SMS and potassium methyl siliconate are water-soluble and their pH value are larger than 12. When the pH value is reduced by acids or CO2, the methyl siliconate forms silanol and slowly condenses to form oligomeric and polymeric siloxane [16]. In this study, CuCl2 was used as the pH-controlling agent. Once it was added into SMS solution dropwise, the solution became blue turbid immediately. In this neutralization reaction, Cu(OH)2 was probably formed as a by-product (Fig. 1), similar to the potassium carbonate and potassium bicarbonate in the previous study.
The blue turbidity was further collaborated through centrifugation and its nano-structure was characterized using TEM. Surprisingly it was observed that lamellar crystal Cu(OH)2 with size of around 10 nm was imbedded in polymethylsiloxane (PMS) particle as seen in Fig. 2a. It was speculated that the wrapping structure was formed by self-assembly since the Cu(OH)2 precipitation and silanol condensation took place simultaneously during the preparation of silanol solution. Herein the particular nano-structure of Cu(OH)2 and PMS was named as Cu-containing nanoparticles (CuNP). As a contrast, for sample prepared in total blue colloidal suspension, parts of PMS particles contained nothing inside and they were connected with CuNP as well, seen in Fig. 2b. In other words, the obtained xerogel was a mixture of pure PMS particles and CuNP, as named PMS/CuNP. It is known that Cu(OH)2 is a metastable phase which easily transforms into CuO [18]. However, CuNP and PMS/CuNP prepared in this study were still light blue after drying in oven at 105 °C for 2 h. From the results of TEM, their good thermal stability might be attributed to the specific nano-sized wrapping structure.
As seen in the FTIR spectra in Fig. 3, three characteristic IR absorption bands of Si–O–Si bonds, including the rocking mode at around 450 cm−1, bending mode at 800 cm−1 and asymmetrical stretching mode at 1075 cm−1 [19], were clearly observed in PMS. The symmetric deformation of –CH3 in PMS was also well identified at 1275 cm−1. For CuNP, the absorption band around 510 cm−1 was assigned to the stretching vibration mode of Cu–O bonds and the broad bands at around 3450−1 might be attributed to the stretching vibration mode of –OH in crystalline Cu(OH)2. The characteristic absorption bands of Si–O–Si and –CH3 was also found in CuNP, although the intensity of these bands was low. The result was consistent with TEM observations that small amount of PMS was present in CuNP. For PMS/CuNP, the above characteristic bands were all observed, confirming that the PMS/CuNP xerogel was a mixture of PMS and CuNP.
The thermal stability of PMS, PMS/CuNP and CuNP was characterized by TGA with air as carrier gas. As shown in Fig. 4, water content in PMS was around 4% and the second stage of mass loss occurred at around 400 °C, which corresponded to the oxidation of –CH3 in PMS. In contrast, a sustained weight loss was found until 500 °C for PMS/CuNP and CuNP and they changed to light black after TG analysis, indicating that crystalline Cu(OH)2 imbedded in PMS in nano-size showed excellent thermal stability. Note that a slight mass loss was also found at 450–500 °C in PMS/CuNP, which belonged to the oxidation of –CH3 in PMS.
After dip-coating in the colloidal suspension and drying, a compound membrane layer composed of PMS/CuNP xerogel formed at bamboo surface. Hydrophobicity of modified bamboo was influenced by several factors, including pH value, SMS concentration, immersion time and dip-coating times.
The hydrophobicity of modified bamboo through dip-coating in SMS solution was unsatisfied, since the WCA was only 118.2 ± 1.3°. Besides, the modified bamboo probably suffered from degradation under high alkaline conditions from SMS solution [20]. With the addition of CuCl2, the pH value decreased gradually. The pH value was an important parameter during the neutralization titration process. The effect of pH value on the contact angle of modified bamboo is shown in Fig. 5. Compared to immersion in the pH = 12 suspension, the bamboo modified in pH = 11 showed a higher WCA of 134.9 ± 2.8° due to more CuNP and PMS produced in the suspension of pH = 11. However, as the pH continued to decrease, the WCA decreased gradually. It is known that lower pH value enhanced the dehydration condensation among methysilanols, leading to flocculent PMS with larger colloidal particle size. In the meantime some crystalline Cu(OH)2 was produced, but not inside PMS, which increased the hydrophilicity of the composite coating layer.
The effect of SMS concentration on hydrophobicity of bamboo was investigated and the final pH value was set at 11, as shown in Fig. 6. When the SMS concentration was lower than 1%, WCA of modified bamboo was less than 100° due to incomplete coverage of the coating layer at bamboo surface. For 2% SMS solution, the WCA increased to 136.3 ± 2.4°. However, WCA dropped slightly to 126 ± 1.4° when 4% SMS was used. Considering that original bamboo had micro-roughness derived from fibrous cell wall, the excess amount of PMS/CuNP adsorbed would diminish the micro-roughness and generate relatively flat surface, thus decreasing the final WCA of modified bamboo.
The effect of immersion time and dip-coating times on hydrophobicity of modified bamboo was investigated under the condition of 2% SMS and pH 11. As shown in Fig. 7, when the immersion time increased from 10 to 120 min, the WCA increased from 115.3° to 136.5°. After one dip-coating for 120 min, the coating layer was dried in air for 10 min and then a second dip-coating was carried out. As shown in Fig. 7, with the increase of dip-coating times, hydrophobicity of modified bamboo enhanced gradually and the WCA reached up to 151.3 ± 1.9° at the fifth dip-coating, which could be called superhydrophobic surface.
Bamboo was dip-coated in the colloidal suspension under optimized conditions and samples prepared in Cu(OH)2 suspension and pure PMS colloidal suspension were prepared at pH = 11 and 2% concentration for comparison. The mildew resistance of original bamboo and bamboo coated with Cu(OH)2, PMS and PMS/CuNP was evaluated according to ASTM D3273-16 [21]. The photographs of mold growth on bamboo surface after 4 weeks of incubation are presented in Fig. 8. Mold started to grow on the untreated bamboo surface within 2 days after inoculation and at least three kinds of mold were observed to cover fully the surface due to the rich content of sugars and starch inside bamboo. For bamboo coated with Cu(OH)2, only Cu-tolerant fungal spores could germinate and grow on the surface, which probably belonged to Trichoderma [8]. PMS-modified bamboo surface showed limited mildew resistance as mold began to grow at the 7th day which might be attributed to a relatively dry surface environment generated by PMS layer. However, at the end of 4th week, its surface was still fully covered with several kinds of fungi, with less distribution density as compared to the unmodified bamboo. In contrast, the mildew resistance grade of the PMS/CuNP-modified bamboo was marked as 10 as no mold was found at the surface. The synergistic effect of hydrophobic surface with antimicrobial agents was also reported by Chen and Li et al. [22]. They found that after polymethylhydrogensiloxane (PMHS) hydrophobic modification, ZnO/bamboo had excellent anti-mildew properties when exposed to Trichoderma viride, Aspergillus niger, and Penicillium citrinum.
The surface morphologies of unmodified bamboo and modified bamboo samples, containing parenchymal cells and vascular bundles are shown in Fig. 9. Unmodified bamboo surface was highly hydrophilic, with WCA 0°. For bamboo modified with Cu(OH)2, nanoparticles evenly covered bamboo surface as shown in Fig. 9b. Since Cu(OH)2 was hydrophilic, WCA of the modified bamboo was still 0°. After dip-coating in pure PMS sol and drying, bamboo surface was covered with cracked PMS layer, which was caused by the shrinkage of PMS gel during drying process. PMS was hydrophobic due to the presence of methyl groups and the original concave–convex cell structure of bamboo provided additional roughness, leading to an improved hydrophobicity with a WCA of 131.7°. It was conjectured that mold hypha could still grow through these cracked PMS layer with a slower growth rate due to the lack of water. With the addition of CuNP inside, the cracks of PMS layer almost disappeared, indicating that the possibilities of mold hypha contacting the bamboo surface were greatly reduced. Besides, SEM image suggested that PMS/CuNP was rougher than the pure PMS layer. The additional nano-size roughness on PMS layer further increased the WCA of modified bamboo.
