Effects of Vapor-Phase-Formaldehyde Treatments on
Thermal Conductivity and Diffusivity of Ramie Fibers
in the Range of Low Temperature
ATSUHIKO YAMANAKA,1 MASATOSHI YOSHIKAWA,2 SHUNZO ABE,1 MASAYUKI TSUTSUMI,1
TAKESHI OOHAZAMA,1 TOORU KITAGAWA,1 HIROYUKI FUJISHIRO,3 KIMIKO EMA,4YOSHINOBU IZUMI,4 SHIGEHIRO NISHIJIMA4
1

Research Center, Toyobo, Co., Ltd., 2-1-1, Katata, Ohtsu, Shiga 520-0292, Japan2

Textiles Products Development Center, Toyobo, Co., Ltd., 50-Innai Daimon-machi Imizu-gun Toyama 939-0295,
Japan

3

Department of Materials Science and Technology, Faculty of Engineering, Iwate University, Ueda 4-3-5,
Morioka 020-8551, Japan

4

Department of Nuclear Engineering, Graduate School of Engineering, Osaka University, Yamadaoka 2-1,
Suita, Osaka 565-0871, Japan

Received 24 February 2005; revised 13 June 2005; accepted 27 June 2005
DOI: 10.1002/polb.20563
Published online in Wiley InterScience (www.interscience.wiley.com).

To understand the inﬂuence to thermal conductivity by bridging in the polymer ﬁbers, the thermal conductivity, and thermal diffusivity of ramie ﬁber and thosebridged by formaldehyde (HCHO) using vapor-phase method (VP-HCHO treatment) were
investigated in the lower temperature range. The thermal conductivities of ramie ﬁber
with and without VP-HCHO treatments decreased with decreasing temperature. Thermal diffusivities of ramie ﬁber with and without VP-HCHO treatments were almost constant in the temperature range of 250–50 K, and increased by decreasing temperature
below 50 K. Thermal conductivity and thermal diffusivity of ramie ﬁber decreased by VPHCHO treatment. The crystallinities and orientation angles of ramie ﬁbers with andwithout VP-HCHO treatment were measured using solid state NMR and X-ray diffraction. These were almost independent of VP-HCHO treatment. Although tensile modulus
decreased slightly by VP-HCHO treatment, the decrease could not explain the decrease inthermal conductivity and diffusivity with decreasing sound velocity. The decrease of thethermal diffusivity and thermal conductivity by VP-HCHO treatment suggested the possibility of the reduction of the mean free path of phonon by HCHO in VP-HCHO treated
C 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2754–2766, 2005
ramie ﬁber. V
Keywords: thermal conductivity; thermal diffusivity; ramie; bridge
ABSTRACT:INTRODUCTION
In applications of polymeric materials, the thermal conductivity has been an important propCorrespondence to: A.
yamanaka@kt.toyobo.co.jp)

Yamanaka (E-mail:atsuhiko_

Journal of Polymer Science: Part B: Polymer Physics, Vol. 43, 2754–2766 (2005)
C 2005 Wiley Periodicals, Inc.
V

2754

erty, for example, for a cool/warm sensation for
clothing ﬁber or wood 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 in cryogenic andheat-releasing materials in electrical equipment

THERMAL CONDUCTIVITY AND DIFFUSIVITY OF RAMIE FIBERS WITH VAPOR-PHASE-HCHO TREATMENTS

has become more important. Furthermore, thedesired features vary, depending upon the application, from insulation for use in cryostat5 tohigh thermal conductivity for use in superconducting coils6 and electronic engineering.7 Investigation of the factors that affect the thermal
conductivity of polymeric materials is essential
for the development of technologies for designing
polymeric materials that possess the required
thermal conductivity.
From previous studies of polymer materials, itis well known that the thermal conductivity ofamorphous polymers is smaller than that of metals and semiconductors.8,9 Therefore, these haveprincipally been used as heat insulators. However, other reports have shown that polymeric
crystals possess a high thermal conductivity inthe direction in which the molecular chains arecovalently bonded, polyethylene crystals being
an example.10,11 The thermal conductivity of electrical insulators is considered to be attributableto phonons,10,12 and the heat in polymers is conducted in the direction of covalently-bonded molecular chains, whereas conduction in the direction of intermolecular chains bonded by Van
der Waals forces is much less. Therefore, the thermal conductivity depends on the crystallization,orientation, crystal size, and defects in the polymer crystal material. Thus, high-crystallization and high-orientation polymers exhibit highthermal conductivity.11–16 For example, highly
crystallized polymer materials, including highstrength polyethylene ﬁber16–19 and high-strength
polypara-phenylene-benzo-bisoxazole ﬁber17 are
known to possess high thermal conductivity similar to that of metals. The thermal conductivity ofpolymers, therefore, ranges from high to lowdepending on the crystal structure or morphology.
The thermal conductivity of solid, electricallyinsulating materials is affected by the scatteringof phonons. The phonon scattering is considered
to be introduced by imperfections on the surfaceof the material, the grain boundaries or intracrystal impurities, or interphonon dispersion in
the material.20 Therefore, in polymers, crystal or
amorphous boundary, defects, the ends andentanglements of the molecular chains can scatter phonons and interfere with the thermaltransmittance. In polymers, it is expected thatthe phonon is also scattered by impurities likethe following; molecular impurities inter molecular chain, chemical branch or graft, chemical
bridge points, chemical impurities copolymerized
in the polymer chain, or chemical boundary in the2755

block copolymer. Therefore, in the case of the thermal conduction of polymer ﬁbers in the ﬁber direction, the mean free path of phonons is inﬂuenced
not only by the crystallization or orientation, butalso by the length of molecular chains and thelength between the impurities mentioned above.
That is to say, the mean free path of phonon is
expected to depend on the concentration of chemical branch, bridge points, and interchain impurities. The changes in the mean free path of phonons result in changes in the thermal conductivity.
In this report, changes in the thermal conductivity
due to bridging or graft in the polymer ﬁbers aredescribed.
Fibers mainly made of cellulose, including
ramie and cottons, are well known for being
bridged with formaldehyde (HCHO),21–28 for example, by vapor–phase-formaldehyde method (VPHCHO treatment).21 This technology is employed
for stabilizing the conformity of cellulose ﬁbers
and is used practically in clothing materials suchas wash and wear shirts.22,26–29 There are bridge
points and graft points combing cellulose chain
and HCHO in the cellulose ﬁbers with VP treatment. It is expected that the phonon is scatteredat the bridge or graft points in cellulose ﬁber,
with VP-HCHO treatment. Therefore, the mean
free path of phonon in ﬁber direction of cellulose
ﬁber is expected to reduce by scattering of phonon at bridge or graft points combined with
HCHO. That is to say, thermal conductivity anddiffusivity are considered to decrease by VPHCHO treatment. In this paper, we report theeffect of VP-HCHO treatment on the thermal
conductivity and diffusivity of ramie ﬁber in therange of low temperature.

EXPERIMENTAL
SampleIn this work, thermal conductivity and diffusivity of raw and VP-HCHO-treated ramie were
measured in the range of low temperature. The
sample preparations are described in the following section.
Ramie Fibers
The ramie ﬁber samples used in this work weremade in China. The ramie ﬁber were scoured
and bleached. The density and ﬁneness of ramieﬁbers were 1.54 and 4.3 dtex, respectively.

2756

YAMANAKA ET AL

The VP-HCHO-Treated Ramie Fibers
Two kinds of ramie ﬁbers were prepared by VPHCHO treatment.21 In the VP-HCHO treatment,
the ramie ﬁbers were washed by steam and were
exhausted in vacuum to remove free HCHO. Details of VP-HCHO treatment are described elsewhere.21 The concentrations of bound HCHO of
the two kinds of VP-HCHO-treated ramie ﬁbers
were 0.7% and 1.6%, respectively. These are
hereinafter abbreviated to VP (0.7) and VP (1.6).
The density of ramie ﬁbers with VP-HCHO treatment was 1.55.
The concentrations of bound HCHO were
measured by iodometric titration of HCHO withhydrolysis of VP-HCHO-treated ramie ﬁbers. Details of measurements of concentration of bound
HCHO are described elsewhere.21 The densities
of ramie ﬁbers with and without VP-HCHO treatment were measured by pycnometer after drying
at 383 K  8 h.
Measurements
Thermal conductivity and diffusivity depend on
not only bridge or graft points but also the crystallinity and orientation angle of crystal.11–16 To
investigate the relative changes of the crystallinity and orientation angle by VP-HCHO treatment, the crystallinities and orientation anglesof ramie ﬁbers with and without VP-HCHO
treatment were measured by solid state high resolution NMR and X-ray diffraction, respectively.
The tensile moduli of the ﬁbers were measured
for relative comparison of sound velocities of theramie ﬁbers with and without VP-HCHO treatment. In this section, the measurements of crystallinities, orientation angles, tensile modulus
are described, and the measurements of thermal
conductivity and diffusivity are described in thefollowing section.
Thermal Conductivity
Thermal conductivity (j) was measured by asteady-state heat ﬂow method.17,30 The measurements of j were carried out on the automatedmeasuring system with thermal controller of a
Gifford–MacMahon (GM) cycle helium refrigerator as a cryostat.30
The ﬁber samples were stored in a desiccatorfor more than three weeks. The ﬁber samples wereprepared by bundling about 400–8500 monoﬁlaments with length of 25 mm. Both ends of the ﬁber

bundle were ﬁxed by adhesive Stycast GT. One
end of the bundle was attached to the cold stage ofa GM refrigerator by mechanical pressing using
indium metal and the adhesive Stycast GT. A
small resistance heater (1 kO) was adhered to theother end of the bundle using GE7031 varnish.
The intermediate positions where thermocouples
were contacted were bound by ﬁne (0.1 mm) Cu
wire. Au (Fe at 0.07%)-chromel thermocouples
were used as thermometers, which were adhered
by GE7031 varnish. The sample space was evacuated to below 103 Pa by an oil diffusion pumpfor heat insulation. Before the measurements, thesample space was kept at high vacuum below 103
Pa for 24 h to dry up the sample.
The automated measuring system of j was
operated in the temperature range of 10–250 K.
The j was estimated by the relation j(mW/cm K)¼ (Q/DT)(L/S), where Q is the heat ﬂow per second, DT is the temperature difference between
thermometers, L is the distance between thethermometers, and S is the cross section of thebundle. Details of the determination of j aredescribed elsewhere.17,18
Thermal Diffusivity
Thermal diffusivity (k) was measured by nonsteady-state heating method.17,18,30 The measurements of k were carried out on the same system
as that for measurements of thermal conductivity. In this method, time evolutions of temperature at two measurement points, the intermediate positions between the cold head and 1 kO
heater, were recorded after applying heat pulse
from the heater, and the measured temperaturechange was compared once for various k values.
The ﬁber samples were stored in a desiccator for
more than a week. The ﬁber samples were prepared and were set in the sample space by thesame way as that for the measurements of thermal conductivity mentioned above. The sample
space was evacuated to below 103 Pa by an oil
diffusion pump for heat insulation and was kept
for 24 h before the measurements for drying up
the sample. The automated measuring system ofk was operated in the temperature range of 10–
250 K. Details of the determination of k aredescribed elsewhere.17,18,30
Water Contents
To measure the water contents of the samples
used in the measurements of thermal conductiv-

THERMAL CONDUCTIVITY AND DIFFUSIVITY OF RAMIE FIBERS WITH VAPOR-PHASE-HCHO TREATMENTS

2757

ity and thermal diffusivity, the samples werestored at high vacuum below 103 Pa for 24 h in
the sample space in the same way for measurement of thermal conductivity. The weights of
those samples were deﬁned as W. The driedsamples were obtained by drying them at 383 K
for 8 h in vacuum at 1 Pa as described elsewhere.31,32 The weights of dried samples weredeﬁned as Wd. The water contents of those samples were estimated by the difference between
W and Wd.

Crystallinity
It is known that the chemical shifts of cellulose
in solid state 13C NMR spectra are different by
the difference in crystal structure, for example
cellulose I and II.33–35 It is also known that theresonance lines of C4 and C6 carbons split into
crystal and amorphous components in solid state
13
C NMR spectra of cellulose.33,34,36,37 The changes in crystal structures and crystallinities ofcellulose ﬁbers by mercerization or liquid ammonia treatment were studied with solid state
NMR.38,39 In this work, the changes in crystal
structure and crystallinity by VP-HCHO treatment are investigated by solid state NMR.
Solid state NMR measurements were carriedout on Brucker Avance 300 (13C, 75.5 MHz). NMR
spectra were measured by cross-polarization (CP),high power proton decoupling, magic angle spinning (MAS) methods. Pulse width was 4 ls andmagic angle spinning rate was 4.5 kHz.

Orientation Angle
Orientation angle of crystallic phase to ﬁber axis
was estimated by X-ray diffraction. The samples
were prepared in the form of a bundle of about
30 ﬁlaments in parallel and they were ﬁxed atboth ends. Rigaku RU-200 (40 kV  100 mA) wasused with X-ray diffraction on Ni-ﬁltered Cu Ka(k ¼ 0.1548 nm). The intensity distribution of(002) diffraction spot was used for estimating orientation angles.

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 crosshead speed was
10 mm/min.

Figure 1. Temperature-dependence of thermal conductivities of ramie ﬁbers with VP-HCHO treatment.
[Color ﬁgure can be viewed in the online issue, whichis available at www.interscience.wiley.com.]

RESULTS AND DISCUSSION
The Change in Thermal Conductivity of Ramie
Fiber by VP-HCHO Treatment
The temperature dependence of thermal conductivity of ramie ﬁbers with and without VPHCHO treatments in ﬁber direction from 10 to150 K are shown in Figure 1. The number of ﬁlaments used in these measurements are morethan 6000. Thermal conductivities of all samples
decrease with decreasing temperature. Thoseramie ﬁbers are electrical insulators. Therefore,
it is considered that thermal conduction of ramieﬁbers is caused by phonon conduction like mostof the polymer materials.
As for the effect of VP-HCHO treatment, thermal conductivity of ramie ﬁber decreases. The
thermal conductivity of ramie ﬁber decreases toabout 50% and 20–25% by the VP-HCHO treatment for the concentration of 0.7% and 1.6%
HCHO, respectively. To clear the changes in thethermal conductivity by VP-HCHO treatment,
the relation between the thermal conductivity at
120, 100, 80, and 60 K and the concentration of
the bound HCHO in the ramie ﬁbers are shownin Table 1. As shown in Table 1, the thermal conductivity decreases by bridge of HCHO at every
temperature. The concentration of bound HCHO
is higher; the thermal conductivity is lower.
This decrease in thermal conductivity is inferred to be caused by the change in several

2758

YAMANAKA ET AL

Table 1. Thermal Conductivity and Thermal Diffusivity of Ramie Fibers With andWithout VP-HVHO Treatment

SampleBound
HCHO (%)

Thermal Conductivity (mW/cm K)
120 K

100 K

80 K

60 K

ThermalDiffusivity at
240 K (mm2/s)

Ramie
VP (0.7)
VP (1.6)

0
0.7
1.6

34.0
18.4
7.0

30.5
16.5
6.0

26.3
14.0
5.5

21.0
11.3
4.8

0.94
0.42
0.23

properties, such as radiation effect, water content, crystallinity, crystal structure, orientation
degree, and mean free path of phonon by bridging or graft reaction with VP-HCHO treatment.
The changes in these properties are investigatedin the following section.
Radiation Effect in the Measurement of Thermal
Conductivity
Temperature dependence of thermal conductivityof various number of ramie ﬁbers in the ﬁber
direction from 10 to 250 K are shown in Figure 2.
In the temperature range below 100 K, theobserved thermal conductivities are independentof the number of ﬁlaments and the slopes of thecurves decrease with increasing temperature. On
the other hand, the observed thermal conductivities depend on the number of ﬁlaments, and theslopes of the curves increase with increasing
temperature in higher temperature range. The
observed thermal conductivity is higher withdecreasing the number of ﬁlaments in higher
temperature range. The dependence of observed
thermal conductivity on the number of ﬁlamentsin the higher temperature range is inferred to becaused by radiation effect. In the temperature
range over 200 K, the observed thermal conductivity show the behavior inﬂuenced by radiation
effect even if the number of ﬁlaments are morethan 5000. Therefore, it is difﬁcult to observe thethermal conductivity in the higher temperature
range more than 200 K. On the other hand, theradiation effect can be neglected in low temperature range in the case where the number of ﬁlaments is more than 6000.
The radiation effect of thermal conductivity
also depends on the side shape of ﬁlament. The
photographs of ramie ﬁbers by scanning electron
microscope are shown in Figure 3. The shape ofthe side view does not change distinctly by VPHCHO treatment in the case of bound HCHO

below 1.6%. Therefore, the radiation effect does
not change by change in shape, with VP-HCHO
treatment. The ramie ﬁbers with VP-HCHO
treatment show the same behavior of ﬁlament
number dependence. Therefore, the radiation
effect can be neglected in the measurements ofthermal conductivity in the temperature range
below 150 K, with more than 6000 ﬁlaments forthe discussion of the effect of VP-HCHO treatment on the thermal conductivity.
Water Content
The water contents of ramie ﬁbers, with andwithout VP-HCHO treatment, used in this work
are smaller than 0.1%. After the measurementsof thermal conductivity, the ramie sample wasdried at 383 K for 8 h in vacuum at 1 Pa andthermal conductivity was measured again. The
temperature dependence of thermal conductiv-

Figure 2. Temperature-dependence of thermal conductivity of ramie ﬁbers with various number of ﬁlaments. [Color ﬁgure can be viewed in the online issue,
which is available at www.interscience.wiley.com.]

THERMAL CONDUCTIVITY AND DIFFUSIVITY OF RAMIE FIBERS WITH VAPOR-PHASE-HCHO TREATMENTS

2759

Figure 3. Side view of ramie ﬁbers with and without VP-HCHO treatment by scanning electron microscope. [Color ﬁgure can be viewed in the online issue, which isavailable at www.interscience.wiley.com.]ities of ramie ﬁbers before and after dry treatment are shown in Figure 4. This ﬁgure showsthat thermal conductivity does not change by dry

treatment compared with VP-HCHO treatment.
Therefore, the inﬂuence of thermal conductivityof 0.1% water is neglected in this article.

2760

YAMANAKA ET AL

Figure 4. Temperature-dependence of thermal conductivity of ramie ﬁbers with and without dry treatment. [Color ﬁgure can be viewed in the online issue,
which is available at www.interscience.wiley.com.]

Thermal conductivity depends on heat capacity, density, and thermal diffusivity as shown in
the follwing formula (1).17,18
j ¼ Cp qk

The resonance lines of C4 and C6 carbons split
into crystal and amorphous components.33,34,36
The peaks of crystal are sharp and those of amorphous are broad. Because the peak splitting of
C4 resonance is larger than that of C6 resonance,
C4 resonance peaks are used for estimation of
crystallinity. The C4 peak separations of ramieﬁbers with and without VP-HCHO treatment are
shown in Figure 6. The peak shape and chemical
shifts do not change by bridge with HCHO (VPHCHO treatment). It is known that a linear relationship exists between the mass fraction of theC4 crystal component peak in solid state NMR
and the crystallinity determined by X-ray analysis.34,36,44 The crystallinities of those ramie ﬁbers
estimated by peak separation are shown in Table 2.
Although the crystallinity of ramie ﬁber increases slightly by VP-HCHO treatment, thischange is not so large compared with that ofthermal conductivity. Therefore it is expectedthat the thermal conductivity of ramie ﬁber is
not much inﬂuenced by the relative change incrystallinity with VP-HCHO treatment.

ð1Þ

In this formula, the parameters are deﬁned asfollows: j, thermal conductivity in the ﬁber direction; Cp, heat capacity at constant pressure; q,density of ramie ﬁber; k, thermal diffusivity inthe ﬁber direction. It is known that heat capacityof cellulose depends on water contents.40 Thechange in heat capacity by 0.1% water is estimated to be about 0.25%, according to the relation40 between heat capacity and water contents.
From this formula, the inﬂuence to thermal conductivity with the change in heat capacity induced by 0.1% water is about 0.25% and it can beneglected in this work. Therefore, it is also considered that thermal diffusivity does not change
by 0.1% water according to formula (1).
Crystallinity
13

C-CP-MAS solid state NMR spectra of ramieﬁbers with and without VP-HCHO treatments
are shown in Figure 5. The assignments of resonance lines are shown in this ﬁgure. It is known
that the crystal structure of ramie is Cellulose I
type.34,41–43 All the NMR spectra show same
peak shape and chemical shifts of the crystal
structure of Cellulose I type.34

Figure 5. 13C-CP-MAS solid state high resolution
NMR spectra of VP-HCHO-treated ramie ﬁbers.
[Color ﬁgure can be viewed in the online issue, whichis available at www.interscience.wiley.com.]

THERMAL CONDUCTIVITY AND DIFFUSIVITY OF RAMIE FIBERS WITH VAPOR-PHASE-HCHO TREATMENTS

2761

ramie ﬁbers with and without VP-HCHO treatments are reported.

Orientation Angle of Crystal Region
An X-ray diffraction photograph of ramie ﬁber
is shown in Figure 7. The orientation angles ofcrystals were estimated by azimuthal intensity
distribution of the (002) diffraction spot. X-ray
diffraction proﬁles of (002) spot of the ramie ﬁbers are shown in Figure 8. The half-widthof diffraction peak changes not much by VPHCHO treatments. The orientation angles are
estimated by the half-width of diffraction peak
and the orientation degrees are estimated byusing orientation angles in the following formula (2).24,45/ ¼ ½ð180  bÞ=180  100

Figure 6. Line-shape analysis for C4 peaks of ramiecellulose with VP-HCHO treatment in 13C-CP-MAS
solid state NMR spectra. A solid line indicates observed curve, and broken lines indicate the estimatedcurves of crystal components and amorphous component. [Color ﬁgure can be viewed in the online issue,
which is available at www.interscience.wiley.com.]

The absolute values of crystallinity estimatedby solid state NMR are a little different fromthose estimated by X-ray.34,44 Therefore, othermeasurements will be necessary for more detailed discussion about absolute crystallinity. In
the following section, the orientation angles ofð2Þ

In this formula, (2) / is deﬁned as orientation
degree and b as the orientation angle.
The orientation angles and orientation degrees of ramie ﬁbers with and without VP-HCHO
treatment are shown in Table 2. It is shown thatthe orientation angle and the orientation degree
do not change by VP-HCHO treatment in thecase of bound HCHO below 1.6%.
As mentioned above, effect of radiation andwater content to the change in thermal conductivity can be neglected, and the crystallinity andorientation degree of crystal do not change distinctly by VP-HCHO treatment.
Therefore, the decreasing of thermal conductivity by VP-HCHO treatment is inferred to becaused by the other changes induced by bridge or
graft points. From formula (1), thermal conductivity depends on density, heat capacity, andthermal diffusivity. The density changes in lessthan 1% by VP-HCHO treatment. Therefore, thechange in density does not contribute to the decrease of thermal conductivity by VP-HCHO
treatment. The change in thermal diffusivity by

Table 2. Crystallinity and Orientation Degree of the Ramie Fibers With andWithout VP-HCHO Treatment

SampleBound
HCHO (%)

Crystallinity(%)

Orientation
Angle (deg)Orientation
Degree (%)

Ramie
VP (0.7)
VP (1.6)

0
0.7
1.6

56
58
62

12.7
11.0
11.8

92.9
93.8
93.4

2762

YAMANAKA ET AL

Figure 7. X-ray diffraction photograph of ramie ﬁber. [Color ﬁgure can be viewed
in the online issue, which is available at www.interscience.wiley.com.]

VP-HCHO treatment is investigated in the following section.

The Change in Thermal Diffusivity by VP-HCHO
Treatment
Temperature dependence of thermal diffusivityof ramie ﬁbers with VP-HCHO treatment in theﬁber direction is shown in Figure 9. The thermal
diffusivities of all ramie ﬁbers show almost constant values in the temperature range of 250 to
50 K and increases when temperature decreases
below 50 K. As for effect of VP-HCHO treatment,
the thermal diffusivity decreases distinctly inthe range of all temperatures. The relation between the concentration of bound HCHO andthermal diffusivity at 240 K is shown in Table 1.
Thermal diffusivity at 240 K decreases to about
45 and 25% by VP-HCHO treatment for the concentration of 0.7% and 1.6% HCHO, respectively.
The decrease in thermal diffusivity agrees withthat of thermal conductivity by increasing bound
HCHO. Therefore, the decreasing of thermal diffusivity is one of the causes for the decreasing
thermal conductivity of ramie ﬁbers by VPHCHO treatment. The cause of decreasing thermal diffusivity by VP-HCHO treatment, such asradiation effect and the graft or bridge points, isinvestigated here.

For investigation of the radiation effect in themeasurements of thermal diffusivity, the temperature dependence of thermal diffusivities of ramie ﬁbers are compared with the different number of ﬁlaments in Figure 10. The behavior ofthermal diffusivity is almost independent of thenumber of measuring ﬁlaments. In this method,
thermal diffusivity is estimated by time-depend-

Figure 8. X-ray diffraction proﬁle of ramie ﬁbers with
VP-HCHO treatment. [Color ﬁgure can be viewed in the
online issue, which is available at www.interscience.
wiley.com.]

THERMAL CONDUCTIVITY AND DIFFUSIVITY OF RAMIE FIBERS WITH VAPOR-PHASE-HCHO TREATMENTS

2763

The thermal diffusivity depends on soundvelocity and mean free path of phonon as shownin the following formula (3).17,18
k ¼ ð1=3Þvl

ð3Þ

In this formula, the parameters are deﬁned asfollows; k, thermal diffusivity in the ﬁber direction; v, sound velocity in the ﬁber direction; l,mean free path of phonon in the ﬁber direction.
The sound velocity in the ﬁber direction isestimated by using the tensile modulus and density in the following formula (4).

Figure 9. Temperature-dependence of thermal diffusivity of ramie ﬁbers with VP-HCHO treatment.
[Color ﬁgure can be viewed in the online issue, whichis available at www.interscience.wiley.com.]ence of temperature at two measurement points
and heat ﬂow is not used.30 This is a different
point from the measurements of thermal conductivity, although thermal diffusivity and conductivity are measured by same system. Therefore,
the radiation effect to thermal diffusivity is muchsmaller than that to thermal conductivity,
although it is not equal to 0. Therefore, the radiation effect is neglected compared with the change
in thermal diffusivity by VP-HCHO treatment.
This result shows that the thermal diffusivity
decreases by the HCHO in the ramie ﬁbers.
It is considered that HCHO molecules exist in
the ramie ﬁber in the following four types; freemolecule, graft combined with cellulose chain,interchain bridge, and intrachain bridge. In thefour types of HCHO, it is considered that the free
HCHO molecules exist in little, because the ramieﬁbers were washed by steam and were exhausted
in vacuum to remove free HCHO, as described in
the experimental section.21 The ramie ﬁbers
treated with similar VP-HCHO method show thestability of conformity for wash and wear
shirts.22,29 Therefore, it is considered that some of
the HCHO molecules exist as the interchain
bridge. The average degree of polymerization of
HCHO in VP-HCHO treatment ﬁber is reported tobe 1.7.46 It is also reported that HCHO is mostly
combined at C6 carbon with cellulose.47 Although,it is difﬁcult to distinct the four type HCHO molecules by the iodometric titration method, it isinferred that most of the HCHO molecules exist asgraft, inter-, and intra-chain bridge.

v ¼ ðE=qÞ1=2

ð4Þ

In this formula, v is deﬁned as sound velocity, E
as tensile modulus, and q as density of ramie. To
discuss the cause of decreasing thermal diffusivity, the possibility of the relative change in soundvelocity is discussed by tensile modulus.
The tensile modulus at room temperature inﬁber direction of ramie ﬁber, VP (0.7) and VP(1.6) are 61, 49, and 52 GPa, respectively. It isreported that the tensile modulus increases
slightly by VP-HCHO treatment in the same way
after drawing in the water.24 In this case, thetensile modulus slightly decreases by VP-HCHO
treatment, in spite of the crystallinity and crystal orientation independence. The two reasonsare considered as following: (1) The disordered

Figure 10. Temperature-dependence of thermal diffusivity of ramie ﬁbers with various number of ﬁlaments. [Color ﬁgure can be viewed in the online issue,
which is available at www.interscience.wiley.com.]

2764

YAMANAKA ET AL

chains ﬁxed by bridged HCHO do not contributeto tensile modulus. Those samples with VPHCHO treatment became brittle and hard. (2)The orientation degree of molecular chain in
amorphous region decreases by VP-HCHO treatment. The opposite case is reported that the tensile modulus of ramie ﬁber increased by the orientation of amorphous region.24 The decreasing orientation degree of amorphous region is able toinduce the decreasing sound velocity and mean
free path of phonon.
The density of those samples is 1.54 and 1.55.
The sound velocities of ramie, VP (0.7), and VP(1.6) at room temperature are estimated as 6400,
5700, and 5900 m/s by the substitution of thenumerical values of E and q into the formula (4).
The sound velocity estimated by tensile modulus
slightly decreases by VP-HCHO treatment,
although the decreasing rate is smaller than thatof thermal diffusivity.
It is known that sound velocity of polymericmaterials depends on temperature.48 On theother hand, the thermal diffusivities of thoseramie ﬁbers show almost constant value in thetemperature range of 50–250 K. Therefore, thethermal diffusivities of the ramie ﬁbers at room
temperature are expected to be almost equal tothose at 240 K. The tensile modulus does notdecrease, for explaining the decrease of thermal
diffusivity by decreasing sound velocity.
Crystal region and amorphous region are
mixed in the ramie ﬁber used in this work. The
mean free path of phonon in crystal region andthat in amorphous region are different. Therefore, it is difﬁcult to discuss the absolute value ofphonon mean free path in the crystal and amorphous region in this work. Therefore, the relativechange in average behavior of phonon mean freepath in the mixture of crystal and amorphous by
VP-HCHO treatment is discussed in this work.
The mean free path of phonon in the ramie ﬁbers
with VP-HCHO treatment can be compared withthat in blank ramie ﬁber relatively by using formula (3) and the sound velocity estimated by formula (4) in the assumption that thermal diffusivities at room temperature are equal to those at
240 K. The estimated mean free path of phonon
decreases with VP-HCHO treatment to 52 and
27% in the case of VP (0.7) and VP (1.6), respectively.
It is considered that all of the four types
HCHO—free molecule, graft combined with cellulose chain, interchain bridge, and intrachain
bridge—scatter the phonon, although the proba-

bilities are different. That is to say, it is considered that the mean free path of phonon reducesby the HCHO, such as the graft or bridge points
in the VP-HCHO treated ramie ﬁbers. It is also
considered that the decrease of orientation in theamorphous region by bridge points reduces themean free path of phonon as mentioned above.
The probability of existence of HCHO in crystal region is considered to be different from thatin amorphous region. It is considered that HCHO
molecules in crystal region contribute effectively
to decreasing thermal diffusivity. For more quantitative discussion, the more detailed discussion
including structural analysis will be necessary.
The decreasing of thermal diffusivity of ramieﬁber by VP-HCHO treatment is inferred to becaused by the reduction of the mean free path ofphonons by HCHO. This result is based on someassumptions. Therefore, the direct measurements of temperature-dependence of sound velocity are necessary for detail discussion of mean
free path.
Heat Capacity
From formula (1), thermal conductivity depends
on not only thermal diffusivity but also heat
capacity. It is known that heat capacity of cellulose depends on crystallinity.49 But in this case,the change in heat capacity by crystallinity does
not contribute to the decrease in thermal conductivity. Heat capacity is able to change by change
in molecular motion induced by bridge or graft
points with VP-HCHO treatment.
The observed thermal conductivities and thermal diffusivities have the experimental error of
about 10% as shown in Figures 4, 9, and 10.
Although the errors are smaller than the change
by VP-HCHO treatment, the quantitative discussion of heat capacities is difﬁcult by the estimation from formula (1) in this work. Therefore, itwill be necessary to investigate the heat capacity
independently to understand the decreasing
thermal conductivity by VP-HCHO treatment.

CONCLUSIONS
Thermal conductivity and thermal diffusivity
were measured by steady-state heat ﬂow method
and non-steady-state heating method for ramieﬁber and those treated with formaldehyde(HCHO) by VP-HCHO treatment, with concentrations of bound formaldehyde of 0.7% and 1.6%

THERMAL CONDUCTIVITY AND DIFFUSIVITY OF RAMIE FIBERS WITH VAPOR-PHASE-HCHO TREATMENTS(VP (0.7) and VP (1.6)). The following conclusions
were drawn:
1. The thermal conductivities of ramie ﬁbers
with and without VP-HCHO treatments decreased with decreasing temperature.
2. Thermal conductivity and diffusivity decreased to about 50% and 20–25% by VPHCHO treatment in the case of VP (0.7)
and VP (1.6), respectively.
3. The crystallinity and orientation degree oframie ﬁber did not decrease by VP-HCHO
treatment.
4. Thermal diffusivities of ramie ﬁber andthose with VP-HCHO treatments were almost constant in the temperature range of
250–50 K, and increased by decreasing temperature below 50 K.
5. The decrease of thermal diffusivity by VPHCHO treatment could not be explained bychange in tensile modulus, although it
decreased slightly by VP-HCHO. Those results suggested the possibility of the reduction of mean free path of phonon by HCHO.
As mentioned above, the thermal conductivity
and thermal diffusivity decrease by bridge or
graft with formaldehyde, although the crystallinty and orientation degree do not decrease.
Those results suggest that thermal conductivityof polymer material could be controlled by bridgeor graft structure.
Many thanks are due to Mr. Atsushi Hamai, the Fiber
Business Unit, Toyobo, and the Research and Development Center for obtaining ramie samples and VPHCHO treatment. Our gratitude is also expressed toProfessor Dr. Fumitaka Horii, Kyoto University, and
Dr. Seichi Ochi, Research and Development Department, Toyobo, Professor Dr. Masao Murano, Saga
University and Dr. Atsushi Kaji as well as Dr. Yuji
Shimizu, Toyobo, for their valuable advice and references with respect to cellulose and its structural analysis. Thanks are also due to Dr. Hiroshige Sugiyama
and Dr. Sonoko Ishimaru, Toyobo for their advice andreferences on the engineering applications of thermal
conductivity. We express our appreciation to Ms.
Sayuri Nagao and Ms. Chie Sawa, Toyobo, and Mr. N.
Nakajima, JEMS, who were very helpful with theexperiments. We would also like to thank Professor
Dr. Manabu Ikebe, Iwate University, Dr. Hiroshi Hirahata, Dr. Yukihiro Nomura and the members of theirgroup, Toyobo Functional Fiber Group, as well as
Mr. Takayuki Matsui, Manager, and Mr. Tokuichi

2765

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.

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