Evaluation of Drying and Anatomical Characteristic of Mongolian
Oak Lumber by Kiln Drying due to storage time after sawing
Yoon-Seong Chang*, Min-Ji Kim, Hyun-Kyeong Shin
and Yeonjung Han
* Corresponding author: Yoon-Seong Chang ; E-mail: jang646@korea.kr
Dept. of Forest Products National Institute of Forest Science
Seoul, 02455, Republic of Korea ABSTRACT
Based on the 2017 statistics of the Korea Forest Service, the amount of annual production of domestic roundwood was 4.5 million ㎥. Twenty eight percent of the production was hardwood roundwood. About 80% of the hardwood was Quercus species. The hardwood roundwoods were mainly used for wood chips (50%) and medium density fiberboard (MDF)(26%). Only 6% of the hardwood roundwoods were used for sawn timber. Most of oak forests in Korea were not managed well. Therefore, diameter of oak trees was relatively small with lots of flaws, such as inside decay, discoloration etc., since many of them were reproduced by sprout forest. Oak trees produced from unmanaged forests are prone to drying defects. To overcome this proneness, drying and anatomical characteristic of Mongolian Oak (Quercus mongolica) lumber (thickness 30 ㎜ x width 100 ㎜ x length 1,900 ㎜) were investigated in this study. Shrinkage and color difference (E) due to kiln dry was measured. In order to evaluate drying defects, the degree of surface check was investigated. Warp of lumber, such as cup, bow, crook, and twist, were also measured after drying process. Tylosis formation between one month and six months after sawing was observed microscopically. In order to develop value-added products from domestic oaks, the yield rate from standing tree to the flooring board were calculated. It is expected that efficient production plan for the value added products from domestic oak species would be proposed based on the results of this study, which are not being used as commercial sawn timber in Korea currently.
1.INTRODUCTION
Along with the pine trees, oak trees are widely distributed on the Korean Peninsula, and most of them are natural or secondary forest. Based on the 2017 statistics of the Korea Forest Service, the area of Oak and mixed forest accounted for about 3.7million ha(61%). However, compared to management and utilization of coniferous forests, it is insufficient to research on those of oak and other broadleaved trees. Oak wood was used for various purposes in low-value utilization such as bed log, wood chip, and firewood in Republic of Korea. Therefore, it is urgent to develop utilization technology for enhancing the value-added domestic oak wood product.
Also, Korean oak trees grow slowly in a poorly managed forest condition unlike European and American oak tree. So the diameter of oak trees was small with lots of flaws (discoloration, insect damage etc.) since many of them were reproduced by sprout forest. Therefore, they are prone to drying defects. To use wood as a material, moisture content (MC) must be reduced through drying process. The common method of drying timber to 12% MC is conventional kiln drying (KD). To reduce drying defects and improve drying speed, pre-surfacing and pre-steaming treatment were applied to KD system. Domestic oak timber with pre-surfacing treatment had increased the drying speed by 7.4% and the drying defects were remarkably decreased compared with untreated timber. However, pre-steaming treatment had no significant difference from untreated timber(Han and Jung 1986; Kang 1992). Northern red oak under microscopic investigation found occasional cracks in the cell walls, and a reduced fiber lumen size in the steamed samples. The reduced lumen size was attributed to a disruption in the warty layer by steaming, resulting in increased swelling of the fiber walls(Kubinsky 1971).
Discoloration after logging can occur during the storage of logs and green timber or during drying(Simpson, 1991). Shortly after felling, the logs may be discolored due to physiological and biochemical activities similar to those in the standing tree, when the surfaces are exposed to oxygen (Bauch, 1984). Since time, temperature and humid storage conditions contribute to this development, rapid sawing and drying are recommended (Simpson, 1991). The objective
of this study is to assess drying and anatomical characteristic characteristics of domestic hardwood timber, especially oak species, dried by KD system due to storage time after sawing.
2.MATERIALS AND METHODS
2.1.PREPARATION OF SAMPLES
The Oak(Quercus mongolica) wood used for manufacturing flooring was harvested at Pyeongchang-gun, Gangwon-do Province, Republic of Korea. 12 oak logs were harvested and transported to a local sawmill where the logs were divided into two groups. The first group was sawn 23 days after harvesting, while the second group was stored on the log yard for another 168 days prior to sawing. The logs were stored at the low temperatures during winter and early spring, average temperature 5.8℃ and RH 58%. The volume of the logs was calculated according to KS F 2163. The below equation was used for calculating the volume of logs.
2
10
6V D L
(1)where, V = volume of log (m3); D = (small) end diameter of log (mm); L = length of log (m)
Figure 1: The mongolian oak log for sawing.
The volume of oak wood which was used in this study was 5.7m3. From twelve logs, 1.4 m3 green boards (30 × 100
× 1,900 mm3) were prepared for Kiln drying. Samples were end-coated with urethane paint in order to minimize the
effect of end drying. Average oven-dry density and initial moisture content were 0.79 (± 0.03) g/cm3, 73.9 (± 7.1) %,
respectively.
2.2. KILN DRYING AND DRYING PROPERTY
Drying was conducted in a 0.7 m3 capacity experimental-scale conventional kiln drying(After 1 and 6 months). Air
flow through the stack was provided by the fan located at one side of the kiln with air velocity of 1.5 m/s. The samples were stacked on 20 mm by 20 mm stickers. The stacks were restrained by placing concrete block on top of the stack to minimize warping. Drying condition was set according to Table 1.
Table 1: Kiln drying schedule(modified T11-B3)
Moisture Step
(%) Dry-bulb Temperature(℃) Wet-bulb humidity(%) Relative Moisture Content(%) Equilibrium
Above 35 65 62 83 14.0
35 to 30 65 54 57 8.0
35 to 30 71 57 51 6.8
20 to 30 71 54 43 5.8
Below 15 76 57 39 5.1
Conditioning 82 76 78 10.8
Shrinkage of boards in width and thickness was determined by measuring its dimension before and after drying. Two measuring baselines were drawn along R and T directions at cross section to measure the length of the same position by digital calliper(CD-20APX, Mitutoyo).
Figure 2: digital caliper (CD-20APX, Mitutoyo)
Color difference(E) before and after drying was measured. The spectrum of reflected light was measured in the visible region (360–740 nm) with a portable spectrophotometer (JP/CM-600d, Konica Minolta). CIEL*a*b* color scale, where L* stands for lightness, a* stands for redness, and b* stands for yellowness, was used to quantify the color changes. In order to evaluate dry defects, the degree of surface check was investigated. Warp of board such as cup, bow, crook, and twist were also measured after drying. The residual drying stress was evaluated by prong test.
2.3. ANATOMICAL PROPERTY
In order to find out why drying was performed smoothly with relatively strong drying condition, the anatomical characteristics according to the storage time (from 1 to 6 months) were conducted. To observe the radial section of the test material, a specimen of 1×1×1cm3 was prepared. glycerin and distilled water were added with a solution made from
1:3 to 4 ratio with a heating mantle, and it was sliced 15 to 20μm thick by using microtome. Samples dyed 1% safranin solution and dehydrated with an alcohol (30, 60, 90, 99.7%). The slice was made of permanent preparation, which was observed by using optical microscope (axio imager M2, Carl zeiss, Germany).
3.RESULTS AND DISCUSSION
3.1. DRYING PROPERTY
Figure 2 shows the drying curve of Kiln drying at one month after harvesting(O) and six months(S) after sawing. The difference in drying speed over time was compared by applying improved drying schedule(T11-B3). After 148 hours of KD at O, average moisture content(AMC) of O was 70%(±6.8), and final moisture content(FMC) reached 9.3%(±1.2). After 103 hours of KD at S, the AMC was 30%, and the FMC reached 9.7%(±1.2). Comparing the drying rate at the same moisture content interval (From 30 to 10%), it was assessed that the drying rate was 0.23%/hr at O and 0.21%/hr at S, which was about 10% slowly.
Figure 4: Drying curve of oak board kiln-dried at storage time
The shrinkage rates of dried oak board in width (tangential direction) and thickness (radial direction) by KD was 6.8%(±1.3) and 4.3%(±0.9), respectively. The amount of cupping, bowing and crooking were negligibly low, and no twisting was detected. It might be placing a concrete loading block on top of the board stack during KD. The difference of color between before and after drying of the KD dried oak board was the greatest (Figure 10). The color difference (ΔE) of KD before and after drying was 8.8 (±1.2). It was deceased in lightness and increased in redness and yellowness. Discoloration of wood during drying may result in loss of quality and value of the oak board. The discoloration reduces the quality of dried board if the color of the dried wood is not within the range that is considered to be natural for the wood species. In order to produce lighter-colored wood products, the initial drying temperature and the drying schedule should be improved for enhancing quality and value of the oak board.
Figure 5: Drying stresses measured by prong test of KD board by prong test((a): 72hr, (b): after conditioning)
The sawing, drying, finishing yields were 28.8 %, 26.1 % and 7.1 %, respectively. The size of final product(flooring) is 22 ㎜(thickness) x 85 ㎜(width) x 1,100 ㎜(length). The cumulative yield was 7.1 %, which means 92.9 % of volume of log was not used for manufacturing flooring. Therefore, it is expected that the use of high value-added products made of domestic oak species such as flooring and furniture could be commercialized if technology development for high production yield with kiln drying and reducing final product size are applied to enhance economic point of view.
Figure 6: The prodution processing for oak flooring. 3.2. ATOMINICAL PROPERTY
A number of tylosis were sharply observed in radial sections from three months after sawing. Tylosis of sapwood is almost same level. Tylosis of heartwood and transition(boundary layer between sapwood and heartwood) are increased during storage time.
Figure 7: Change of tylosis formation between one month and six months after sawing
Compared to the previous results of drying, it is thought to be due to prompt drying before the formation of tylosis within one month, which did not produce additional tylosis after sawing. Therefore, it is deemed that drying work can be performed within a short period of time after logging to control the associated dryness. It was believed that domestic oak trees in korea can be used for high value-added purposes with low drying defects if they are properly managed and processed.
4.CONCLUSION
Even though adopting modified drying schedule, there were no observation of obvious wood defects. It is expected that the modified schedule can reduce drying time of hardwoods. In further studies, the improved schedule for hardwood drying by KD will be suggested. The results applied that it is possible to include a domestic species in the traditional range of wood flooring products. It is expected to expand the utilization of domestic oak trees based on the
appropriate final product size for enhancing production yield and development of drying and anatomical research results in the future.
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