Abstract

Wood resource availability in China is limited, whereas Chinese wood consumption continues to rise. Since the 1980s, main wood supply countries such as Australia and South Africa began to implement forestry protection measures. Resource availability became even more tense after the State Forestry Administration of China initiated the timber protection policy in 1998. One study forecasted that China would have one of the largest plantation forest areas in the world (Long 2012). Indeed, fast-growing species, such as Chinese fir (Cunninghamia lanceolata (Lamb.) Hook.), eucalyptus, aspen, and pine, among others, have been extensively planted in the southern regions of China, mainly Hainan, Guangdong, and Guangxi, in the early 1970s. The plantation area has reached over 7,000 km² (Hisada 2001) in total and hosts high-quality trees such as beech and mahogany (Chomcharm and Skaar 1983). However, these fast-growing trees present relatively poor wood mechanical properties, such as hardness or elasticity, and a lack of decorative patterns (Rhatigan 2003). Therefore, the wood derived from these trees cannot meet industrial requirements and would not perform satisfactorily during service.

Many methods such as chemical or physical modification have been developed to improve the mechanical properties, dimensional stability, and durability of wood (Fadl and Basta 2005, Nicholson and Hoffman 2006, Park and Wilderman 2010, Avramidis et al. 2011). Impregnating wood with urea formaldehyde (UF) resin is a traditional chemical modification of wood that can increase the strength and dimensional stability of wood (Mdiftekhar et al. 2004). However, impregnated lumber from trees that are quickly grown results in tissue with large built-up stresses and a higher proportion of juvenile wood. These wood-quality issues result in poorer dimensional stability and longer drying times (Doe and Lee 2000) Large-scale commercial application of lesser-used species like Chinese fir can be prohibited if the proportion of juvenile wood is high, coupled with lower density. During the drying process, shrinkage differences in both the tangential and radial directions will increase checking and downgrading (Zhou and Li 2004).

Wood drying is one of the most important steps in solid wood processing. To the best of our knowledge, the drying behavior of Chinese fir wood modified by UF resin has not been extensively studied. Thus, the drying characteristics of Chinese fir wood modified by UF resin were determined in the present study to provide a useful method for enhancing the drying properties of low-quality wood and adding value to Chinese fir.

Materials and Methods

Methods

The drying characteristics of untreated and UF-modified wood were characterized by assessment of the classical 100°C-test method, which is a fast and simple method to determine the drying schedule of wood (Tsing 1965, Wang 2009). Before drying, the dimensions of the sample were measured (Fig. 1a) and the initial characteristics of all the samples were recorded, including the initial mass and dimensions. All the samples were placed in an oven operating at a temperature of 100°C ± 2°C. The mass, initial checks and internal checks, and dimensions of each sample were recorded in the drying process (Fig. 1b). The drying experiment ended once the mass remained constant. Finally, the samples were removed from the oven and the final mass, dimensions, deformations, and warp were measured and recorded (Fig. 1c) following the methodology described in the literature (Tsing 1965, He 1998).

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To observe the penetration depth and uniformity of the resin impregnation, the pit structure on the radial cell wall was evaluated using scanning electron microscopy (SEM) and fluorescence microscopy. The dried samples were cut into small specimens by using a sliding microtome (Leika, ESM-100L) to obtain a size of 10 by 10 by 5 mm (longitudinal by radial by tangential) for the SEM observations. The specimens were mounted on aluminum stubs, sputter coated, and analyzed on a microscope (Questar, 450 Quanta^TM) operating at an accelerating voltage of 15 kV. The fluorescence microscopy experiments were performed on 20-μm-thick slices, which were softened by soaking in glycerol for 10 days and then dehydrated by using a gradient concentration of ethanol (30%, 50%, 75%, 95%, and 100%). The samples were then dyed with toluidine blue (Avramidis and Siau 1987, Yu et al. 2002) for 15 minutes. A fluorescence microscope (Olympus BX51, Japan) was used to observe the samples and the micrographs were analyzed with the Image-Pro Plus 6.0 software. The area of the chromogenic reaction (i.e., voids filled with UF resin) was determined by counting the total pixels of the image of the sample after impregnation compared with the ones of the untreated sample.

Results and Discussion

The characteristics of the wood samples, including the initial dimensions, internal checks, cross-section deformation, warp, and drying rate, were monitored before, during, and after the drying process to estimate the preliminary drying schedules of the treated and untreated wood samples. The drying defects and drying rates were graded and are listed in Table 1. All samples developed light end checks early in the drying process, whereas the end checks developed throughout the drying period. Some surface checks were also detected after drying.

Table 1 Grades of the drying defects and velocities of the urea formaldehyde (UF)–treated and untreated Chinese fir wood.

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The grades for the initial check, internal check, cross-section deformation, warp, and drying rate of the modified wood were 2, 2, 3, 2, and 2, respectively, which means relatively medium rank in the grade standard. The impregnated and untreated samples presented identical grades for the cross-section deformation and warp parameters. However, the impregnated wood resulted in lower grades for the initial check and dried faster compared with the untreated sample that served as the control experiment. The grade of the internal check was lower for the treated wood than for the untreated sample. A slower drying rate was observed in the later stage for the untreated wood. The drying had less impact on the impregnated wood as shown by the lower grade of the initial check than on the untreated wood. This observation could result from the curing of the resin on the surface, which helped to inhibit wood from cracking, although internal checks appeared later in the drying process. According to the above-described observations we assumed that the treatment with UF resin consolidated the Chinese fir as the treatment affected the drying velocity.

Impregnated Chinese fir exhibited the fastest initial drying rate, which decreased in the later stage of drying. The experiment was repeated to establish the relation between MC and drying time. The recorded data are plotted in Figure 2. When the impregnated wood samples were dried after 0.5, 1.0, 1.5, 2.5, and 3.5 hours, the calculated moisture contents were 49.8, 41.2, 33.3, 27.5, and 22.4 percent, respectively. The drying curve of the impregnated wood decreased significantly after 3.5 hours of drying at high MC, followed by a decelerated decline of the MC. However, the treated sample dried more slowly than the untreated sample. At the initial drying stage, water contained within the wood structure evaporated more easily in the untreated wood than in the case of treated wood (Long 2012). Therefore, it was predicted that the drying speed accelerated at relatively high MCs. When the internal water passages were blocked because of the impregnation of the UF resin, the water evaporation was limited, and the drying speed was decreased when the MC fell below 15.3 percent.

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To evaluate the penetration and distribution of the UF resin within the wood structure, the samples were analyzed by SEM and fluorescence microscopy (Bolton et al. 1988). The SEM images of the radial wall of both the impregnated and untreated wood are displayed in Figure 3. The majority of the lumens, voids, and even microvoids within the wood structure were filled with the UF resin, confirming the efficiency of the pressure impregnation in the exchange of water molecules by UF resin. Moreover, the UF resin penetrated the tracheids and canals within the structure, wood fiber lumens, and intercellular spaces. The SEM results confirmed a uniform penetration of the UF resin within the wood structure, which lowered the drying rate.

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Treatment with toluidine blue inhibited the autofluorescence of wood and highlighted the UF resin fluorescence (Kamke and Lee 2007). Therefore, the penetration and the distribution of the resin inside the wood structure were characterized by a fluorescent color reaction (Gindl et al. 2002) between toluidine blue and the UF resin. The impregnated specimens characterized by monochromatic color are shown in Figure 4. The area of the chromogenic reaction was analyzed quantitatively by using the Image-Pro Plus 6.0 software and accounted for 67.8 percent of the total area. The results showed that the UF resin was distributed evenly in the central locales, indicating that the wood subjected to the UF impregnation possessed relatively higher permeability compared with untreated samples. An uneven luminescence and lack of color were observed in some parts within the wood and could be explained by the quenching effect of the fluorescent dye or the variability of the wood structure itself. The findings further validated that the consolidation of the wood structure by the UF resin contributed to the low drying speed at a later drying stage.

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The wood drying conditions depended on the degree of the drying defects during the 100°C-test and could be used for development of drying schedules (Christian and Anders 1997). According to the protocol of the kiln-drying swan timber (Standardization Administration of China [SAC] 2012a), the checking grades correlated with practical conditions (humidity and temperature) and procedures during kiln drying. The drying conditions could be predicted on the basis of the attributed grades of the drying defects of the UF resin–impregnated wood. The suggested drying schedules for the UF resin–impregnated wood with a thickness of 25 mm are listed in Table 2.

Table 2 Suggested drying schedule for the urea formaldehyde resin–impregnated Chinese fir.^a

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The predicted values of the drying schedule of the UF resin-impregnated wood were the following: an initial drying temperature of 60°C, a dry-bulb and wet-bulb temperature difference set to 3°C, and a final drying temperature of 80°C. The predicted values for the untreated wood were an initial drying temperature of 60°C, a dry-bulb and wet-bulb temperature difference of 3°C, and a final drying temperature of 85°C. According to the results, a mild drying schedule should be adopted and the drying temperature should not exceed 50°C. In addition, the relative humidity should be high at the early stage of drying to minimize surface checking. The usual temperature of the wet-bulb temperature depression was nearly zero at the early stage of the drying (GalPerin 1995). During the period in which the MC decreased to 35 percent from the initial value, the drying temperature should increase by 5°C to 7°C to ensure the temperature uniformity. In contrast, the temperature depression should increase by 20 to 50 percent when the MC decreased to 3 to 5 percent. As the drying process continues, the drying temperature should keep increasing, although the maximum wet-bulb temperature depression should not exceed 25°C to 30°C (He and Lin 1990). We applied the predicted drying schedule to verify the process experimentally using a 2-m³ kiln dryer. Finally, the drying characteristics (warp, internal checks and cross-section deformations, and drying velocity) of the UF resin–impregnated wood were in good agreement with the drying characteristics recorded when using the sawn timber (SAC 2012b). In the practical drying process, a moisture detector would be inserted inside the timber to continuously record the moisture changes, providing the evolution of the drying stage. Therefore, this drying schedule is moisture-based.

Conclusions

• The grades of the initial check, internal check, cross-section deformation, warp, and velocity of the UF resin–modified Chinese fir wood were 2, 2, 3, 2, and 2, respectively, which means relatively medium rank in the grade standard. The UF resin–impregnated and untreated (as control sample) wood exhibited identical grades for the cross-section deformation and warp. Compared with the untreated sample that served as the control experiment, the UF resin–impregnated wood presented fewer initial checks and faster initial drying velocity. Yet more internal checks and slower drying velocities were observed at later stages of drying.

• The average drying velocity of the UF resin–impregnated Chinese fir wood was 38.9 percent lower than that of the untreated sample. This observation could be caused by the consolidation of the wood structure due to the deep penetration of the UF resin as illustrated in the SEM and fluorescence micrographs. Therefore, the internal passages were hindered, prohibiting the water evaporation.