat release (THR), the smoke produce rate (SPR), the total smoke release (TSR), the carbon monoxide production rate (COP) and the carbon dioxide production rate (CO2P).4. Mildew resistance testFor the laboratory anti-mold test, the bamboo was processed into specimens with the dimension of 50× 20 × 5 mm3. The specimens were inoculated with Aspergillus niger, and then placed in an incubator at 25 °C, 85 RH % for 28 days. The anti-mildew property of Aspergillus niger was evaluated according to the National Standard GB/T18261-2013. At least 6 replicates of each sample were performed. The changes of extracellular electrical conductivity and extracellular pH of the spore suspensions of Aspergillus Niger were evaluated to determine the cell membrane permeability. The extracellular conductivity and extracellular pH of Aspergillus Niger were measured by micro conductivity meter and pH meter respectively. A 200 μL spore suspension (1 × 105 CFU/mL) was mixed with 40 mL of PDB and cultured for 48 h at 25±2 °C and 150 rpm. After centrifugation, the mycelium was washed and collected. Weigh the same amount of mycelium and add Zn2+-guanazole solution. The extracellular conductivity and extracellular pH of Aspergillus Niger were measured at 0, 1, 2, 4 and 6 h with sterile water as the control.