Journal of Nanomaterials & Molecular NanotechnologyISSN: 2324-8777

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Research Article, J Nanomater Mol Nanotechnol Vol: 3 Issue: 3

Effect of Nano-ZnO on Decay Resistance and Artificial Weathering of Wood Polymer Composite

Siroos Habibzade*, Asghar Omidvar, Mohammad Reza Mastery Farahani and Mehdi Mashkour
Department of Wood and Paper Engineering, Gorgan University of Agricultural Sciences and Natural Resources, Gorgan, Iran
Corresponding author : Siroos Habibzade
Department of Wood and Paper Engineering, Gorgan University of Agricultural Sciences and Natural Resources, Gorgan, Iran
E-mail: [email protected]
Received: February 11, 2014 Accepted: April 08, 2014 Published: April 12, 2014
Citation: Habibzade S, Omidvar A, Farahani MRM, Mashkour M (2014) Effect of Nano-ZnO on Decay Resistance and Artificial Weathering of Wood Polymer Composite. J Nanomater Mol Nanotechnol 3:3. doi:10.4172/2324-8777.1000146


Effect of Nano-ZnO on Decay Resistance and Artificial Weathering of Wood Polymer Composite

Wood polymer nanocomposite (WPNC) was prepared by impregnation of styrene and Nano zinc oxide into poplar wood (Populus deltoids Marsh.). WPNCs containing different loading of nano zinc oxide, namely 0, 0.5, 1 and 1.5 percent (by weight of monomer), were made. Then the effect of addition of nanoparticles on the decay resistance and artificial weathering were studied. The WPNCs were subsequently exposed to the artificial weathering test with duration of 200, 400 and 800 hours and decay test according to a modified ASTM D1413 standard. The results showed that the amount of color change of samples treated with nano zinc oxide was less than untreated one, also the decay resistance of the composite against the fungi with increasing of concentration was improved.



Wood polymer nanocomposite; Nano-zinc oxide; Decay resistance; Artificial weathering


Wood polymer composites (WPC) are prepared by impregnating of wood with vinyl monomers (such as, styrene, methyl methacrylate and etc.), followed by free radical bulk polymerization in the lumen and cell walls [1]. When, the WPCs exposes outdoors are susceptible to degradation from sunlight, moisture, fungal attack and microbial colonization [2-4]. Although, polymers are generally resistant to fungal attack [5,6]. In contrast to polymer, wood is susceptible to fungal decay [7]. Despite the styrene monomer into wood, is mostly placed in the lumen and a little penetration in the cell wall [8] so that fungi can access the cell wall within wood of WPCs. Therefore, WPCs need to be protected against fungi and UV irradiation when they are used in outdoors.
Nanotechnology was used in many applications [9-11]. The practical application of nanotechnology was introduced by impregnation of solid wood with metal nanoparticle suspension for heat treatment and wood preservation [12,13]. It was followed by applying mineral nanofibers as fungicide and fire-retardants [14,15], spectroscopy analysis [16], as well as improving thermal conductivity in wood-composite panels [17]. Recently, the introduction of inorganic nanocomposites to organic polymers has attracted much attention because organic-inorganic nanocomposites offer an effective way to improve the decay resistance, physical and mechanical properties and UV stabilizer [18-22]. However, the retention of homogeneous dispersion of nanosized particle in the preparation process of nanocomposites is very difficult, because of strong tendency for nanoparticles to agglomerate to prevent the formation of agglomerated nanoparticles in polymers, the combination of inorganic particle with polymers is usually accomplished by surface modification [23].
It is know that surface modification of nanoparticles by grafting polymers onto it is an effective way to improve its dispersability in polymer matrix as well, and hence ameliorate the polymer matrix, thus enhancing the properties of the resulting composites [19,23,24].
Nano-zinc oxide is naturally occurring elements in the environment that having a long history of UV stabilization, antibacterial, antimicrobial properties and etc [25-28]. There are studies reporting the use of ZnO as a wood preservative. In addition, ZnO has also been used to wood plastic composite protection [20]. The objective of this study is to evaluate the influence of nano zinc oxide on UV and fungal resistance of wood polymer nanocomposite.


Poplar wood (Populus deltoids L.) was collected from Gorgan province, located in the Northern part of Iran. The samples were selected in Gorgan forest along an altitudinal gradient 450 meters above the sea level. The range of temperature and RH were 17.8°C and 69% respectively The chemicals used to produce wood polymer nanocomposites (WPNCs) were styrene (ST), benzoyl peroxide (BP) and glycidyl methacrylate (GMA) were supplied by Merck, Germany. ZnO nanopowder, <50 nm, 90 m2\g, was purchased from Nano Pars Lima Co, Tehran, Iran. Tetrahydrofuran (THF, anhydrous) and Vinyl trichloro silane (VTCS) were purchased from Sigma-Aldrich chemical company.
Sample preparation: The poplar wood sample used for the study were prepared from clear defect-free sapwood, cut into block of 19×19×19 mm and 50×50×5 mm (radial × tangential × longitudinal) for decay and weathering tests respectively. The replication was 7.
Surface modification of nano-zinc oxide: At first nanoparticles were modified with silane coupling agent. The surface modification of ZnO nanoparticles was carried out as follows: Firstly, a 1.0 g portion of ZnO nanoparticle, 1.0 mL of VTCS, and 40 mL of toluene were added into a three-necked round-bottom flask (250 mL) and refluxed at 80°C for 6 h under mechanical stirring. Secondly, the precipitate was centrifuged and extracted with ethanol for 12 h to remove the residual silane. At last, the precipitate was then dried in vacuum for 48 h. From the above treatment, the double bonds were introduced onto the surface of the ZnO nanoparticle [23].
Preparation of ST\ZnO Nanocomposite: The monomer solution was used for wood polymer nanocomposite production: styrene (ST) mixture containing benzyl peroxide catalyst (polymerization initiator) and glycidyl methacrylate (GMA) as cross-linker. Monomer solutions were used with the addition of ethanol in order to adjust the two different polymer loading levels in wood polymer nanocomposite. Then the different amounts of modified ZnO nanoparticles (0/5, 1 and 1/5 wt% of ST) were dispersed in ST monomer in presence of minimum amount of solvent (THF). The dispersion was achieved using ultrasonic instrument for 30 min at room temperature.
Impregnation and polymerization procedure: All the samples were oven dried at 100°C to constant weight before treatment and weights were measured. The samples were then placed in an impregnation chamber followed by application of load over each sample to prevent them from floatation during of monomers-vacuum was applied for a specific time period for removing the air from the pores of the wood before addition of monomers/ZnO mixture. Then, the dispersion of ST monomer and ZnO nanoparticles was added from a dropping funnel to completely immerse the wood samples. The samples were then kept in the chamber at room temperature for another 30 min after attaining atmospheric pressure. After immersion, vacuum was applied for a short time period in order to adjusting polymer load. After impregnation, samples were taken out of the chamber and excess chemicals were wiped from wood surface. The samples were then wrapped in aluminum foil and cured at 90°C for 24 h in an oven. The cured samples were then soxhelt extracted using chloroform to remove homopolymers, if any, formed during polymerization. Finally the samples were dried and weights were measured.
Weight percent gain: Weight percent gain (WPG) after polymer loading was calculated according to the formula WPG (%) = (W2 - W1)/W1× 100
Where W1 g/cm3 is oven dry weight of wood blocks before polymer treatment and W2 g/cm3 is oven dry weight of blocks after polymer treatment.
Decay Test: The treated and untreated samples with dimensions 19×19×19 mm were dried at 103°C for 24h, weighted and autoclaved at 121°C for 20 min. Then the samples were exposed to the white rot fungus Trametes versicolor (L.: Fr.) Pilat and brown rot fungus Coniophora puteana (Schum.: Fr.) P. Karsten in a soil block decay test for three months in accordance with a modified ASTM D 1413 test standard [29].
Artificial UV light radiation: The samples, after impregnation with ST/ZnO mixture were put into artificial weathering system. In this system, UV radiance and spray of water perform as a round cycle. The cycle was closed and followed a vaporization cycle of 4 hours for water spray, 20 hours exposure to UV radiation. UV ray radiated on samples was from UVA type to wave length range of 320 to 400 nm. From these samples, ones were picked that only one side of them was exposed to UV ray.
In this study, for survey effect of this nano material, on prevention of wood color change during time, the samples were exposed to artificial weathering test duration of 200, 400 and 800 hours. Then, with completion of weathering in each stage, the samples color changes were measured to survey optical severance effect of this material by increasing of time in treated samples in comparison to untreated ones
Color change measurement: The sample color changes were evaluated via visual control and photography by using of color measurement system (spectrophotometer) according to standard of ISO- 7724-2. For color change measurement, of L*, a* and b* parameters were calculated for treated and untreated samples before and after weathering change calculated by following formula:
ΔE* = (Δl*2+Δa*2+Δb*2)1/2
Color measurements were determined according the CIE L*a*b* system of three parameters. The L* axis represents the lightness and varies from 100 (white) to zero (black). The a* coordinates represent chromaticity with +a* for red and –a* for green; and the b* coordinates represent chromaticity with +b* for yellow and –b* for blue. Thus; a*, b* are coloration characteristics and Δa*, Δb* and ΔL* are changes of a*, b* and L* before and after weathering respectively.
Scanning electron microscopy: The mini-blocks were fixed overnight with 3% glutaraldehyde in 0.1 M phosphate cacodylate buffer. Then they were rinsed in the same buffer three times for 30 min. Post-fixation was carried out with 1% osmium tetra-oxide in 0.1 M phosphate cacodylate buffer for 4 h. Fixed specimens were washed with distilled water three times for 30 min each. Dehydration with an increasing ethanol percentage (10, 30, 50, 70, 80, 90 and three times 100%) was carried out for 30 min (each step). Critical point drying was applied with CO2 at 42°C. The specimens were coated with Au/ Pd at 0.4 Torr pressure and 20 mA for 3 min before examination with a Jeol JSM-5200 SEM at 15-20 kV [30].
Statistical analysis: Statistical analysis was conducted using SPSS software program, version 16. One-way ANOVA was performed to discern significant difference at the 95% level of confidence. Grouping was then made between treatments using the Duncan’s test.

Results and discussion

Weight percent gain (WPG%)
The values of WPG for WPC and WPNC samples for decay resistance and artificial weathering were measured and given in table 1.
Table 1: Average weight gain of specimens after polymerization for fungal decay resistance and weathering.
SEM results
Figure 1 shows the SEM micrographs of untreated (Figure 1a) treated wood samples (Figure 1b-d). The empty cell wall and pits were observed in untreated wood (Figure 1a). In treated wood (Figure 1b), these empty spaces were occupied by the ST polymer. The impregnated nanoparticles as white spots were either located in the wood (Figure 1c). In Figure 1d, It can be observed, nano zinc oxide was nearly homogeneously dispersed in the composite. There also was nearly uniformity in the distribution of nano zinc oxide in the composite and no aggregation of nanoparticles observed.
Figure 1: SEM images of wood (a) untreated and treated with (b) ST, (c) ST/ ZnO (0.5%), and (d) ST/ZnO (1.5%).
Nano zinc oxide has been dispersed in the styrene acrylonitrile copolymer [21]. In this research, it was shown that nano zinc oxide did not agglomerate in the wood polymer composite.
Decay resistance
Significant differences were observed in weight loss in the treated and untreated samples after decay resistance tests against white-rot fungus T. versicolor and brown-rot fungus C. puteana (Figure 2).
Figure 2: Weight loss (WL) due to decay as a function of nano zinc oxide.
As can be observed, ZnO nanoparticles decreased weight loss of the WPNC as the nano zinc oxide loading increased. However, T. versicolor caused lower weight loss than C. puteana. Many brown rot fungi are heavy metal tolerant due to their ability to produce organic acids, notably oxalic acid. When in contact with wood preservatives containing heavy metals, these acids can form insoluble metal salts and detoxify the preservatives [31]. In addition, Bordes et al. (2009) reported the presence of the additives, the diffusion path for enzymes was tortuous and hindered. In other words, the decay resistance against the fungi was achieved as a result of the toxicity of nano-zinc oxide toward the fungi [20,28,32].
Weight loss due to artificial weathering
As shown in Figure 3, the samples treated with nano zinc oxide showed less weight loss compared to the untreated samples. As can be observed, the rate of weight loss was highest for untreated wood samples and lowest for 1.5 treated samples. The chain scission of the polymer occurred on exposure to UV irradiation. The chain scission decreased the density if an entanglements of polymer chain and as a result weight loss was observed, on the other hand, nano zinc oxide has UV shielding ability against UV irradiation. Therefore, nanoparticles were protected chain scission in wood polymer nanocomposite [21,33].
Figure 3: Weight loss of wood polymer nanocomposite after artificial weathering test.
Color change after impregnation
The samples after impregnated with nano zinc oxide almost didn’t show any color change at three concentrations, 0.5, 1, 1.5. Therefore, the brightness of surface of samples after impregnation was increased slightly. In addition, the highest change color was obtained in highest concentration of nano zinc oxide (Table 2).
Table 2: Colour change of samples after impregnated nano zinc oxide.
Color change after artificial weathering
The CIE l*a*b* color specification of treated and untreated samples are presented in Figure 4. Figures 4a-c show that the Δa*, Δb* and ΔL* values of all WPCs increased with increasing exposure time. Among, the untreated samples exhibited the most severe color change. In contrast, the untreated samples showed the less color change against artificial weathering. It could be seems, the photo-sensitive aromatic groups in the side chain of polymer lead to surface yellowing upon UV exposure. On the other hand, Pigmented molecules in polymer may accelerate UV absorption and subsequently leads to photodegradation [33]. When the wood is exposed to UV radiation, it can undergo photo-degradation which mainly involves photoinduced breakdown of lignin to water soluble products as leads to the formation of free radicals species [34,35] which are responsible for woods color change. Fabiyi et al. [33] observed a strong correlation between composite lightness and wood lignin degradation. Another possible reason for color change of the composites is the removal of water soluble extractives by the rainfall that also has been claimed to impart color wood [23,27,36]. In contrast, the nano zinc oxide to be an important factor to reach a good protective efficiency against photo discoloration. Effect of artificial weathering on ΔE change of treated and untreated samples are shown in Figure 5.
Figure 4: Colors changes of wood polymer nanocomposite after artificial weathering test.
Figure 5: Colors changes of wood polymer nanocomposite after artificial weathering test.
The results showed that there were significant differences in the ΔE change of treated and untreated samples, as the ΔE change of treated samples were lower and visibly brighter then untreated control ones. As can be observed, the maximum and minimum color changes were found untreated and high concentrations treated samples, respectively. Therefore, the UV absorption ability of nano zinc oxide added in wood could be higher, which would explain this result. It can be used as a UV-shielding material because of its strong ultraviolet absorption [21,26,37]. The effect of UV-shielding ability of ZnO nanoparticle on PET was reported in the literature. It also should be added, the weathering is a surface phenomenon by UV radiation, moisture, and other factor such as temperature. Nano zinc oxide with high distribution on surface wood cause increasing of the contact surface and improving of the optical properties prevents UV radiation effect on the wood surface, lignin especially; and prevents the formation of free radicals. This properties, improve resistance against wood color change.


The following conclusions could be drawn from the results of the present study:
1- Presence of nano zinc oxide in wood polymer composite can reduce the lightness and total color change due to less lignin degradation and lack of color change after impregnation.
2- The decay resistance test showed that the wood polymer composite containing nano zinc oxide (0.5, 1 and 1.5 concentration) exhibited favorable activity against T. versicolor and C. puteana Consequently, nano zinc oxide can be enumerate as an important material in the formation of materials related to the protection of wood polymer composite and recommended use to wood polymer composite in the outdoor applications.


In this research, we appreciate the Engr. Hamid Reza Mehri Iraei, expert in pulp and paper industry for his great help in analyzing of data.


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