Journal of the Korean Wood Science and Technology
The Korean Society of Wood Science & Technology
Original Article

Effects of Hot-Air Heat Treatment on the Surface Color of Phyllostachys bambusoides Bamboo

Hyoung-Woo LEE2,, Eun-Ju LEE2
2College of Agriculture and Life Science, Chonnam National University, Gwangju 61186, Republic of Korea
Corresponding author: Hyoung-Woo LEE (e-mail: dryingeng@daum.net, ORCID: 0000-0001-6451-325X)

© The Korean Society of Wood Science & Technology.

Received: Aug 24, 2021; Accepted: Oct 16, 2021

Published Online: Nov 25, 2021

Abstract

We investigated color changes on the outer surfaces of Phyllostachys bambusoides bam-boo by heat treatment under three different temperatures (180℃, 200℃, and 220℃) for three different durations (60 min, 90 min, and 120 min). A method of predicting the bam-boo surface color change after heat treatment was developed to provide valuable information and increase the added value of domestic bamboo products. The three average color parameters L*, a*, and b* decreased, and the overall color changes increased as the severity factor increased. The values of L* × a* × b* were highly related to the severity factor, and the optimal duration time for the desired bamboo surface color with a certain heat-treatment temperature could be estimated.

Keywords: bamboo; Phyllostachys bambusoides; heat-treatment; hot-air; color change

1. INTRODUCTION

Bamboo has been widely used as the material for industries in construction, furniture, household goods, musical instruments and handicrafts (Zhang et al., 2018). Recently, Shin et al. (2018) developed bamboo/PLA bio-composites for 3D printer filament. Galih et al. (2020) evaluated the possibility of cross-laminated timber (CLT) with bamboo laminated board as the core layer. However, bamboo is lignocellulosic material and rich in sugar and starch, which leads to its high moisture absorption and poor biological durability. Therefore, enhancing the durability of bamboo is needed (Yuan et al., 2020).

There are mainly two methods of modifying the bamboo, chemical and physical methods. The chemical methods generally include the addition of fungicides and flame retardants, while the physical methods include various types of heat treatments. Qi et al. (2019) investigated the effects of a novel mold inhibitor specially for bamboo on the properties of bamboo fiber-based composites (BFBCs). Kang et al. (2017) developed a new drying technology in order to quickly and massively dry bamboo tubes without crack and check. They impregnated bamboo tubes with the solution of PEG-1000 before high-temperature drying.

The culm of bamboo is covered by its hard epidermis with a wax-coated skin which has a function of preventing moisture loss from the bamboo (Ariffin and Sukri, 2019). Chang et al. (2015) found that giant bamboo (Dendrocalamus giganteus Munro) culm waxes are composed of saturated and unsaturated hydrocarbons, including alkanes, alcohols, ketones, aldehydes, fatty acids, and minor aromatic compounds. Therefore, several results have revealed that bamboo is difficult to treat with preservatives (Kamarudin and Sugiyanto, 2012). Moreover, environmental concerns are encouraging the search for new methods of preservation of bio-materials such as wood and bamboo. Thermal treatment has been investigated since the middle of 20th century for the purpose of avoiding toxic effects of chemical treatment which involves chemical substances. Shin et al. (2004) investigated physical and mechanical properties of Phyllostachys pubescens bamboo treated with hot water (95℃) and microwave irradiation (700 W). Maulana et al. (2017) evaluated the effects of steam treatment on physical and mechanical properties of bamboo oriented strand board.

High-temperature treatment of bamboo changes its structure and chemical composition because of decomposition of hemicelluloses, ramification of lignin, crystallization of cellulose, and removal of certain extractives. Thus, bamboo becomes less hygroscopic and these changes decrease water re-absorption, improve the dimensional stability, and increase weathering durability. Meng et al. (2016) investigated the effect of heat treatment (180℃ and 200℃) on the chemical composition of bamboo slivers. Their results showed a decrease in the content of holocellulose, as well as an increase in the contents of lignin and extractives.

Color is an attribute of visual perception, which can be one of the important factors that influence customers when choosing bamboo and wood products. Color preferences survey conducted by Hidayat et al. (2017) revealed that their respondents expressed a stronger preference for the darker color of heat-treated woods than the original wood color. Thermal treatment is also a suitable method for color homogenization and colorization (Varga and Van Der Zee, 2008). Zhang et al. (2013) found that the color of bamboo was darkened and all three color parameters (L*a*b*) were significantly changed after the heat treatments at seven temperature levels (100 ~ 220℃) for four duration times (1 ~ 4 hours).

In this study heat treatment was performed under different temperatures and duration times to investigate the color changes on the outer surfaces of bamboo. To determine optimum conditions of heat treatment for the color desired, the method to predict the color change of bamboo surface after the heat treatment was developed. The results are expected to be able to provide valuable information for increasing the added value of domestic bamboo products.

2. MATERIALS and METHODS

2.1. Material preparation

The culms of Phyllostachys bambusoides bamboo used for this study were collected around Damyang-gun, Jeollanam-do, Republic of Korea. Bamboo species was verified according to the producer information. The bamboo culms were cut into 15 cm-long cylindrical specimens with no node on both ends. One hundred specimens were selected and the average circumference, thickness and moisture content of these specimens were 76.64 mm, 5.54 mm and 80.4%d.b., respectively.

These specimens were immersed into plastic tub filled with tap-water, where they remained for ten days. The tap-water was changed each day to avoid attacks from bacteria and the fermentation of the material. This water immersion was performed to extract starch from specimens, making them less susceptible to biological attacks and discoloration. The specimens extracted by water were exposed to the open air over seventeen days to dry. Average moisture content of air-dried specimens was 26.3%d.b..

2.2. Heat treatment procedure

All the samples were oven-dried at 105℃ before the heat treatment. Out of all one hundred specimens, thirty-six specimens with similar sizes and weights were selected for heat treatment test. The specimens were thermally treated at three temperature levels (180, 200 and 220℃) with three duration times (60, 90 and 120 min.) for each assay.

For the torrefaction of biomass, a severity factor (SF) is used to integrate the effects of reaction times and temperatures into a single variable (Lee et al., 2012). The SF used in this study is defined as the following equation:

S F   = log t - exp T H - T R 14 . 75
(1)

where SF is the severity factor, PIC8878 the heat treatment time in minutes, PIC8879 the heat treatment temperature in ℃, and PIC887A the reference temperature (= 100℃). The severity factors applied in this study were calculated as Table 1.

Table 1. Severity factors applied to the heat treatment of bamboo specimens
Temperature Heat treatment time
30 min. 60 min. 90 min. 120 min.
180℃ 3.83 4.13 4.31 4.43
200℃ 4.42 4.72 4.90 5.02
220℃ 5.01 5.31 5.49 5.61
Download Excel Table

Three specimens for each assay were put into the forced convection drying oven (HB-502M, Han Baek Scientific Co.). In the preliminary tests, it was recognized that rapid cooling after heat treatment induced severe cracks on bamboo. Bamboo undergoes the thermal expansion during the heat treatment. And rapid cooling of this heat treated bamboo induces rapid shrinkage and stress. Therefore, the stepwise heat treatment schedule was adapted to minimize these thermal stresses in specimens (Fig. 1). In the preheating stage oven temperature was set at 100℃ and a temperature of 100℃ was maintained for 30 min. Then the oven temperature was increased to 150℃ and was maintained at 150℃ for 30 min. In the heat treatment stage oven temperature was set at each heat treatment temperature and this temperature was maintained for each heat treatment duration time. After the heat treatment stage was finished, cooling stage was started. Oven temperature was lowered to 150℃ and 100℃ in sequence and each set temperature was maintained for 30 min., respectively. Finally, heater of oven was turned-off, while fan was running until the oven temperature reached room temperature.

wood-49-6-566-g1
Fig. 1. Sample of stepwise heat treatment schedule for bamboo.
Download Original Figure
2.3. Color measurements

The surface color changes of specimens due to drying and heat treatment were measured by a portable colorimeter (NR100, 3nh) fitted with a measuring aperture diameter of 8 mm. The CIE PIC888B, PIC888C and PIC888D color parameters (according to the international Commission on Illumination) of the specimens were obtained directly from the colorimeter and were used for color evaluation. Three positions on the outer surface of each specimen were selected and marked with a circle of diameter of 12 mm to measure the color changes at each identical position. The overall color changes (PIC888E) of the specimens were calculated according to the following equation:

E * = L * 2 + a * 2 + b * 2
(2)

where PIC888F is the overall color change and PIC88A0, PIC88A1 and PIC88A2 represent the changes in PIC88B2, PIC88B3 and PIC88B4 of specimens, respectively. A lower PIC88C5 means lower color change.

3. RESULTS and DISCUSSION

3.1. Color changes due to drying

Table 2 shows the surface color changes of bamboo due to air- and oven-drying. All the three color parameters, PIC88C6, PIC88C7 and PIC88C8, increased as bamboo dried. Dark green surface color of green bamboo turned into light green color after air-drying, and then into yellow color after oven-drying at 105℃. As bamboo dried, lightness (PIC88D8) increased gradually. Besides, the bamboo surface showed a tendency to turn reddish and yellowish by increases in chromaticity parameters PIC88D9 and PIC88DA. Color parameter PIC88DB was kept lower than 0 value after air-drying. This means green color of bamboo surface was maintained even after the air drying indoor. However, after oven-drying color parameter PIC88EC exceeded over 0 value, and the color of bamboo surface lost green color completely. Overall color changes PIC88ED of air-dried bamboo surface due to oven-drying was 13.62, which was much higher than that of green bamboo surface due to air-drying, 1.97. In the meantime wax layers on the bamboo outer surfaces melted and became sticky during oven-drying. After cooling these melted wax layers were hardened again and shiny outer surfaces appeared.

Table 2. Average color parameters of bamboo surfaces before and after air- and oven-drying
Color parameters PIC88EE PIC88FF PIC8900 PIC8901
Green 43.00 -5.70 17.50 -
Air-dried 44.70 -4.90 18.10  1.97
Oven-dried 45.80 8.10 22.00 13.62
Download Excel Table
3.2. Color changes due to heat treatment

Fig. 2 demonstrates that surface color of oven-dried bamboo gets darker as heat treatment temperature increases. All the average color parameters PIC8911, PIC8912 and PIC8913 decreased as heat treatment temperature increased, except at 180℃ for 60min. As expected, the average overall color changes PIC8914 increased as severity factor increased (Table 3). This results are in a good agreement with the findings of Lee et al. (2018), who observed that all the color parameters decreased as heat treatment temperature increased when they treated moso bamboo at 150 ~ 210℃ for 1 ~ 2 hours.

Table 3. Average color parameters of bamboo surfaces after heat treatments
Heat treatment conditions Color parameters
Temperature Time SF PIC8995 PIC89A6 PIC89A7 PIC89A8
180℃  60 min
 90 min
120 min
4.13
4.31
4.43
43.19
44.59
41.56
7.05
9.23
9.06
19.54
21.25
18.88
2.35
2.56
5.24
200℃  60 min
 90 min
120 min
4.72
4.90
5.02
32.65
30.25
29.31
7.60
5.44
4.95
 8.61
 5.11
 4.37
18.72
19.88
24.80
220℃  60 min
 90 min
120 min
5.31
5.49
5.61
31.26
28.70
27.66
6.38
4.27
1.96
 6.30
 2.93
 1.19
25.11
28.95
28.27
Download Excel Table
wood-49-6-566-g2
Fig. 2. Heat treated bamboo specimens.
Download Original Figure

According to the measured PIC8993 (lightness coordinate) values of the bamboo specimens, the heat treatment was decreasing the PIC8994 as the severity factor increased, indicating the bamboo has a tendency to change to darker colors due to the severer heat treatment. Schulz et al. (2021) suggested that the well-known darkening of the wood color due to thermal treatments is related to the thermal degradation in hemicellulose and cellulose, which normally reflect visible light with high wave numbers, leading to a perception of a light color.

The measured PIC89B9 (yellow/blue coordinate) values of the bamboo specimens implied that the heat treatment caused PIC89BA value reduction, indicating the surface color of the bamboo tended to change from yellow to blue color.

The results also indicate minor increase of the PIC89BB (red/green coordinate) values when the duration time increased at the heat treatment temperature of 180℃, but decreases occurred when the temperatures and duration times for heat treatment increased further. Zhang et al. (2021) presented the oxidation of phenolic compounds, the presence of reduced sugars and amino acids or the caramelization of holocelluse components as the main causes of color changes of bamboo with the increasing of the temperature.

Table 4 shows the average weight losses induced by heat treatment. At the heat treatment temperature of 180℃, weight loss rates did not exceed 1%. However, as the severity factor increased weight loss rates increased. Specimens lost 9.1% of oven-dry weight by heat treatment at 220℃ for 120min.

Table 4. Average weight loss rates of bamboo due to the heat treatment
Heat treatment conditions Weight loss
Temperature Time SF Weight loss (g) Weight loss rate (%)
180℃  60 min
 90 min
120 min
4.13
4.31
4.43
 0.3
 0.3
 0.7
0.3
0.2
0.6
200℃  60 min
 90 min
120 min
4.72
4.90
5.02
 2.8
 4.4
 6.4
2.4
3.7
5.9
220℃  60 min
 90 min
120 min
5.31
5.49
5.61
 4.3
 7.2
12.2
3.4
6.2
9.1
Download Excel Table
3.3. Pre-estimation of surface color after heat treatment

As mentioned previously, all the three color parameters PIC89DF, PIC89E0 and PIC89E1 decreased as the severity factor increased. Therefore, it is expected to pre-estimate the final surface color of bamboo after heat treatment with a certain severity factor. However, the integration of three color parameters into one parameter is needed to simplify the pre-estimation process. Fig. 3 shows the values of PIC89F1 × PIC89F2 × PIC89F3 were highly related to severity factor as the following regression equation (R2 = 0.9976).

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Fig. 3. Relationship between severity factor and the value of PIC89CC × PIC89CD × PIC89DE of the bamboo surface after heat treatment.
Download Original Figure
y = - 3499 x 4 + 54401 x 3 - 281947 x 2 + 487208 x + 1
(3)

where PIC8A2A means the value of PIC8A3B × PIC8A3C × PIC8A3D and PIC8A3E means severity factor, respectively.

It would be possible to pre-determine the optimal conditions of heat treatment for the desired bamboo surface color after heat treatment if that color could be pre-estimated. In this study the algorithm was developed to pre-determine the optimal duration time at a certain temperature for heat treatment as Fig. 4. Firstly, the desired values of the three color parameters PIC8A4F, PIC8A50 and PIC8A51 for the surface color of bamboo after heat treatment are determined respectively. Then, the desired value of PIC8A52 × PIC8A62 × PIC8A63 is calculated to estimate the severity factor for this color using equation (3). Finally, the optimal duration time are estimated using equation (1) after selecting the temperature for this heat treatment process. Table 5 shows the results estimated for the heat treatment temperature of 200℃.

Table 5. The optimal duration times estimated for the heat treatment at 200℃ according to the values of PIC8A15 × PIC8A16 × PIC8A17
PIC8A18 × PIC8A28 × PIC8A29 Severity factor Duration time (min.)
900 4.898  90
800 4.925  96
700 4.960 104
600 5.010 116
500 5.285 219
400 5.369 266
300 5.410 292
200 5.440 313
100 5.464 331
Download Excel Table
wood-49-6-566-g4
Fig. 4. The algorithm for pre-determining the optimal heat treatment condition for the desired bamboo surface color after heat treatment.
Download Original Figure

There were various contradictory reports on the resistance of weathering of heat-treated wood, and other reports proclaimed the necessity of painting with UV resistance (Kim, 2018). Therefore, more advanced studies are necessary to develope the methods for maintaining the color of heat-treated bamboo.

4. CONCLUSION

Heat treatment tests were conducted to investigate the effects of temperature (180, 200 and 220℃) and duration time (60, 90 and 120 min.) on the change of the surface color of Phyllostachys bambusoides bamboo. All the three average color parameters PIC8A64, PIC8A75 and PIC8A76 decreased and the average overall color changes PIC8A77 increased as the severity factor increased. Through the analysis of the results the algorithm was developed to estimate the optimal duration time for the desired bamboo surface color at a certain heat treatment temperature.

ACKNOWLEDGMENT

This study was carried out with the support of ‘R&D Program for Forest Science Technology (Project No. 2020262A00-2022-AC02)’ provided by Korea Forest Service (Korea Forestry Promotion Institute).

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