Thermal Conductivity of Ramie Fiber Drawn in Water in
Low Temperature
Atsuhiko Yamanaka,1 Shunzo Abe,1 Masayuki Tsutsumi,1 Tooru Kitagawa,1 Hiroyuki Fujishiro,2Kimiko Ema,3 Yoshinobu Izumi,3 Shigehiro Nishijima3
1

Research Center, Toyobo, Co., Ltd., Katata, Ohtsu, Shiga 520-0292, Japan
Department of Materials Science and Technology, Faculty of Engineering, Iwate University, Morioka 020-8551, Japan3
Department of Nuclear Engineering, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan
2

Received 22 March 2005; accepted 30 September 2005
DOI 10.1002/app.23675
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: To understand the effect of extension of molecular chain in amorphous region in polymer ﬁbers tothermal conductivity, the thermal conductivity, tensile modulus and crystal orientation angle of ramie ﬁbers and thosedrawn by the stress of 17.4 kg/mm2 (water treatment) in thewater were investigated. The tensile modulus of ramie ﬁber
in ﬁber direction increased from 61 to 130 GPa by drawing
in the water. The crystal orientation angles of ramie ﬁber
with and without water treatment were measured by X-ray
diffraction. The orientation degrees of ramie ﬁbers without
and with water treatment were estimated as 92.9 and 93.6%,
respectively. Therefore, the tensile modulus increases twotimes as that of blank ramie by water treatment although
crystal orientation angle does not change distinctly. The
increasing of tensile modulus of ramie ﬁber by water treat-

INTRODUCTION
In applications of polymeric materials, the thermal
conductivity has been an important property, for example, for a cool/warm sensation for clothing ﬁber orwood products, or in the thermal insulation of plastics, including styro-forms.1– 4 With the recent development of superconducting and electronic engineering technologies, the thermal conductivity of structural and insulating materials used as composites incryogenic and heat-releasing materials in electricalequipment has become more important. Furthermore,
the desired features vary, depending upon the application, from insulation for use in cryostat5 to highthermal conductivity for use in super conducting
coils6 and electronic engineering.7 Investigation of thefactors that affect the thermal conductivity of polymeric materials is essential to enable the formation oftechnologies for designing polymeric materials thatpossess the required thermal conductivity.
Correspondence to: A. Yamanaka (Atsuhiko_Yamanaka@
kt.toyobo.co.jp).
Journal of Applied Polymer Science, Vol. 100, 2196 –2202 (2006)© 2006 Wiley Periodicals, Inc.

ment was explained by extension of the molecular chains in
the amorphous region. Thermal conductivities of ramie ﬁbers with and without water treatment were measured in
the ﬁber direction in the temperature range from 10 to 150 K.
Thermal conductivity of ramie ﬁber in the ﬁber direction
increased by water treatment. The increasing ratio of thermal conductivity by water treatment agreed to that of soundvelocity induced by increasing tensile modulus. Those results suggest that thermal conductivity of polymer ﬁber
increase by the extension of molecular chains in the amorphous region. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100:2196 –2202, 2006

Key words: thermal conductivity; ramie; sound velocity;drawing in water; amorphous

From previous studies of polymeric materials, it iswell-known that the thermal conductivity of amorphous polymers is smaller than that of metals andsemiconductors.8,9 Therefore, these have principally
been used as heat insulators. The thermal conductivityof electrical insulators is considered to be attributableto phonons,10,11 and the heat in polymers is conductedin the direction of covalently bonded molecular
chains, whereas conduction in the direction to intermolecular chains bonded by van der Waals forces ismuch less. It is considered that thermal conduction in
amorphous polymer is largely contributed by the intermolecular-chains conduction. On the other hand,thermal conductivity of extended molecular chain is
expected to be high in chain axis. In fact, it has beenreported that polymeric crystals possess a high-thermal conductivity in the direction in which the molecular chains are covalently bonded regularly, polyethylene crystals being an example.10 –16 Therefore, thethermal conductivity depends on the crystallization,orientation, crystal size, and defects in the polymer
crystal material. Thus, high-crystallization and highorientation polymers exhibit high-thermal conductivity.11–16 For example, highly crystallized polymer materials including high-strength polyethylene ﬁber11,17,18,19 and high-strength polypara-phenylene-

THERMAL CONDUCTIVITY OF RAMIE FIBER DRAWN IN WATER

benzo-bisoxazole (PBO) ﬁber17 are known to possess
high-thermal conductivity similar to that of metals.
Therefore, the thermal conductivity depends on the
crystal structure or morphology, and it is expected to
be possible to control the thermal conductivity.
Ramie ﬁber is one of the natural cellulose ﬁbers
used as the ﬁber material for clothes as like cotton. It
is known that the tensile modulus of ramie ﬁber increases by drawing in the water (hereinafter water
treatment).20 It is reported that the tensile modulus isincreased by the increasing of orientation degree ofmolecular chains in the amorphous region of ramieﬁber by water treatment.20 It is also known that thetensile modulus of cotton ﬁber increases by drawing
in the water.20 But, crystal orientation degree also
increases by drawing in the water in the case of cotton.20 It is considered that hydrogen bonding interchain is cut by water molecules invading into cellulose
ﬁber.20 Thus, the cellulose molecular chains are expected to become easy to move.20 In this state, thecellulose molecular chain is considered to be extended
easily by drawing force.20 This increasing of tensile
modulus by water treatment is inferred to be causedby orientation of molecular chain by extension in thewater.20 In this study, we investigate the thermal conductivity of ramie ﬁbers with water treatment in theﬁber direction to effect of orientation of molecular
chains in amorphous region on thermal conductivity.

2197

the extension of the molecular chain by water treatment. And, the crystal orientation angles were measured by X-ray diffraction for estimation of contribution of crystal region to molecular orientation. After
those measurements of orientation of molecular chain
by water treatment, the thermal conductivities oframie ﬁbers with and without water treatment were
measured. In this section, the measurements are described.
Tensile modulus
The tensile test of a single ﬁber was carried out on
small tensilon TOYO SOKKI UTM-II. The span length
was 10 mm, and the cross head speed was 10 mm/
min.
Orientation angle
Orientation angle of crystallic phase to ﬁber axis wasestimated by X-ray diffraction. The samples were prepared in the form of a bundle of about 30 ﬁlaments inparallel, and they were ﬁxed at the both ends. Rigaku
RU-200 (40 kV ⫻ 100 mA) was used with X-ray diffraction on Ni-ﬁltered CuK␣ (␭ ⫽ 0.1548 nm). The
intensity distribution of (002) diffraction spot wasused for estimating orientation angles.
Thermal conductivity

EXPERIMENTAL
SampleIn this work, thermal conductivities of ramie ﬁbers
with and without water treatment were measured in
the range of low temperature. The sample preparations were described in the following.
Ramie fiber
The ramie ﬁber samples used in this work were madein China. The ramie ﬁbers were scoured and bleached.
The speciﬁc gravity and ﬁneness of ramie ﬁbers were
1.5 and 4.3 dtex, respectively.
Ramie fiber drawn in water (water treatment)
Ramie ﬁbers described earlier were dipped into thewater and were weighed with 5 g by a single ﬁlament(17.4 kg/mm2) in water for 12 h. They were dried bynaturally with weighing for 24 h after taking out fromwater with weighing.
Measurements
First, the tensile modulus of the ramie ﬁbers with andwithout water treatment were measured to certiﬁcate

Thermal conductivity (␬) was measured by a steadystate heat ﬂow method.17,18,21 The measurements of ␬
were carried out on the automated measuring systemwith thermal controller of a Gifford-MacMahon (GM)cycle helium refrigerator as a cryostat.21 The ﬁbersamples were prepared by bundling about 6000 monoﬁlaments with length of 25 mm. Both ends of the ﬁber
bundle were ﬁxed by adhesive STYCAST GT. One endof the bundle was attached to the cold stage of a GM
refrigerator by mechanical pressing, using indium
metal and the adhesive STYCAST GT. A small resistance heater (1 k⍀) was adhered to the other end of thebundle using GE7031varnish. The intermediate positions where thermocouples were contacted werebound by ﬁne (0.1 mm) Cu wire. Au (Fe: 0.07%)chromel thermocouples were used as thermometers,
which were adhered by GE7031 varnish. The sample
space was evacuated to below 10⫺3 Pa by an oildiffusion pump for heat insulation. The automatedmeasuring system of ␬ was operated in the temperature range from 10 to 150 K. The measurements werestarted after keeping the sample space in high vacuum
below 10⫺3 Pa for 24 h to dry up the sample. The ␬ wasestimated by the relation ␬(mw/cm K) ⫽ (Q/⌬T)(L/S), where Q is the heat ﬂow per second, ⌬T the temperature difference between thermometers, L the distance between the thermometers, and S the cross sec-

2198

YAMANAKA ET AL.

water.20 From those results, the tensile modulus oframie ﬁber increases to twice by water treatment, andit suggests the extension of molecular chains of ramieﬁbers.
To investigate the contribution of crystal orientation
to molecular orientation by water treatment, crystal
orientation angles and degrees of ramie ﬁbers withand without water treatment are measured by X-ray
diffraction in the following section.
Orientation angle

Figure 1 Stress–strain curves of ramie ﬁbers. [Color ﬁgure
can be viewed in the online issue, which is available atwww.interscience.wiley.com.]tion of the bundle. Details of the determination of ␬
are described elsewhere.17,18
RESULTS AND DISCUSSION
Tensile modulus
The stress–strain (s–s) curves of ramie ﬁbers with andwithout water treatment observed by the tensile test
are shown in Figure 1. This ﬁgure shows that thetensile modulus and strength of ramie ﬁber increaseby drawing in the water very much. The tensile modulus of those ramie ﬁbers are estimated by the inclinations of the s–s curves. The estimated tensile modulus of ramie ﬁber is 61 GPa and that of water-treated
one is 130 GPa. To clear the change of tensile property
by water treatment, the tensile modulus of the ramieﬁbers with and without water treatment are shown in
Figure 2. The tensile modulus of blank ramie ﬁber
shown in Figure 2 is almost equal to that reported inprevious papers.22,23 On the other hand, the tensile
modulus of the ramie ﬁber with water treatment is
twice as high as that of blank ramie ﬁber. This increasing of tensile modulus suggests that the molecular
chains are extended and that the contribution of molecular chain to tensile modulus in ﬁber direction increases by the water treatment. The tensile modulus oframie ﬁber with water treatment is almost equal tothat of crystal of natural cellulose in chain axis.24 –26 It
is also reported by Abe et al. that the tensile modulusof ramie ﬁber with water treatment is almost equal to
crystal tensile modulus of natural cellulose.20 This result is inferred to be caused by full extension of molecular chain of ramie cellulose by drawing in the

The X-ray diffraction photographs of ramie ﬁbers withand without water treatment are shown in Figure 3.
The orientation angles of crystals were estimated byazimuthal intensity distribution of the (002) diffraction
spot. X-ray diffraction proﬁles of (002) spot of theramie ﬁbers are shown in Figure 4. The half-width ofdiffraction peak changes not so much by water treatments. The orientation angles are estimated by thehalf-width of diffraction peak, and the orientation degrees are estimated by using orientation angles in thefollowing formula (1).20,27

␾ ⫽[(180⫺ ␤ )/180]⫻100(1)

In this formula (1), ␾ is deﬁned as orientation degree, and ␤ as orientation angle.
The estimated orientation angles and orientation
degrees of ramie ﬁbers with and without water treatment are shown in Figures 5 and 6, respectively. The
orientation angle of ramie ﬁber without water treatment is 12.7°, and that with water treatment 11.5°,
respectively. The orientation degrees of ramie ﬁbers

Figure 2 Tensile modulus of ramie ﬁbers. [Color ﬁgure canbe viewed in the online issue, which is available at www.
interscience.wiley.com.]

THERMAL CONDUCTIVITY OF RAMIE FIBER DRAWN IN WATER

2199

Figure 4 X-ray diffraction proﬁle of ramie ﬁbers. [Colorﬁgure can be viewed in the online issue, which is availableat www.interscience.wiley.com.]

Figure 3 X-ray diffraction photographs of ramie ﬁbers.

without and with water treatment estimated by theformula (1) are 92.9 and 93.6%, respectively. Those
results agree to that by Abe et al., which are reported
in previous article.20 It is shown that the crystal orientation angle and degree do not change by water
treatment. On the other hand, it is reported that theorientation degree of crystal increases by water treatment in the case of cotton ﬁber.20 In the case of ramieﬁber, the crystal is highly oriented in blank ﬁber.
Therefore, it is considered that the orientation degreeof crystal region increases not so much by drawing in
the water.20
With above-mentioned tensile test and X-ray diffraction, the tensile modulus increases two times asthat of blank ramie by water treatment although crystal orientation angle does not change distinctly. Those
results agree to the report by Abe et al.20 In thisstudy,20 it is considered that the molecular chains in
the amorphous region are extended by water treatment and that those extended chains contribute to theincreasing tensile modulus. In the case of cotton ﬁber,
which is natural cellulose ﬁber composed of CelluloseI type crystal as like ramie ﬁber, it is known thatspin-lattice relaxation time of 13C of amorphous peakin C4 resonance in solid-state NMR spectrum becomes
longer by water treatment.20 It suggested the decreaseof molecular motion of amorphous region in cotton
ﬁber.20 And, it was reported as an evidence of orientation of molecular chains in amorphous region.20
In the next section, the thermal conductivities oframie ﬁbers with and without water treatment areinvestigated in the range of low temperature.

Figure 5 Orientation angles of ramie ﬁbers. [Color ﬁgure
can be viewed in the online issue, which is available atwww.interscience.wiley.com.]

2200

Figure 6 Orientation degree of ramie ﬁbers. [Color ﬁgure
can be viewed in the online issue, which is available atwww.interscience.wiley.com.]

YAMANAKA ET AL.

Figure 8 Temperature dependence of the ratio of thermal
conductivity of ramie treated with water to that of blank.
[Color ﬁgure can be viewed in the online issue, which isavailable at www.interscience.wiley.com.]

Thermal conductivity
The temperature dependence of thermal conductivities of ramie ﬁbers with and without water treatment
in ﬁber direction from 10 to 150 K is shown in Figure
7. The thermal conductivities of both samples decrease
with decreasing temperature. Ramie is an electricalinsulator. Therefore, it is considered that thermal conduction of ramie ﬁbers is caused by phonon conduction as like most of polymeric materials.
We investigated the effect of water treatment tothermal conductivity in the following. Thermal conductivity of ramie ﬁber increases by water treatment

very much. It is inferred to be caused by the extensionof molecular chain in the ﬁber direction with drawing
in the water. The increasing of the thermal conductivity of ramie ﬁber by water treatment is caused by theorientation of molecular chains in the amorphous region since the molecular chains are extended by water
treatment in the amorphous region as described earlier.
To compare the thermal conductivity between theramie ﬁbers without and with water treatment, theratio of thermal conductivity of ramie treated withwater to that of blank is estimated by the curves in
Figure 7. Temperature dependence of the estimatedresults is shown in Figure 8. From this ﬁgure, theincreasing ratio of thermal conductivity of ramie ﬁber
by water treatment is the range from 1.25 to 1.46.
Especially, the increasing ratio is almost constant in
the range from 1.40 to 1.46 in the temperature range
from 40 to 150 K. The cause of increasing thermal
conductivity by water treatment is discussed in thefollowing.
The thermal conductivity is shown in the following
formula (2) in the case that thermal conduction iscaused by phonon.15,17–19,28

␬ ⫽(1/3)Cp ␳ vl

Figure 7 Temperature dependence of thermal conductivities of ramie ﬁbers. [Color ﬁgure can be viewed in the online
issue, which is available at www.interscience.wiley.com.](2)

In this formula, the parameters are deﬁned as follows: ␬, thermal conductivity; Cp, heat capacity atconstant pressure; ␳, density of ramie ﬁber; v, soundvelocity in ﬁber direction; l, mean free path of phononin the ﬁber direction.

THERMAL CONDUCTIVITY OF RAMIE FIBER DRAWN IN WATER

The thermal conductivities of ramie ﬁber without
and with water treatment are deﬁned as ␬(b) and ␬(w),respectively. Figure 8 shows that ␬(w)/␬(b) is almost
equal to 1.4 (␬(w)/␬(b) ⱌ 1.4). The effect of increasing
the tensile modulus by water treatment on the thermal
conductivity is investigated in this section. In the formula (2), the sound velocity v is contributed by tensile
modulus. The relation between sound velocity andtensile modulus is shown in the following formula (3).
v ⫽ 共E/ ␳ ) 1/2(3)

In this formula, v is deﬁned as sound velocity as likeformula (2), E as tensile modulus, and ␳ as density oframie. The ratio of sound velocity of ramie ﬁber withwater treatment to that of blank ramie ﬁber is shownin the following formula (4) introduced by formula (3).
v共w兲/v共b兲 ⫽ 共E共w兲/ ␳ 兲 1/2 /共E共b兲/ ␳ 兲 1/2⫽ 共E共w兲/E共b兲兲 1/2(4)

In this formula (4), the parameters are deﬁned asfollows: v(b) and v(w), sound velocities of blank ramieﬁber and ramie ﬁber with water treatment; E(b) andE(w), tensile modulus of blank ramie ﬁber and ramieﬁber with water treatment; respectively.
The ratio v(w)/v(b) is estimated as 1.46 by substitution of the numerical values of E(b) and E(w) into theformula (4). The numerical value of E(b) and E(w)were used 61 and 130 GPa, respectively. Those values
were measured by the tensile test in the front section.
The numerical value of v(w)/v(b) (⫽ 1.46) is almost
equal to the increasing ratio of thermal conductivity
by water treatment (␬(w)/␬(b) ⱌ 1.4). Therefore, theincreasing of thermal conductivity by water treatment
is inferred to be caused by the increasing of soundvelocity.
That is to say, the heat capacity Cp and mean freepath of phonon l do not change by water treatment
since the crystal size, crystallinity, and crystal orientation angle do not change by water treatment. The
increasing of thermal conductivity by water treatment
is inferred to be caused by the following. By the water
treatment, the molecular chains in the amorphous region are extended in the ﬁber direction. The extensionof molecular chains in the amorphous region induces
the increase of the tensile modulus in the ﬁber direction. And, the thermal conductivity increases by only
the increasing the sound velocity induced by the increasing of tensile modulus on the ﬁber direction.

2201(water treatment). And, the effects of thermal conductivity by drawing in the water were investigated. The
following conclusions were drawn.
1. The tensile modulus of ramie ﬁber in ﬁber direction increased from 61 to 130 GPa by drawing in the water.
2. The orientation angle of ramie ﬁber did notchange by drawing in the water.
3. The increasing of tensile modulus of ramie ﬁber
by water treatment was explained by extensionof the molecular chains in the amorphous region.
4. Thermal conductivities of ramie ﬁbers with andwithout water treatment decreased by decreasing temperature.
5. Thermal conductivity of ramie ﬁber in the ﬁber
direction increased by water treatment. Therefore, thermal conductivity increases by extension of molecular chains in the amorphous region.
6. The increasing ratio of thermal conductivity bywater treatment range from 1.25 to 1.46 in thetemperature range from 10 to 150 K, especially,
almost constant from 1.40 to 1.46 in the temperature range from 40 to 150 K.
7. The increasing ratio of thermal conductivity bywater treatment agreed to that of sound velocity
induced by increasing tensile modulus. Therefore, the increasing of thermal conductivity oframie ﬁber by water treatment was explainedby the increasing of sound velocity with increasing of tensile modulus.
With the above-mentioned, it was found that thethermal conductivity of ramie ﬁber increased bywater treatment. Those results suggest that thermal
conductivity of polymer ﬁber increase by the extension of molecular chains in the amorphous region.
The authors acknowledge Mr. Atsushi Hamai, the Fiber
Business Unit, Toyobo for obtaining ramie samples. We
express our appreciation to Ms. Sayuri Nagao, Toyobo, andMr. N. Nakajima, JEMS, who were very helpful with theexperiments. We also thank Professor Dr. Manabu Ikebe,
Iwate University, Dr. Hiroshi Hirahata, Dr. Yukihiro Nomura and the members of their group, Toyobo Functional
Fiber Group, as well as Mr. Toshiyuki Matsui, Manager, and
Mr. Tokuichi Maeda and other members of the Toyobo
Dyneema Division in addition to Dr. Takaharu Ichiryu, Zylon Business Unit, and Dr. Keiji Yukimatsu, President of
Nippon Dyneema for their support. They also acknowledge
Dr. Seiichi Ochi, Dr. Hiroshige Sugiyama and Dr. Sonoko
Ishimaru, Toyobo for their advice and references on theengineering applications of thermal conductivity.

CONCLUSIONS
Thermal conductivity, tensile modulus, and crystal
orientation angles were measured for ramie ﬁbers andthose drawn in the water by the stress of 17.4 kg/mm2

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