Effect of moisture content and frequency variation on dielectric properties of bamboo (Phyllostachys heterocycla cv. pubescens)

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EFFECT OF MOISTURE CONTENT AND FREQUENCY VARIATION 
ON DIELECTRIC PROPERTIES OF BAMBOO 
(Phyllostachys heterocycla cv. pubescens) 
Nguyen Thi Huong Giang1, Tran Van Chu2 
1,2Vietnam National University of Forestry 
SUMMARY 
Moisture content of bamboo and frequency are the most important factors that affects dielectric properties of 
bamboo material. Dielectric properties of bamboo is one of the most important factors to determine the high-
frequency hot pressing process parameters of glued laminated bamboo... Therefore, study on dielectric 
properties of bamboo has important significance. Bamboo was adjusted moisture content under laboratory 
conditions for 0-18%. Effect of moisture content and frequency variation on dielectric properties of bamboo 
was determined by using the 4294A Precision Impedance Analyzer with the 16451B. Dielectric properties 
including dielectric constant (e’) and dielectric loss tangent (tan d) have been done in the moisture content 
range from 0% to 18% and in the frequency range from 60 Hz to 6 MHz. The results showed that the dielectric 
constant (e’) and tan d increase with the increasing moisture content and decrease with the increasing 
frequency. Dielectric constant and tan d increased slowly with the moisture content below fiber saturation point 
(FSP), increased sharply with the moisture content around the FSP. Dielectric constant and tan d decreased 
obviously with the frequency below 6 kHz, but changed slowly when it above 6 kHz. 
Keywords: Bamboo, dielectric constant, dielectric loss tangent, frequency, moisture content. 
I. INTRODUCTION 
Bamboo is a natural material. It has been 
used traditionally as an engineering-structural 
material for fabrication of village houses in all 
stages of human culture development. In order 
to utilize bamboo effectively under modern 
scientific and technological conditions it is 
necessary to study its properties. Bamboo is a 
main material for bamboo-based panelsand a 
wide range of bamboo products, including 
bamboo articles for daily uses and bamboo 
carbon (Zhang, 1995; Zhang et al., 2001). 
Dielectric constant and dielectric loss 
tangent is important factor of the dielectric 
properties of bamboo. It has important 
implications in the high-frequency and 
microwave heating technology of bamboo 
processing applications. Applications of 
dielectric properties of bamboo and wood in 
high-frequency and microwave heating 
technology to determined drying, glueing, 
softening and moisture content of bamboo and 
wood (Yin, 1996). 
Electric properties of both wood and WPC 
were measured under different moisture 
contents and relative humidities. It showed that 
dielectric constant of wood increased 
significantly with moisture content but no 
significant difference was observed in the case 
of WPC within the range of moisture contents 
studied (Khan et al., 1991). 
Dielectric constant and tan d values of 
different sections of bamboo cut from outer 
skin to the central core have been determined 
at different temperature range and frequency 
range (Chand et al., 2006). It has been found 
that dielectric constant and tan d increased with 
increase of temperature and decreased with 
from the center core to periphery outer surface 
with increase of frequency. 
The estimation of dielectric loss factor 
which is considered a very important feature 
for bamboo industry and wood industry, 
properties of different wood species was done 
by using soft computing algorithms as a 
function of both ambient electro-thermal 
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conditions applied during drying of wood and 
basic wood chemistry (Iliadis et al., 2013). 
Dielectric constant and dielectric loss 
tangent of bamboo culm increased slowly with 
the moisture content below fiber saturation 
point (FSP), increased sharply with the 
moisture content around the FSP, and when 
above the FSP, it had a linear relation with the 
moisture content. Dielectric constant of grain 
direction was higher than that of other two 
directions. It decreased obviously with the 
increase of frequency, but changed slowly 
when it above 6 kHz. Bamboo culm age, 
different part of culm had no evident effect on 
dielectric constant (Xu et al., 2012). 
Bamboo or wood-like materials such as 
WPC can be used as an important insulating 
material for special applications. All untreated 
woods had a higher dielectric constant than 
their polymer composites. It is therefore 
postulated that the presence of polymers has 
led to a decrease in the number of polarizable 
units (Chia et al., 1986). 
Dielectric properties of wood block treated 
at various temperatures up to 800°C were 
measured in the range from 20Hz to 1MHz and 
from -150 - 20°C. These results suggested that 
the electric conductivity decreased with 
increasing temperature up to 400°C and a 
small volume fraction of particles with large 
conductivity is formed at microscopic levels in 
the cell walls (Sugimoto et al., 2004). 
At present, study on dielectric properties of 
wood quite widely. However, very little work 
has been done on the dielectric properties of 
bamboo. 
This study determined dielectric constant 
and dielectric loss factor of bamboo at 
different moisture contents and frequencies. 
The main purpose is to provide the dielectric 
properties of bamboo to determine the 
parameters of high frequency press technology. 
II. RESEARCH METHODOLOGY 
2.1. Materials 
The bamboo (Phyllostachys heterocycla cv. 
pubescens) trees [6 years old, diameter ranging 
from 7 to 12 cm] were collected from 
Zhejiang, China. Approximately, the same 
amount of bamboo semicircular fragments was 
cut from the bamboo stem to prepare flat-
rolled. Bamboo samples were cut from these 
bamboo strips with a diameter of 50 mm and 
thickness of 5 mm. Uniformity of test sample 
surfaces were polished by using a sanding 
paper. Total of test samples were 12 samples. 
2.2. Experimental methods 
2.1.2. Moisture adjustment 
Moisture adjustment was conducted in 
drying cabinet. Based on experimental 
requirements, all samples were put into drying 
cabinet and the use of thermostat humidity 
cabinet to adjust moisture content of bamboo 
samples. All samples were conditioned for 0% 
to 18% relative humidity to adjust. Moisture 
adjustment times were 3 times, every time was 
3 days. Moisture content adjustment 
parameters of bamboo samples in Table 1. 
Table 1. Moisture content adjustment parameters of Bamboo 
Moisture 
content 
(%) 
Adjustment parameters 
Time 1 Time 2 
Temperature (0C) Humidity (%) Temperature (0C) Salt solution 
0 100 2 0 ÷ 2 20 - 
6 35 40 20 KNO3 
12 35 78 20 NaCl 
18 35 98 20 MgCl2 
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The moisture content (MC) of the samples 
were calculated according to the following 
formula: MC (%) = [(m1-m0)/m0]×100, where 
m1 is the weight of the sample before drying, 
and m0 is the weight of the sample 
immediately after drying. 
2.1.2. Experimentalmethod 
Figure 1 displays the flow chart when using 
the 16451B for permittivity measurements. 
When using an impedance-measuring 
instrument to measure permittivity, the parallel 
plate method is usually employed. An 
overview of the parallel plate method is shown 
in Figure 2. 
The parallel plate method, also called the 
three terminal method in ASTM D150, 
involves sandwiching a thin sheet of material 
or liquid between two electrodes to form a 
capacitor. The measured capacitance is then 
used to calculate permittivity. In an actual test 
setup, two electrodes are configured with a test 
fixture sandwiching dielectric material. The 
impedance- measuring instrument would 
measure vector components of capacitance (C) 
and dissipation (D) and a software program 
would calculate permittivity and loss tangent. 
Figure 2. Parallel plate method 
2.1.3. Measurement of Dielectric 
The measurements of dielectric constant 
(e’) and tan (d) values of bamboo samples 
were made by using a Agilent 4294A Precision 
Impedance Analyze with the 16451B, in the 
moisture content range from 0% to 18% and 
frequency range from 60 Hz to 6 MHz. 
e’ was calculated by using the following 
equations: e’ = (ta×Cp)/(A×e0), where Cp (F) is 
equivalent parallel capacitance, ta (m) is 
average thickness of test sample, A (m2) is area 
of Guarded electrode, and e0 = 8.854×10
-12 
Compensate the 
residual impedance 
Set the measurement 
conditions 
Adjust the electrodes Compensation 
for adjustment 
Insert the material Cp-D measurement Calculate permittivity 
Prepare the 
dielectric material 
Attach the guarded 
electrode 
Connect the 16451B Cable length 
compensation 
Figure 1. Measurement procedure flow chart for the 16451B 
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[F/m]. Each sample had tested with 3 times. 
Value of e’ and tan d were averaged. 
III. RESULTS AND DISCUSSION 
3.1. Dielectric constant (e’) 
The change of dielectric constant as a 
function of moisture content at several 
frequencies for bamboo is shown in Figure 3. 
It is visible that dielectric constant of bamboo 
is directly related to treatment severity, which 
depends on the moisture content. e’ increased 
with increasing moisture content showing 
anomaly at the transition MC from 0% to 18%. 
e’ decreased with increasing frequency from 
60 Hz to 6 MHz. e’ increased with increasing 
severity of moisture content treatment. With 
the same moisture content condition, in 
general, e’ of treated bamboo sample decreased 
in the order of the frequencies from small to 
large. It is quite the reverse, with different 
moisture content conditions on the same 
bamboo sample, in general, e’ of treated 
bamboo sample increased in the order of the 
treatment moisture contents 
(0%<6%<12%<18%). Moisture content is the 
dominating factor over duration of adjusting in 
increasing e’. The same dielectric constant can 
be obtained at lower treatment frequency with 
lower moisture content or by using higher 
treatment frequency with higher moisture 
content. For example, with the same treatment 
time were nine days, dielectric constant of 
bamboo samples were about 6.0 0.5 when 
moisture content at 6% for 60Hz but only 
required 20% at 6 MHz. 
Dielectric constant of the bamboo in the dry 
state has lowest value (2.0) and has highest 
value 2.19 with different frequency. 
Dielectric constant of the bamboo at MC 
18% has the lowest value (6.68) with 
frequency at 6 MHz and it has the highest 
value (61.34) with frequency at 60 Hz. 
Figure 3. Variation of Dielectric constant e' for Bamboo at different moisture contents 
and frequencies 
Table 2 presents the two-way analysis of 
variance (ANOVA) results of the e’ of 
bamboo. Moisture content and frequency 
showed significant effects on dielectric 
constant, (P-value < 0.0001). In addition, these 
two factors showed significant interaction on 
the dielectric constant of bamboo. 
-
10.00
20.00
30.00
40.00
50.00
60.00
70.00
0 2 4 6 8 10 12 14 16 18 20
Moisture content (%)
D
ie
le
c
tr
ic
 c
o
n
s
ta
n
t 
e'
60Hz 600Hz 6KHz 60KHz 600KHz 6MHz
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Table 2. Two-Factor Without Replication results of dielectric constant of bamboo 
Source df F-value P-value 
f 5 42.70 < 0.0001 
MC 3 158.29 < 0.0001 
f×MC 15 13.66 < 0.0001 
f – Frequency. 
MC – Moisture content. 
f×MC – Interaction of frequency and moisture content. 
This increase of e’ is due to the increased 
mobility of water dipoles in bamboo. Water 
has OH molecules and OH of water acts as a 
dipole (Chand et al., 1994). These dipoles 
contribute to the e’ behaviour of the bamboo. 
The bound water content of bamboo gradually 
increased when the moisture content of 
bamboo increased, e’of water is relatively high 
( 81) (Liu et al., 2004), lead to e’ increases 
with increasing of water in bamboo. When 
moisture content of bamboo is lower than the 
fiber saturation point, the bound water of 
bamboo fibers has not been in a saturated state. 
Therefore, freedom degree of functional 
groups in bamboo molecules are quite small, 
kinetic energy of molecule is small that effect 
the electrical conductivity, the dielectric 
constant increases quite slowly. Dielectric 
constant decreased when moisture content is 
lower than 6% with frequency variation and 
which increased quickly when moisture 
content is larger than 12% with high frequency 
value (> 6 KHz). The moisture content of 
bamboo is near the fiber saturation point, the 
movement speed of molecules bamboo is 
faster, the electrical conductivity increased to 
make dielectric constant increased. At lower 
frequencies, because the water molecules's 
dipolar are absorbed, lead to e’ values in the 
bamboo is high. 
3.2. Dielectric loss tangent d 
The change of tan d value is shown in 
Figure 4. It is visible that dielectric loss 
tangent of bamboo was observed increasing 
with increasing moisture constant and 
decreasing with increasing frequency. Tan d 
decreased when moisture content is lower than 
6% and increased quickly when moisture 
content is larger than 12%. Tan d increased 
slowly with the moisture content below fiber 
saturation point (FSP), increased sharply with 
the moisture content around the FSP. Tan d 
decreased sharply at the low frequency (< 6 
KHz) and decreased slowly at the high 
frequency (> 6 KHz). 
Figure 4. Variation of Dielectric loss tangent d for Bamboo sample at different moisture contents 
and frequencies 
-
0.30
0.60
0.90
1.20
1.50
1.80
2.10
2.40
2.70
0 2 4 6 8 10 12 14 16 18 20
Moisture content (%)
D
ie
le
c
tr
ic
 l
o
s
s
 t
a
n
g
e
n
t 
d
60Hz 600Hz 6KHz
60KHz 600KHz 6MHz
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Table 3 presents the two-way analysis of 
variance (ANOVA) results of the tan d of 
bamboo. Moisture content and frequency 
showed significant effects on dielectric loss 
tangent (P-value < 0.0001<). In addition, these 
two factors showed significant interaction on 
the dielectric loss tangent of bamboo. 
Table 3. Two-Factor Without Replication results of dielectric loss tangent of bamboo 
Source df F-value P-value 
f 5 14.85 < 0.0001 
MC 3 37.60 < 0.0001 
f×MC 15 5.69 < 0.0001 
f – Frequency. 
MC – Moisture content. 
f×MC – Interaction of frequency and moisture content. 
This decrease of tan d is mainly due to the 
reduction of the hydroxyl group content in 
bamboo. At lower frequency, a section of 
water molecules and free radicals in molecular 
organization of bamboo moved and actived 
when the electric current changes, tan d 
decreased sharply. Water molecules and free 
radicals in molecular organization of bamboo 
moving speed to late to keep up with changing 
frequency, the number of actived free radicals 
are reduced, conduction of electric current 
inside bamboo decrease, tan d decreased 
slowly. The lossy dielectric can be represented 
by the circuit analog of a resistance in parallel 
with a capacitor minimizes (Goodman et al., 
1991). At higher frequencies, the capacitor 
offers low reactance minimizes the conduction 
losses in the resistor. Hence, value of dielectric 
loss decreases at the higher frequencies 
(Vijendra Lingwal et al., 2003; Shiraneet al., 
1954). The tan d decrease from at all 
frequencies. 
IV. CONCLUSIONS 
Dielectric properties that include dielectric 
constant (e’) and dielectric loss tangent (tan d) 
have been done in the moisture content range 
from 0% to 18% and in the frequency range 
from 60 Hz to 6 MHz. From the above results, 
we can give some conclusions: 
(1) Dielectric constant (e’) and tan d exist in 
bamboo. Low moisture content (MC < 6%) 
and high frequency variation (> 6 KHz) are 
less effective on dielectric properties, but they 
are very effective on dielectric properties a 
thigh moisture content (MC > 12%) and low 
frequency variation (<6 KHz). Dielectric 
constant was small when the bamboo in the dry 
state with different frequency value. Dielectric 
constant of the bamboo at MC 18% was lowest 
value (6.68) with frequency at 6 MHz and it 
was highest value (61.34) with frequency at 60 
Hz. Tan d decreased when moisture content is 
lower than 6% and increased quickly when 
moisture content is larger than 12%. 
(2) Dielectric constant (e’) and tan d 
increased with the increase of moisture content 
and decreased with the increase of frequency. 
Dielectric constant (e’) and tan d increased 
slowly with the moisture content below fiber 
saturation point (FSP) and they increased 
sharply with the moisture content around the 
FSP. 
(3) Dielectric constant (e’) and tan d 
changed obviously when the frequency is 
changing, and decreased with increasing 
frequency. At lower frequency, tan d decreased 
sharply. At higher frequency, tan d decreased 
slowly. Dielectric constant and tan d decreased 
obviously with the frequency below 6 KHz, but 
they changed slowly when it is above 6 KHz. 
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(1986). A preliminary study on the dielectric constant of 
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dependence of dielectric behaviour of sisal fibre. J. 
Mater. Sci. Lett, 13, 156-158. 
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T.G. (1991). In Ceramic Materials for electronics; 
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(1954). Dielectric properties and phase transitions of 
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ẢNH HƯỞNG CỦA ĐỘ ẨM VÀ TẦN SỐ ĐẾN ĐẶC TÍNH ĐIỆN MÔI 
CỦA TRE (Phyllostachys heterocycla cv. pubescens) 
Nguyễn Thị Hương Giang1, Trần Văn Chứ2 
1,2 Trường Đại học Lâm nghiệp 
TÓM TẮT 
Độ ẩm của tre và giá trị tần số là những nhân tố quan trọng nhất ảnh hưởng đến đặc tính điện môi của tre. Đặc 
tính điện môi lại là một trong những nhân tố quan trọng nhất dùng để xác định các thông số công nghệ của quá 
trình ép nhiệt cao tần ván ghép khối tre. Vì vậy, việc nghiên cứu đặc tính điện môi của tre có ý nghĩa vô cùng 
quan trọng... Trong bài viết này, độ ẩm của nguyên liệu tre được điều chỉnh từ 0 - 18% trong điều kiện phòng 
thí nghiệm. Sau đó sử dụng thiết bị 4294A kết nối với máy phân tích trở kháng 16451B để xác định ảnh hưởng 
của độ ẩm và tần số đến đặc tính điện môi của tre. Đặc tính điện môi bao gồm hằng số điện môi (e’) và góc tổn 
thất điện môi (tan d) được xác định trong phạm vi độ ẩm từ 0 - 18% và tần số từ 60 Hz - 6 MHz. Kết quả 
nghiên cứu cho thấy, hằng số điện môi (e’) và góc tổn thất điện môi (tan d) tăng khi độ ẩm của tre tăng và giảm 
khi tần số tăng. Hằng số điện môi (e’) và góc tổn thất điện môi (tan d) tăng chậm khi độ ẩm dưới điểm bão hòa 
thớ gỗ (FSP), tăng mạng khi độ ẩm tre gần với điểm bão hòa thớ gỗ FSP. Hằng số điện môi (e’) và góc tổn thất 
điện môi (tan d) không tăng rõ ràng khi tần số ở dưới 6 KHz, nhưng lại thay đổi chậm khi tần số trên 6 KHz. 
Từ khóa: Độ ẩm, góc tổn thất điện môi, hằng số điện môi, tần số, Tre. 
Received : 05/8/2017 
Revised : 24/9/2017 
Accepted : 05/10/2017