Preparation, Test, and Analysis of a Novel Aluminosilicate-Based Antimildew Agent Applied on the Microporous Structure of Wood.

Fungi play a considerable role in the deterioration of lignocellulose materials, as their activities either affect the esthetic properties or lead to decay of the host materials. The new generation of organic-inorganic preservatives, which are copper-based but chrome- and arsenic-free, is a subject of many research works. Mildew fungus prevention, treatment of affected materials, and their successive conservation are essential to the woodworkers. To prevent degradation and prolong the service life of wood, a sol-gel organic-inorganic procedure was employed in this study. Aluminum sulfate (Al2(SO4)3), copper sulfate (CuSO4·5H2O), and boric acid (H3BO3) were introduced into phosphoric acid (H3PO4) and water glass as an antimildew agent, with different treatment concentrations (0.7, 1.4, and 2%). Wood was inoculated with Aspergillus niger and Trichoderma viride after new treatment based on the inorganic preservative. The changes in wood surface, structural chemistry, and the crystalline structure of the treated wood were examined by scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, and X-ray diffraction (XRD), respectively. The growth of the two mildew fungi showed distribution, and evidence of mildew covering only the untreated wood surfaces and an increase in the crystallinity of wood was observed after the process. The study suggests that the two mildew fungi investigated herein could be prevented by sol-gel coating with a Si-Al-Cu-P antimildew agent.

Introduction

Wood is the most important sustainable construction and lightweight material due to its low carbon footprint, ease of use, biodegradability, and cost efficiency.[1] However, it also possesses some drawbacks such as poor dimensional stability due to moisture, outdoor ultraviolet photodegradation, and high susceptibility to biological attack.[2,3] These have resulted in the development of various facile industrial and practical processes that are energy-intensive and targeted toward improving wood durability.

The coating and impregnation processes that make use of solvents, resins, and preservatives have often been employed.[4,5] Impregnation of wood with inorganic materials decreases the number of hydroxyl groups that can absorb moisture by hydrogen bonding, so the fiber saturation point (FSP) and the equilibrium moisture content (EMC) are decreased, which can result in excellent dimensional stability and great resistance to biological degradation; the most commonly used impregnation methods for inorganic precursors are soaking and immersion, followed by drying. These are the most appropriate methods for wood treatment with inorganic compounds.[6] Also, thermal treatment is one of the most adopted methods for wood durability improvement.[7]

In addition, several milder processes, with little industrial usability, have been developed, such as wood impregnation with natural compounds (polymerized natural unsaturated oils, plant extracts, and polymers such as chitosan).[8−10]

In the last few years, attempts have been made to investigate the potential of organic and inorganic biocides for wood protection.[11−14] Majorly to enhance wood properties, such as durability, weathering, and dimensional stability,[15] gelatin solution (sol–gel) techniques are being used. The sol–gel process is a wet-chemical technique used for producing glassy coatings. In this process, the solution evolves gradually toward the formation of a gel-like network containing liquid and solid phases. The application of the sol–gel technique for wood modification is considered a great potential way of obtaining value-added and improved products.

A large number of research results have shown that inorganic–organic antimildew agents have relatively good comprehensive prevention and antimold discoloration abilities.[16,17]

Boron compounds, for example, are very effective in mold control.[18] Wood is often impregnated with inorganic salts such as ammonium sulfate, sodium tetraborate, and boric acid.[19] Water glass comprises different proportions of sodium oxide (Na2O, 10.6%) and silicon dioxide (SiO2, 26.5%); the Na2O/SiO2 solution exhibits the lowest viscosity, a ratio close to 1.8 that provides excellent antimold properties and improves the density and strength of wood.[20−22] Also, inorganic nanomaterials such as silicate (SiO2)[23] and titanium dioxide (TiO2)[24] have received extensive attention because of their excellent mechanical, thermal, and optical properties, as well as their low toxicity.[25] Besides, sol–gel TiO2 and Al–Si have great potential to be used as protective agents for stabilizing wood against photodegradation.[16,17]

For example, Qin and Zhang have performed the treatment of wood with inorganic titanium dioxide in cell lumens playing an important role in wood antibacterial efficacy.[26] Sol–gel treatment improved wood lumen substances and the hydrophobicity of the wood[27] and also resulted in increased resistance to biological and abiotic damage.[28]

The sol–gel technique, a wet-chemical approach, was developed two decades ago for preparing inorganic materials that can be used for wood modification against biological degradation by Aspergillus niger and Trichoderma viride species that are among the most abundant fungi worldwide. Growth conditions include a wide range of temperature (6–55 °C) and relatively low humidity.[29] Typically, mildew appears on wood surfaces as black or greenish-brown. These molds grow on untreated wood to get enough food and cause considerable damage.

The growth of fungal aerial structures depends on the translocation of water and nutrients from the vegetative mycelium.[30] Treatment of wood becomes very necessary to increase the shelf life of the materials in service. Despite the progress recorded, there is still a need for further research on developing new-generation chemical preservatives that are copper-based but chrome- and arsenic-free. In this research, a new approach for protecting wood materials from mildew degradation through a nonbiocidal inorganic system that forms a Si–Al–Cu–P gel on the microporous structure of wood cells is described. The influence of Si–Al–Cu–P will be particularly advantageous for wood that has been exposed to moisture, without environmental or health effects, and subsequent treatment with water glass as an antimildew agent was tested using the sol–gel technique.[20] The objectives of this work include the formation of new environmentally friendly preservative treatment and providing an easy and cost-effective antimildew treatment process.

Experimental Section

Wood Sampling and Experimental Design

Sapwood samples blocks of dimensions 50 × 20 × 5 mm3 (L × R × T) with 6–10 annual growth rings per 10 mm were prepared from Pinus massoniana specimens to perform mildew resistance. They were obtained from Minhou, Fuzhou City. Aluminum sulfate (Al2(SO4)3), phosphoric acid (H3PO4, 85%), cupric salt (CuSO4·5H2O, 99%), sodium borate (Na2[B4O5(OH)4]·8H2O, 99.5%), and water glass (Na2SiO3) were purchased from Tianjin Fuchen Chemical Co. Ltd. Boric acid (H3BO3, 99.5%) was purchased from Sinopharm Chemical Reagent Co. Ltd. All of the reagents in the experiments were of analytical grade. According to the standard test method for mold on the unseasoned lumber (laboratory method, ASTM D 4445-03), two kinds of molds were selected: A. niger strain MB#284309 and T. viride strain MB#181950 purchased from Beijing Zhongke Quality Inspection Biotechnology Co., Ltd.

Preparation of the Inorganic Si–Al–Cu–P Sol–Gel

The production processes for the Si–Al–Cu–P antimildew solution were performed in two phases: the first was the preparation of the Al–Cu–P water-soluble solution.

In two triple-necked flasks, we have added in each flask (0.7 and 2% w/w, respectively) H3BO3 (7.8 g) and CuSO4·5H2O (2.76 g) to form two distinct solutions. The mixture was constantly stirred at 70 °C for at least 30 min or until the solution turned clear blue, and the pH value was 1.5. Afterward, the second phase involved the preparation of an antimildew aqueous solution based on Na2SiO3. The solution was prepared by adding 3 g of sodium borate (Na2[B4O5(OH)4]·8H2O, 99.5%) into the flask with 125 mL of water and adding 127 g of water glass solution (Na2SiO3, 1.4% w/w). The solution obtained was continuously stirred in a flask at 70 °C until it became transparent, and the pH value was 10.

Preparation of the P. massoniana Sample

Potato dextrose agar (PDA) for the mildew was prepared using 200, 20, and 20 g of potato, glucose, and agar, respectively, in accordance with GB/T18261-2013 standards.[31] The potato was boiled in 1000 mL of water and filtered. The solution was transferred into a conical flask. Thereafter, glucose and agar were added and autoclaved. The P. massoniana sample (50 × 20 × 5 mm3, L × W × H) without visible defects was arranged into 17 specimens for each concentration gradient. The P. massoniana samples were kiln-dried until it reached 6.92% moisture content before Si–Al–Cu–P treatment.

Optimization Modeling and Analysis of Si–Al–Cu–P

The retention of the antimildew solution in wood samples was tested according to ASTM D 5583-06 (Standard test method for detection and estimation of retention of wood preservatives by Aspergillus bioassaying (withdrawn 2010)).[32] The dry wood samples were then put into a beaker filled with the Al–Cu–P sol–gel solution at 70 °C for 6 h; afterward, the specimens were dried at 95 °C for 4 h. The specimens were further treated with Na2SiO3 at 70 °C for 20 min and also dried after treatment at 100 °C for 4 h. The retention of Si–Al–Cu–P was evaluated by eq . Mildew tests were performed in Petri dishes. The centers of the PDA media were inoculated with A. niger and T. viride. The Petri dishes were incubated in the darkness at 28 °C and 85% relative humidity for 7 days to allow the spores of fungi to grow. The treated and untreated P. massoniana samples were placed on fungal mycelia and incubated at 28 °C for 28 days.

Response surface methodology (RSM) was used to optimize the parameters of the H3BO3, CuSO4·5H2O, and Si–Al solution concentrations for the effective treatment process. For the RSM, Design-Expert software trial version 8.0.5 was used to design the experiments. The Box–Behnken design (BBD) was adopted to carry out the test at the level of three factors m(CuSO4·5H2O), m(Si–Al), and m(H3BO3) with respective values of 2, 1.4, and 0.7%. The test designs were coded as X1, X2, and X3. Tables and S1 reveal the results of the response surface average mold control effectiveness (AMCE) of two types of molds A. niger and T. viride.

Table 1

Code and Level Factors

	code and levela
factors	–1	0	1
X1	0	0.35	0.7
X2	0	0.7	1.4
X3	0	1	2

Signs mean combinations at two levels: (−1, 0), low level; (0, 1), high level.

Determination of the Antimold Solution

The details for the determination of the retention of the sample were based on the different treatment compounds of CuSO4, Na2SiO3, and H3BO3 and were evaluated according to ASTM D 5583-06.[32] To obtain the best sample optimization, the chemical retention of the antimildew agent was determined using the following equationwhere R is the retention, kg/m3; m = m2 – m1 is the solution absorbed by P. massoniana samples, g; C is the solution strength, %; v is P. massoniana volume, cm3.

Antimold Performance

The antimildew performance was evaluated according to GB/T18261-2013.[31] The antimildew effectiveness was examined using visual observation of three replicates per treatment (Table ). The calculation of prevention and control effectiveness was based on the formula belowwhere E is the mildew control effectiveness (MCE), %, D1 is the average infection value of the Si–Al–Cu–P treatment sample, and D2 is the average infection value of the untreated wood sample.

Table 2

Grade for Infection of the Wood by Mildew

average infection value (%)	infection grade	surface infection value
0	full mildew resistance	normal surface
0–10	strong mildew resistance	the mycelium covers 1/4 of the surface of the wood sample
11–24	medium mildew resistance	the mycelium covers 1/2 of the surface of the wood sample
25–44	slight mildew resistance	the mycelium covers 3/4 of the surface of the wood sample
>45	no mildew resistance	the mycelium covers greater than 3/4 of the surface of the wood sample

Material Characterization

The chemical composition of the wood specimen surface was determined using Fourier transform infrared (FTIR) spectroscopy (MIR-FIR spectrum VERTEX 70 Instrument). The KBr pellet method was employed by grinding 4 mg of dried wood powder (200 mesh) and potassium bromide powder (1:100) to produce thin sheet disks. The crystallization of wood and the Si–Al–Cu compound was determined by X-ray diffraction (XRD, X’Pert PRO Malvern Panalytical Ltd.). The wood powder (200 mesh) was pressed into 10 × 10 × 1 mm3 size on a slide. The tests for cellulose crystallinity and chemical bond formation between wood and chemical compounds by Cu Kα radiation were carried out at wavelength λ = 1.790 nm. The method of calculating the crystallinity index (CI) was used as mentioned in Chen et al.[33] The curves of thermogravimetry (thermogravimetry/derivative thermogravimetry (TG/DTG)) were obtained using a thermogravimetric analyzer (NETZSCHSTA449F3, Germany). Under a high-purity nitrogen atmosphere, approximately 10 ± 0.5 g of wood powder heated at 10–20 °C/min was applied for a temperature range from 20 to 800 °C; the reported results were the average of six specimens.

The morphologies of the wood samples treated with Si–Al–Cu–P and untreated wood samples were characterized using scanning electron microscopy (SEM; Hitachi UHR FE-SEM SU8010, Japan). The cross section of wood samples were cut with a microtome and sprayed with carbon 15 nm before image acquisition, at 50 μm, 15 kV, and variable pressure (5–10 Pa). To reveal the distribution of mildew chemical elements in the wood, EDS Supra 55 (Supra 55 Zeiss, Germany) was used at an acceleration voltage of 15 kV and variable pressure (5–10 Pa). The mapping of the wood sample was performed with a distance of 10 mm, and the time of capture was 600 s. The pore size distribution of wood realized by nitrogen absorption–desorption was analyzed using a JW-BK132F (Beijing). From the sample size of 50 × 20 × 5 mm3, the weight of 0.5–1 g of powder specimen was determined using a blast dryer, under the condition of 103 ± 3 °C. The pore size of wood was calculated according to the BJH formula. The surface porosity of wood was calculated according to the Brunauer–Emmett–Teller (BET) equation.

The measurement of X-ray photoelectron spectroscopy (XPS) was performed on an ESCALAB 250Xi (Thermo Fisher Scientific Company, Waltham, MA). Experiments were performed at ambient temperature in an ultrahigh vacuum system with Al Kα radiation (λ = 1486.6 eV), a power of 300 W, and 500 μm high-sensitivity spectroscopy.

Results and Discussion

Antimildew Resistance

The ability of A. niger and T. viride to grow on untreated and treated samples without and with Si–Al–Cu–P was investigated and is presented in Figures and 2 and Table S3. A total of 17 samples were set for each concentration gradient to verify the reliability of Si–Al–Cu–P antimildew agents. The antimildew properties of the treated sample after 28 days of incubation are shown in Figure .

Figure 1

Average results of three different chemical compounds (CuSO4, Na2SiO3, and H3BO3) obtained after exposure to T. viride (a) and A. niger (b) on treated and untreated wood.

Figure 2

Untreated and optimized wood samples exposed to A. niger and T. viride after 28 days of exposure.

The results showed that the mildew degree of P. massoniana samples treated with the Si–Al–Cu–P chemical reagent was less, some of them were almost free from mold infection, and the mildew resistance was more than 2%, in accordance with Table .

The infection effects of mildew in the untreated wood sample are shown in Figures (1-0) and 1. It can be seen that mildew have grown successfully on the surface of untreated wood samples, and consequently the average mold control effectiveness, AMCE, is 0%, with the average effectiveness rating of 4. Figure (1-1) shows that the sample treated with H3BO3 (1.4% w/w) failed to provide adequate protection against A. niger growth. The average mold control effectiveness was 52.11%, with the lowest mildew control effectiveness of 3% after 28 days. When the Na2SiO3 mildew inhibitor is added at a ratio of 1.4%, the control efficacy of the antimildew of the wood sample against A. niger increased to a certain extent, rising to 81%. After 28 days of exposure, its control efficacy against A. niger was below 2%. In the sample treated with CuSO4, Figure (1-3), after 28 days of testing, the infection value (MCE) was 1%, and the AMCE went up to 95.15%, showing mold control effectiveness (A. niger). The reason for the slower growth of molds due to the presence of cupric salt combined with other compounds provides excellent efficiency or can suppress the growth of mold.[34,35]

The AMCE, shown in Figure a and Table S1, of P. massoniana treated with Si–Al–Cu–P at levels of 0.7, 1.4, and 2% was tested against T. viride, reaching 48.50, 67.41, and 87.65%, respectively. Hence, the CuSO4 (2% w/w) antimold solution effectively possesses the excellent antimildew property for wood. Therefore, it can be concluded that the Si–Al–Cu–P-impregnated wood possessed excellent antimildew property to various mildew, including A. niger and T. viride.

Optimization and Operational Factors of the Wood Sample

The response surface methodology (RSM) was used to optimize the parameters of the CuSO4, H3BO3, and Si–Al solution concentrations for the effective treatment process. To obtain the most appreciable antimildew property of wood, the three dependent variables CuSO4·5H2O, H3BO3, and Si–Al were used to assess the average mold control effectiveness (AMCE%) of the treatment and correlated with the proposed regression model.[36]where AMCE is the average mold control effectiveness; X1, X2, and X3 are masses of CuSO4, Si–Al, and H3BO3, respectively.

Analysis of variance (ANOVA) and complex coefficient (R2-pred) analysis results are presented in Tables and 4. The coefficient for this experiment is approximately 0.9438. Analysis of the data in Table shows that the model is extremely significant, indicating that the fitting is good and the model can well reflect the relationship between factors and response values. Also, the p-value is greater than 0.05, which indicates that the model is not significant. This conforms to the requirements of the experiments. The model value of the probability of variance (p ≤ 0.0001) shows that the quadratic polynomial model has a functional significance of the curvature.[16,37] Predicted values for the inorganic Si–Al–Cu–P adsorption capacity are also displayed in the energy-dispersive spectroscopy (EDS) profile. Thus, these influences on the adsorption capacity of Si–Al and CuSO4·5H2O provide a valuable antimildew efficiency. Figure b shows the 3D response plots and 2D contour lines from the results, which indicate that the interaction effect between Si–Al and H3BO3 exhibited the influence of AMCE%, and the response surfaces and contour plots are similar to those observed in Figure c. The interactive effect between CuSO4·5H2O and H3BO3 both is significant, according to ANOVA. The contour plots are not close to the circle. Each curve represents a region of constant-response 2D contour line defined by the factorial ranges, in this case, 70–90 for (a) and (b) and 60–90 for (c).

Table 3

Box–Behnken Design and Response Values

test	X1 (%)	X2 (%)	X3 (%)	infection rate (%)
1	2.00	2.80	0.00	94.08
2	2.00	1.40	2.80	67.31
3	4.00	1.40	0.00	58.76
4	2.00	0.00	4.00	91.64
5	2.00	1.40	2.00	69.81
6	2.00	2.80	4.00	76.14
7	2.00	1.40	2.80	88.23
8	2.00	0.00	1.40	74.12
9	0.00	0.00	2.00	65.13
10	4.00	2.80	2.00	76.14
11	0.00	1.40	4.00	60.12
12	4.00	0.00	2.80	52.78
13	2.00	1.40	2.00	54.67
14	0.00	2.80	2.00	93.54
15	2.00	1.40	400	92.71
16	4.00	1.40	2.80	58.38
17	0.00	1.40	0.00	51.24

Table 4

Analysis of Variance (ANOVA) Quadratic Model

source	squares	df	mean square	F-value	p-value
model	16 759.60	9	405.87	30.86	<0.0001
X1	354.80	1	4.49	0.34	0.5776
X2	1726.08	1	198.7	15.11	0.006
X3	1.02	1	280.61	21.34	0.0024
X1X2	26.63	1	54.17	4.12	0.082
X1X3	406.63	1	279.73	21.27	0.0024
X2X3	48.51	1	2.3	0.17	0.6886
X12	3833.02	1	903.98	68.74	<0.0001
X22	1637.82	1	802.72	61.04	0.0001
X32	4429.30	1	828.21	62.97	<0.0001
residual	92.06	7	13.15
lack of fit	70.52	3	23.51	4.37	0.0942
pure error	21.54	4	5.38
cor total	3744.92	16

Figure 3

Three-dimensional (3D) response surfaces and two-dimensional (2D) contour lines predicting conversion for the maximum AMCE% in 0.7, 1.4, and 2% concentration values. (a, b) Effects of mass rate of Si–Al and CuSO4. (c, d) Effects of mass rate of Si–Al and H3BO3. (e, f) Effects of mass rate of CuSO4 and H3BO3.

Effectiveness of Chemical Retention

The retention amounts were determined to assess the chemicals retained after the inorganic sol–gel preservative treatment (Figures a and 4b). The effectiveness of the chemical was evaluated based on its concentration and retention.[38] Cr and Cu were tested to be good fixative chemicals for wood treatment. However, how much chemical is retained in the wood after treatment still remains one of the utmost priorities of any preservative treatment. In the meantime, Radivojevic and Cooper have found that when the fixation time was longer, the retention of Cu components in the preservative increased.[39] Therefore, the major concern in this research is how much amounts of these chemicals were retained after the treatment since Cu is already included in the composition. Treated wood samples absorbed about 0.055 and 0.052 g of CuSO4 for concentrations of 0.7 and 2%, respectively, as shown in the graph (Figure S1b). Usually, the difference in the rate of Cu absorption is due to the structural variation of the wood.[40−42] Despite similarities, the chemical retention of Al2(SO4)3, H3PO4, H3BO3, and Na2SiO3 showed a greater absorption rate in treating P. massoniana. A detailed observation of the P. massoniana cell wall could probably reveal some differences, leading to various rates of preservative chemicals retained.[43] Fungal growth after inoculating the wood specimens with the mold was visually examined (Tables and 3). A. niger and T. viride were able to grow on the wood specimens without the Si–Al–Cu–P treatment (Figure ). In comparison, the selected fungi were highly sensitive to the presence of Si–Al–Cu–P. Regardless of the disparity in the rate of the chemical retained by P. massoniana wood during treatment, the result has shown that the selected mold would find it difficult to feed on the treated wood medium. Therefore, the mechanism of the reaction between wood and Si–Al compounds is described in Figure a.

Figure 4

(a) Simplified mechanism of the reaction between wood and Si–Al compounds. (b) Different elemental mapping images Si–Al–Cu–P.

Physicochemical and Thermal Analyses

FTIR analysis was conducted to determine the functional group present on the wood surface treated with Si–Al–Cu–P. The spectrum of the hybrid samples shows the features of the reagent Si–Al–Cu–P compound, indicating that it is successfully coated on the wood surface (Figure b). Treated and untreated Pinus wood samples showed strong O–H stretching with vibration bands between 3650 and 3200 cm–1. Similarly, the same spectra were observed in treated and untreated spruce at the same frequencies. The intensity of the characteristic peaks at 3600 cm–1 decreased or the peaks disappeared after treatment. Two distinct characteristic peaks appeared at 2863 and 2852 cm–1 for the wood specimens treated with Si–Al–Cu–P, which are related to the asymmetric and symmetric stretching vibrations of C–H. There was no special peak in the region between 2710 and 1900 cm–1. The main functional groups for the organic materials, such as cellulose, hemicelluloses, and lignin, could be found in the region of 1800–400 cm–1.[44−46] The peaks of lignin and hemicelluloses[47] were weakly stretched at 1731 and 1191 cm–1 for the C=O stretching vibration. The characteristic peaks for cellulose appeared at 609 and 897 cm–1.[48]A. niger and T. viride can colonize and damage wood polymers such as cellulose, hemicellulose, and lignin, which could result in a high mass loss in the wood.[49] However, upon modification of the O–H groups occurring in the lignin and cell walls of P. massoniana and spruce, the O–H groups required for the growth of fungi were drastically removed and the mildew resistance of the wood increased.[50,51] The crystalline structures of the Si–Al–Cu–P-treated wood and untreated wood specimens were analyzed by X-ray diffraction (XRD, Figure a). The diffraction with three significant peaks located at 2θ values of 16.2, 22.3, and 34.5° corresponds to (101), (110), and (200) planes of the crystalline Iβ form of native cellulose.[52] The XRD data show that the Si–Al–Cu–P sol–gel treatment, after turning off the burner at 800 °C, does not destroy the crystalline structure of the treated Pinus, but the diffraction peaks become much weaker. Interestingly, the crystalline region of the cellulose shows stronger packs of microfibrils after the successful sol–gel impregnation of wood fibers. The improved crystalline peaks observed could be associated with strong synergistic properties of the sol–gel and the Pinus fibers because the Si–Al–Cu–P compound showed a less crystalline region when they stand alone. The weak peaks observed at the crystalline angle 2θ of 34.5°, corresponding to the (200) plane, could be ascribed to the association of the amorphous area of the Si–Al–Cu–P compound and the interacting nonlinear cellulose phase. Figure c,d presents the thermogravimetric and differential thermogravimetric (TG and DTG, respectively) curves of untreated and Si–Al–Cu–P-treated wood. Three clear weight loss regions can be seen from the TG curve for wood samples (a total weight loss of 82.69%), amounting to weight losses of 4.12% (10–240 °C), 49.16% (240–500 °C), and 29.41% (500–800 °C), respectively. The untreated wood samples without Si–Al–Cu–P compounds show an initial slight weight loss between 10 and 240 °C associated with the evaporation of water in the wood sample.

Figure 5

(a) X-ray powder diffraction (XRD) patterns of Si–Al–Cu–P compound-treated wood and untreated wood. (b) Fourier transform infrared (FTIR) spectra of the untreated and treated wood specimens. (c, d) TG and DTG curves of untreated (A) and Si–Al–Cu–P compound-treated (B) wood.

Additionally, a curve of untreated wood and Si–Al–Cu–P-treated wood subjected to heat appears in the temperature range of 20–800 °C, as shown in Figure c,d. When P. massoniana is subjected to Si–Al–Cu–P treatment, significant changes in thermal degradation characteristics can be seen from the DTG curve, as shown in Figure c. The maximum weight loss–thermal decomposition curves appear at 333 °C for the Pinus wood treated with Si–Al–Cu–P compounds and at 289 °C for the Pinus wood without Si–Al–Cu–P compounds, respectively, indicating that Si–Al–Cu–P as a wood antimildew could affect the thermal stability and effectively enhance the protection properties of P. massoniana specimens. However, the total weight loss of treated Pinus is lower than that of the untreated wood.

Microstructure Analysis of Si–Al–Cu–P Antimildew Impregnated Wood

SEM-energy-dispersive X-ray (EDX) analysis was used to examine the morphology of wood after 28 days of exposure to mildew fungi in a controlled environment that is necessary for their activities (Figure a,b,d,e). After 28 days of exposure, the Si–Al–Cu–P sol–gel-treated Pinus showed a smooth surface with no visible activities of the A. niger mildew (Figure a,b). However, the treated wood sample (Figures b and 1) shows that the phosphorus, aluminum, and cupric compounds could be seen in the inner and outer sections of the wood, which could be an indication of the excellent antimildew property of treated wood samples.

Figure 6

(a, b) Scanning electron microscopy (SEM) images of the wood treated with Si–Al–Cu–P compounds and (d, e) untreated wood sample infected by A. niger mildew. (f) Distribution and different weight ratios of the untreated sample after 28 days of exposure. (c) Nitrogen adsorption–desorption isotherm and micropore distribution of the Si–Al–Cu compound, as calculated by the BJH method.

Meanwhile, untreated P. massoniana specimens had a rough surface showing, clearly, the highest coverage of molds of A. niger on the wood surface (Figure e,f). Therefore, Si–Al–Cu–P sol–gel treatment can protect the wood structures from mildew fungi without changing their natural look.

The Si–Al–Cu–P compound penetrated deeply in the microscopic pores of the wood, making it resistant to mildew invasion. It is well known that the structural integrity of wood materials can be lost as a result of biological damage, according to the mapping of the elements in the cross section (Figure b). The interaction between wood and chemical compounds, H3PO4 (P), CuSO4 (Cu), and Si–Al (Al), could be seen in the cell structure, indicating that there was sufficient adhesion. This meant that there was an excellent performance against mildew. Mildew fungi grow on wood and decrease the life of material; the rate of damage depends on wood morphology, moisture in the environment, and effectiveness of the preservative chemicals.[53] The antimildew effectiveness of Si–Al–Cu–P observed in this study corresponds to that obtained for the commercially available nanoparticles used to investigate the antifungal activity of TiO2 against A. niger on Paulownia wood surface.[54] The fungus resistance of pine sapwood specimens impregnated with alcoholic solutions of TiO2 was also tested against two fungi (Coniophora puteana and Poria placenta).[55] The degree of mold invasion by the mycelium did not exceed 1/4 of the surface of treated samples, which is in the range of values categorized for mildew resistance (8–10%), as shown in Table . The spectrum scans of EDX quantify and examine the distribution of chemical compounds present inside the treated wood cell sample. The elemental distribution on the surface of treated wood before and after exposure to A. niger and T. viride molds were shown by energy-dispersive X-ray (EDX) analysis of the treated sample is shown in Figure S2; the spectra show C, O, Al, P, Cu, and Si elements with aspect ratios of 50.95, 42.41, 0.12, 0.33, 0.53, and 0.12%, respectively. Compared with those in Figure f, the spectra are slightly different because the sample is covered with mildew, which explained the presence of C, N, O, S, and K elements in the weight ratios of 50.95, 4.87, 42.41, 0.56, and 0.23, respectively. In addition, a typical nitrogen adsorption–desorption isotherm (BET) of the Si–Al–Cu–P material is reported in Figure c. N2 adsorption–desorption curves for the sample provide clear evidence for hysteresis effects of Si–Al–Cu–P on wood samples having a predominantly microporous surface, which could be classified as a mesoporous structure by the hysteresis loop. Our results are very similar to those of Chen et al.[33] The average pore diameter was 5.02 nm, and the total pore volume and specific surface area were 1.78 × 10–2 cm3/g and 9.11 m2/g, respectively; we can, therefore, conclude that the combination of the Si–Al–Cu–P compound and hierarchical porosity can improve antimildew properties to treated wood sample over the untreated wood sample.

X-ray Photoelectron Spectroscopy Analysis

To further analyze and clarify the chemical bonding between Si–Al–Cu–P and wood samples, the surface chemistry of Si–Al–Cu–P and wood was investigated by XPS. Figure a shows the typical C 1s peak at a binding energy of 285.5 and the peak of O 1s at 530.18 eV. Figure a are corresponding to the following groups: C–C or C–H, C–O, and C=O, respectively.[56] For Si–Al–Cu–P-treated wood, Figure c clearly illustrates three others peaks, Al 2p, Si 2p, and B 2p, which are supposed to be the main elements of the hybrid Si–Al–Cu–P antimildew agents; Figure c indicates that the carbon content was quite low in the wood sample. The characteristic Al 2p and Si 2p peaks of Si–Al–Cu–P were present at 77.24 and 103.28 eV, as shown in Figure b. The B 1s peak appears at the binding energy at 190.48 eV. When the electron density increases because of the chemical environment of Si–Al–Cu–P, the binding energy of the inner electron decreases, as a result of the electronegativity effect.[33]

Figure 7

Typical X-ray photoelectron spectroscopy (XPS) spectra. (a) C 1s and O 1s XPS spectra of untreated wood, (b) B 1s XPS spectrum, (c) XPS spectrum of treated wood, and (d) Si–Al 2p XPS spectrum of treated wood.

Conclusions

To solve the problem of poor mildew performance of wood products, this study proposed a new treatment method for wood based on inorganic materials that were prepared using the concurrent sol–gel method. The solution is less toxic, environmentally friendly, and insoluble in water. These materials can be quickly processed. The antimildew property of wood samples shows that they exhibited efficient mildew resistance after 28 days of exposure to A. niger and T. viride.

Hence, these results suggest that this method can potentially be used as a natural antimildew treatment against mildew fungi. The presence of Si–Al–Cu–P homogeneously distributed inside the cell wall was indeed confirmed by EDX images. Moreover, the degree of crystallinity of Pinus wood was enhanced by Si–Al–Cu–P treatment; most importantly, the treatment does not negatively alter the crystalline nature of the wood. All of these results demonstrate that Si–Al–Cu–P compounds have a remarkable influence on wood properties, after the treatment.

The simulation results obtained by the RSM method proved the suitability and effectiveness of the antimildew agent.