Formation and properties of polyelectrolytes/TiO₂ composite coating on wood surfaces through layer-by-layer assembly method

Introduction

Wood is a popular material for building designers and users. However, wood has also disadvantages, such as hygroscopic swelling, discoloration and degradation during weathering, and amenability to microbial decay (George et al. 2005). There are many attempts to mitigate these effects, including heat treatment (Hakkou et al. 2005), drop-coating (Liu et al. 2013a), impregnation (Nami Kartal et al. 2007; Cai et al. 2008), atom transfer radical polymerization (Yu et al. 2013), etherification (Chang and Chang 2006), sol-gel deposition (Tshabalala et al. 2003; Mahltig et al. 2008; Wang et al. 2011), and hydrothermal synthesis of inorganic coating (Li et al. 2010; Liu et al. 2013b). The introduction of nanomaterials on wood surfaces has gained importance in the last decades, as this approach leads to wood protection by improved hydrophobicity, self-cleaning surfaces (Fu et al. 2012), UV stability (Yu et al. 2010), and better fire resistance (Wang et al. 2014).

Layer-by-layer (LBL) assembly of nanomaterial coatings is a facile and versatile method developed in the 1990s (Decher 1997; Decher et al. 1998). The principle of this approach is that two media with opposite electric charge are applied alternately to assemble a conformal and functional structure, which is controlled at the molecular level via film thickness and chemical properties of the surfaces (Ariga et al. 2007). LBL can be performed under ambient conditions and without expensive apparatus and was employed to inorganic nanoparticles (Kotov et al. 1995; Malikova et al. 2002), multiwall carbon nanotubes (Correa-Duarte et al. 2005), organic polyelectrolytes, polysaccharides (Richert et al. 2004; Podsiadlo et al. 2005), DNA (Decher et al. 1994; He and Bayachou 2005), and proteins (Lvov et al. 1995; Cai et al. 2005). LBL assembly was successful on Teflon (Kotov et al. 1995), glass slides (Malikova et al. 2002; Richert et al. 2004), silica, polystyrene, melamine spheres (Correa-Duarte et al. 2005), and carbon nanotubes (Du et al. 2007). TiO₂ and SiO₂ nanoparticles and halloysite clay nanotubes were LBL deposited on lignocellulosic microfibers or cellulosic fibers (Lu et al. 2007).

Wood surfaces are amphiphilic and rich in hydroxyl groups manifesting in electronegativity in aqueous solution. Thus, wood can assemble polycation layers, which is the basis for subsequent polyanion-type layers and nanoparticles. Renneckar and Zhou (2009) demonstrated that wood surface modification through noncovalent attachment of amine containing water-soluble polyelectrolytes and LBL-assembled films provides a path to create functional surfaces in a controlled manner.

In this study, a polyelectrolyte-based coating with the adjunction of nano-TiO₂ particles was designed for wood surface modification. Poly(allylamine hydrochloride) (PAH) was first immobilized on wood substrate as an interfacial layer, and alternate LBL assembly of poly(styrene sulfonic acid) sodium salt (PSS) and TiO₂ nanoparticles was then deposited to obtain multilayer coatings. The UV-shielding capability and photocatalytic activity of the coating should be tested. The superhydrophilicity of the coating and its transformation to hydrophobicity are also a focus of the study.

Materials and methods

Poplar wood (Populusussuriensis Kom.) was cut into specimens of 10 (L)×10 (T)×10 (R) mm³ in size. All specimens were cleaned ultrasonically and dried at 60°C. PAH (M_w=8000∼10 000) was purchased from Zouping Mingxing Chemicals Co., Ltd. (Shandong, China), and PSS (M_w=70 000) was purchased from Alfa Aesar Chemicals Tianjin Co., Ltd. (Tianjin, China). TiO₂ (anatase, particle size <25 nm, 99.8%, hydrophilic) and rhodamine B (RhB, C₂₈H₃₁ClN₂O₃, M_w=479) were purchased from Aladdin Co., Ltd. (Shanghai, China). Methylene blue trihydrate (MB, C₁₆H₁₈ClN₃S·3 H₂O, M_w=373.90), HCl (35.38%), and stearic acid (C₁₈H₃₆O₂, M_w=284.48) were purchased from Kemiou Chemical Reagent Co., Ltd. (Tianjin, China).

All chemicals were prepared as aqueous dilute solution with an adjusted pH to 2.5 with HCl, in which the colloidal TiO₂ and PAH solutions are positively charged, whereas PSS is negatively charged. The specimens were first immersed in 3 g l^-1 deionized aqueous solution of PAH for 15 min and dried. The cationic polyelectrolyte basis layer is crucial for ensuing formation of multilayers. After PAH patching, the LBL cycle was executed by alternately soaking the specimens in 3 g l^-1 homogeneous anionic PSS solution and 1 g l^-1 TiO₂ colloidal solution for 15 min. The specimens were rinsed with deionized water three times after each immersion and dried at 60°C. Each exposure deposited a reproducible quantity of material and reversed the charge on the surface, leaving the surface primed for the next layer of polymer (Decher et al. 1998). The repeats were continued until the targeted multiple layers of the coating were achieved (Figure 1). According to the number of PSS/TiO₂ bilayers, the specimen was labeled as PAH(PSS/TiO₂)_n-coated wood (the subscript letter n denotes the number of PSS/TiO₂ bilayers). The specimen assembled with PSS/TiO₂ multilayers was further soaked in 10 mmol l^-1 stearic acid for 15 min and dried at 60°C to endow the coating with hydrophobicity.

Figure 1:

Schematic illustration of the layer-by-layer (LBL) assembly process of PAH(PSS/TiO₂)_n coating on wood surfaces.

The surface morphology and chemical composition of the samples were characterized by scanning electron microscopy (SEM, Sirion 200 microscope, FEI Inc., Eindhoven, The Netherlands) and energy disperse X-ray analysis (EDXA). The water contact angle (CA) was measured by optical video contact angle tester (OCA20, Data Physics, Filderstadt, Germany), and every specimen was tested three times in different locations, and the data are presented as mean values with their StD.

The original wood and the PAH(PSS/TiO₂)_n-coated woods were submitted to UV-Vis spectrometry (TU-1901, Purkinje General Instrument Co., Ltd., Beijing, China). The UV stability of the coating was tested with exposure times between 0 and 1000 h. The extent of photochromism was appraised by means of a portable spectrophotometer (NF333, Nippon Denshoku Industries, Tokyo, Japan) and evaluated according to the CIE L^*a^*b^* system. ΔE^*, the parameter for denoting the total color changes, was calculated according to Equation (1) as follows:

The photocatalytic activity of the coating was tested with 5 mg l^-1 RhB and 5 mg l^-1 MB dye solutions. The specimens were immersed in 10 ml of the dye solutions and were UV-irradiated for 2, 5 and 20 h, respectively. Photocatalytic degradation of RhB and MB dyes was characterized by means of UV-Vis spectrometry, as indicated above. The degradation efficiency (η%) was calculated as:

where A₀ is the original absorbance of dye solution, and A_t is the absorbance of dye solution after t UV-irradiation time.

Results and discussion

A stepwise growth of PAH(PSS/TiO₂)_n coating on the wood substrate was observed by SEM. Figure 2a shows the top view morphology of the original wood with clear differentiation of microstructural details. The deposition of PAH and PSS films obscured gradually the ultrastructural features of the cell wall (Figure 2b and c). At high magnification 5000×, the PAH and PSS films appear smooth and mask the surface topography. After the assembly of PSS film, the surface potential becomes negatively charged, as stated before (Lu et al. 2007; Renneckar and Zhou 2009), and renders possible the fixation of TiO₂ nanoparticles. Homogeneous distribution of charges leads to a uniform in situ assembly of TiO₂ nanoparticles. After the TiO₂ layer was assembled, the top surface became slightly rough. At high magnification, a compact and uniformly distributed nanoparticle layer is visible (Figure 2d).

Figure 2:

SEM morphology images of the (a) original wood, (b) PAH-coated wood, (c) PAH/PSS-coated wood, and (d) PAH(PSS/TiO₂)₁-coated wood.

The difference in morphology among the specimens coated with different number of PSS/TiO₂ bilayers on a PAH-treated wood is clearly visible on the low- and high-magnification SEM images (Figure 3). The coating surface with one PSS/TiO₂ bilayer composed of several homogeneously distributed nanoparticles appears smooth at low magnification (Figure 3a). Along with increasing PSS/TiO₂ bilayers, the coating surface gradually turns from smooth to rough and the films mask the fine ultrastructural details of the cell wall but the microscale features of wood are still visible (Figure 3b–d). The high-magnification image of the PAH(PSS/TiO₂)₇ coating shows the emerging microparticle agglomerations of TiO₂ (Figure 3d insert), which enhance the microscale roughness of the surface. The EDXA of the PAH(PSS/TiO₂)_n-coated wood confirms that the mass fraction of Ti increased from 2.6% to 12.2% along with the number of PSS/TiO₂ bilayers from 1 to 7 (Figure 3e).

Figure 3:

Low- and high-magnification SEM images of the specimens coated with (a) 1, (b) 3, (c) 5, and (d) 7 PSS/TiO₂ bilayers on a PAH-treated wood substrate. (e) EDXA spectra of original wood and PAH(PSS/TiO₂)_n-coated woods. (f) The diffuse reflection spectra of original wood and PAH(PSS/TiO₂)_n-coated woods.

Figure 3f shows that the PAH(PSS/TiO₂)_n-coated woods are 7%–15% less dark (manifested as higher reflectance) than the original wood; thus, the introduced coating brings about a shielding effect. The curves of PAH(PSS/TiO₂)_5-7-coated woods are very similar; thus, more than seven layers do not have an additional positive effect.

To evaluate the UV stability of the coated woods, the color changes were measured as a function of UV irradiation time up to 1000 h (Figure 4). Obviously, the color data are changed in comparison to the uncoated wood. The formation of colored unsaturated carbonyl compounds results in reddening and yellowing of wood surfaces, and this effect is already mitigated in presence of merely one PSS/TiO₂ bilayer. In case of three PSS/TiO₂ bilayers, the effect is more pronounced. It can be safely concluded that TiO₂ multilayers create a kind of thick armor, which prevents wood components from photochemical degradation by elevated diffused reflection on the TiO₂ nanoparticles. The multifilms of PAH and PSS protect the wood surface via TiO₂ nanoparticles, which blocks the free radicals to react directly with wood components. Additionally, the white color of TiO₂ with high reflectance is not changed by light, in contrast to the light-sensitive wood components.

Figure 4:

Changing parameters Δa^*, Δb^*, ΔL^*, and ΔE^* of original wood, PAH(PSS/TiO₂)₁-coated wood, and PAH(PSS/TiO₂)₃-coated wood.

TiO₂ particles have a well-known photocatalytic activity to degrade organic molecules, which could be useful in reducing organic pollution on wood surfaces. The pollution was simulated in the present work by means of RhB and MB dyes. The effects of UV irradiation on dyed solutions with PAH(PSS/TiO₂)_n-coated woods are illustrated in Figure 5. The black curves correspond to pure dye solutions. RhB solution has a main absorption peak at 552 nm and MB at 665 nm. When the PAH(PSS/TiO₂)₁₀-coated wood was added in the dyed solutions and irradiated with UV light for 2, 5, and 20 h, the resultant absorption spectra changed (Figure 5a [RhB] and Figure 5b [MB]). The degradation efficiency calculated according to Equation (2) shows that RhB was degraded by ca. 18%, 52%, and 85% and MB, by 64%, 70%, and 86% with prolonged irradiation time, respectively. The photodegradation of RhB is interpreted, as an example, in Figure 6. The energy gap of TiO₂ between the valence and conduct bands is 3–3.2 eV. When UV irradiated, TiO₂ nanoparticles adsorb oxygen molecules and yield superoxide radical anions. HOO^· radicals arise via protonation. The ^·OH radicals are yielded when valence band holes (h⁺) are trapped and HOO^· further absorbs an electron (e^-). All the active radicals, such as O₂^-, HOO^·, and ^·OH, are intimately related to degradation of organic components (Wu et al. 1998). RhB molecules are probably peroxidized or hydroxylated by active radicals to small molecules, and thus, the chromophores are destroyed.

Figure 5:

UV-Vis absorption spectra of (a) 5 mg l^-1 RhB solution, (b) 5 mg l^-1 MB solution, and those of PAH(PSS/TiO₂)₁₀-coated wood under UV irradiation of 2, 5, and 20 h.

Figure 6:

Schematic illustration of the photocatalytic activity and UV shielding mechanism of PAH(PSS/TiO₂)_n coating.

TiO₂ nanoparticles impart wood superhydrophilicity, visible on a water droplet spreading promptly, while the CA, if measureable, is very small (Kommireddy et al. 2005). The PAH(PSS/TiO₂)_n coating was also superhydrophilic, and this effect could be applied for self-cleaning and anti-fog effects. By means of stearic acid modification, the superhydrophilicity can be altered to hydrophobicity with CAs around 140° (Figure 7). According to the Wenzel model, this is due to the TiO₂ nanoparticles uniformly assembled on the wood surfaces, resembling a topography of alternating “valleys” and “hills”. The “hills” elevate the secondary roughness (Hsieh et al. 2011). In addition, the dense packing of stearic acid changes the polarity of surface and decreases the surface energy (Zhang et al. 2006).

Figure 7:

Water contact angle of original wood and PAH(PSS/TiO₂)₆-coated wood with a stearic acid modification.

Conclusions

It was successfully demonstrated that multilayered coating on wood with TiO₂ nanoparticles can be effectuated by means of the LBL method based on polyelectrolytes PAH and PSS. The coating masks the fine ultrastructural features of the cell wall, while the microscale features are intact and well visible. The anatase TiO₂ in the assembled coating lowers photochromism and affords UV protection. The assembled coating is helpful for the catalytic degradation of the dyes RhB and MB. The assembled coating is superhydrophilic and can lead to self-cleaning and anti-fog effects. However, the superhydrophilicity could be altered to hydrophobicity after stearic acid modification. The present paper is an example of the facile development of polymer-inorganic coatings with tailor-made surface properties of wood.