Journal of Analytical and Applied Pyrolysis, 24 (1992) 147-161
147
Elsevier Science Publishers B.V., Amsterdam
Thermal characterization analysis of milkweed floss
Pei Gu, Rita K. Hessley and Wei-Ping Pan
Department of Chemistry, Western Kentucky University, Bowling Green, KY 42101 (USA)
(Received February 18, 1992; accepted July 6, 1992)
ABSTRACT
The thermal analysis of milkweed and several individual commercial materials of similar
chemical composition has been carried out. By combining a two-stage thermal process
(pyrolysis-combustion) with a thermogravimetric analyzer and a Fourier transform infrared
spectrometer, it has been possible to identify the major organic volatile products of pyrolysis
and to correlate gas evolution with the decomposition of the individual components
(cellulose, hemicellulose and lignin) of the floss. During pyrolysis, acetic acid, formic acid
and methanol are formed in addition to CO2 and H20. The data also show that pyrolytic
decomposition of the three chemical constituents of milkweed occur without any apparent
synergistic interaction. The combustion of milkweed produced CO2 and H20, as expected,
but the removal of the waxy coating from the fibers results in an increased susceptibility to
combustion. The coating displays no effect on the process of pyrolysis.
Cellulose; hemicellulose; lignin; milkweed; pyrolysis.
INTRODUCTION
Milkweed plants are distributed throughout the USA and grow on
practically all types of soil. They produce a seed fiber which is often called
"vegetable silk", but is more properly known as milkweed floss [1].
Milkweed has characteristics which make it an attractive commercial
product. Namely, it is light in weight, an efficient thermal insulator, water
resistant and less expensive to produce than comparable synthetic materials
[2]. Thus, it is important to investigate the thermal properties of the
milkweed fiber. As part of this investigation we also compared the thermal
stability of milkweed floss with other materials which are of similar chemical
composition. These include cellulose, lignin, and several kinds of hemicellulose. The focus of the study was to correlate the effect the chemical
compositon of milkweed has on its thermal decomposition in two atmospheres, nitrogen and air. So far we have been able to show that milkweed
Correspondence to: Professor W.-P. Pan, Department of Chemistry, Western Kentucky
University, Bowling Green, KY 42101, USA.
0165-2370/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
148
P. Gu et al. ] J. Anal. Appl. Pyrolysis, 24 (1992) 147-161
fiber displays a high degree of thermal stability, that its constituents are
degraded without any apparent interaction, and that oxidative thermal
decomposition of milkweed does not result in the formation or release of
any toxic volatile organic compounds.
EXPERIMENTAL
Milkweed floss was obtained from Natural Fiber Corp., Ogallala, NE.
The chemical composition of the fiber determined by Natural Fiber Corp.,
is 55% cellulose, 24% hemicellulose, 18% lignin.
Nine varieties of hemicellulose were obtained from Sigma; a Kraft lignin,
lignin AT, was donated by Westvaco; a sample of cellulose was obtained
from Sigma.
A Seiko T G / D T A 200 was used for monitoring the T G / D T A data. A
heated gas line was installed to connect the Seiko T G / D T A thermal analysis
instrument to the Perkin Elmer 1600 FT-IR spectrometer [3]. A 10cm
heated (250°C) gas cell with KRS-5 windows was used for all IR analysis.
Prior to obtaining TG data, all the samples were heated from 25 to 110°C
to remove residual moisture. Data were then collected in the temperature
range 100°-600°C, using a gas-flow rate of 50 ml min -1 air, or 100 ml min -1
nitrogen. The choice of the flow rate was to prevent ignition of the samples.
The mass of all samples was 6 - 1 0 mg. An initial FT-IR analysis was carried
out, during which a spectrum of the gas stream was acquired at 1 min
intervals. In subsequent trials, IR data were acquired by sampling the gas
stream at 5 min intervals.
All thermal analyses were made on samples of whole milkweed (as
received) and on milkweed from which the waxy coating had been removed.
These samples were obtained by exhaustive Soxhlet extraction of the whole
milkweed fiber using a mixture of methanol and hexane (65/35% (v/v)) [4].
The milkweed and cellulose extraction thimbles were dried at 105°C for
2 - 3 h to remove surface moisture. Weighted samples were extracted at
reflux for 24 h. After drying to constant weight at 105°C, the amount of
material extracted was determined by difference in mass. Soxhlet extraction
of 12 samples of whole milkweed showed an average weight loss of 4.2%.
RESULTS A N D DISCUSSION
Figures 1 and 2 show the T G / D T A curves of whole and extracted
milkweed pyrolysis from 100 to 600°C at the heating rate 5°C min 1. The
curves reveal that pyrolysis of milkweed undergoes two reactions subsequent to moisture loss: slow, endothermic decomposition between 100 and
300°C results in the larger weight loss (av. 71% for the whole and extracted
milkweed), while high-temperature (above 300°C) decomposition accounts
for an additional small weight loss of about 10%.
149
G.C. Galletti et al./ J. Anal. Appl. Pyrolysis, 24 (1992) 147-161
i5 •
iiO
9.51
O-
85
c
.<
s
-i5.
60
o
I--
Setko
Q
-30-
-
-45 "
-6
5
35
t0
50
Instruments
200
Inc.
350
TENP C
500
650
(Heating)
Fig. i. Thermograph of whole milkweed in nitrogen.
30-
t2.5
itO
O.
7.5
85
2.5
6O
tR
L9
-60 -
-2.5
-gO-
-7.5
35
t0
50
Setko Instruments
200
Inc.
350
TENP C
(Heating)
500
650
Fig. 2. Thermograph of extracted milkweed in nitrogen.
The T G / D T A curves shown in Figs. 3 and 4 reveal that, in air, milkweed
and extracted milkweed undergo three reactions subsequent to moisture
loss: a small exotherm is associated with the major weight loss, 58% for
milkweed and 62% for extracted milkweed, between 100 and 300°C; rapid
combustion between 300 and 450°C results in loss of most of the remaining
150
P. Gu et al./ J. Anal. Appl. Pyrolysis, 24 (1992) 147-161
IOC
0
oE -soo ~
43(
i05
3~
80
55
21e
N
(.9
I.-
-200
iO~
-300
-iC
50
Setko Inutrumnts
Inc.
30
'
200
'
350
TENP C (Heating)
500
650
Fig. 3. Thermograph of w h o l e m i l k w e e d in air.
9C,
550
iiO
C,
400
80
-9C,
250
5O
c
s
N
(D
(9
I--
O
-t8©
-27C
20
iO0|
-50 I
50
Seiko Instrulents
Inc.
|
200
i
350
TENP C (Heat tng)
-i0
!
500
650
Fig. 4. Thermograph of extracted milkweed in air.
sample mass (38% and 32%, respectively); a small weight loss is recorded at
temperatures between 450 and 600°C (av. 0.8% and 0.6%).
Figure 5 shows the effect of heating rate on /'max during pyrolysis in
nitrogen. Figure 6 plots Tmax as a function of heating rate during oxidation
of the whole and extracted milkweed. Each value represents the average of
G.C. Galletti et al./ J. Anal. Appl. Pyrolysis, 24 (1992) 147-161
151
Tm,
36O
34O
32O
3OO
i
t0
~HeatingP~telC/minJ4b
fib
60
Milkweed --+-Milkweed2~r
Fig. 5. Heating rate vs. Tm.x for whole and extracted milkweed in nitrogen.
T~
300
280
240
o
lb
3b
Heating PatelC/min.]
fib
60
Milkweed --+- Hilkveed 2~r
Fig. 6. Heating rate
vs. Tmax
for whole and extracted milkweed in air.
three trials. The relative standard deviation is around 2%. These Figures
show that while heating rate has a pronounced effect on the observed
thermal stability of milkweed under combustion conditions, pyrolysis in
nitrogen is not affected significantly by the removal of the wax. The fact
that milkweed floss possesses this natural combustion inhibitor increases its
attractiveness for commercial use. We also observed that when the heating
rate was higher than 5°C min -~, thermolysis was sufficiently rapid that
distinctions in the chemical processes, apparent at lower heating rates, were
lost. Based on this observation, all subsequent temperature-programmed
analyses were carried out at heating rates of 5°C m i n Table 1 summarizes the results of T G A analyses of eleven commercial
samples carried out under conditions identical to those used to study the
combustion of the milkweed floss. The data in Table 1 show the Tmax and
weight loss for each reaction observed for the individual commercial samples. G u m Karaya was chosen as the model hemicellulose material, because
its initial weight loss, its Tmax value and its overall thermal profile matched
that of milkweed most closely. This conclusion is supported by data shown
in Table 2, which summarize the T G A data of whole and extracted
152
P. Gu et al. / J. Anal. Appl. Pyrolysis, 24 (1992) 147 161
TABLE 1
TGA percentage weight-loss summary for samples of hemicellulose, cellulose and lignin in
air a
Sample
Cellulose
Lignin AT
Gum Damar (dewaxed)
Gum Damar (whole)
Gum Arabic
Gum Locust Bean
Gum Tragacanth
Gum Accroides
Gum Mastic
Gum Elemi
Gum Karaya
Tmaxl (°C)
% wt. loss
100 300°C
300-410°C
410-600°C
6
14
21
23
47
54
54
16
21
30
54
85
42
57
55
17
28
18
19
58
59
12
11
43
24
24
32
19
28
63
25
5
23
333
397
476
318
466
285
437
494
400
371
277
a Celluloseand all hemicellulosesamples were obtained from Sigma; Lignin AT was supplied
by Westvaco, Charleston Heights, SC. All samples were used as received except for grinding
for ease of weighing. Data represent the average of one trial.
milkweed. The thermal behaviors of these materials do not lend themselves
to any simple correlation with the chemical content of milkweed. However,
the data suggest that the initial combustion reactions involve primarily
hemicellulose, and that the stability o f cellulose is intermediate between the
hemicellulose component and the lignin constituent [5]. This is illustrated in
Fig. 7, which is a composite of the individual T G A curves for milkweed,
extracted milkweed, gum Karaya, cellulose and the Kraft lignin. F r o m this
composite it is clear that in the first thermal reaction of milkweed, decomposition of a constituent similar to gum K a r a y a (and of a small a m o u n t of
cellulose) is taking place. In the second thermal stage of milkweed combustion (300-410°C) the bulk of the weight loss is due to the decomposition of
cellulose, while reactions occurring above 410°C involve only small amounts
of residual lignin.
Table 3 summarizes the T G data for the combustion of two-component
mixtures of commercial samples. The weight loss was calculated for each
component of the mixture from the T G data obtained for each individual
component in each temperature region. It was expected that if the reaction
of each component is independent of the other, the difference between the
calculated weight loss and the observed weight loss will be within experimental error of the instrumentation, for multiple trials. There is some
ambiguity about the additive behavior of gum K a r a y a and lignin, and gum
K a r a y a and cellulose during combustion, but the data are conclusive in
G.C. Oalletti et al. / J. Anal. Appl. Pyrolysis, 24 (1992) 147 161
r.)
o
P
I
r~
P
O
P
O
7
P
O
I
O
e~
.o
P
o
e.,
I
=o
z
P
I
~D
e.,
~D
153
154
P. Gu et al./ J. Anal. Appl. Pyrolysis, 24 (1992) 147-161
i00 -
itO
-50
80
50
aR
(9
. . . . .
-
.
-
-
20
. . . .
~._-~
Ce|luloso
........
~-~Ltgntn
~-
-I001
,
50
~lko
In|truNnt|
,
200
Inc.
~
~0
C
-t0
,
650
500
(~ottng)
Fig. 7. A composite T G A profile of all samples.
TABLE 3
Potential synergistic relationship of components during combustion a
Sample
% wt. loss
100-300°C
300-410°C
410-600°C
1:1 G. Karaya/lig0inobs
1 : 1 G. Karaya/lignincalc
% difference
31
34
3
30
27
3
26
32
6
1 : 1 G. Karaya/celluloseobs
1:1 G. Karaya/celluloseca~o
% difference
25
30
5
51
48
3
16
17
1
1 : 1 Lignin/celluloSeobs
1:1 Lignin/celluloseoaic
% difference
24
10
14
52
63
!1
24
27
3
3: 1 Lignin/celluloseobs
3: 1 Lignin/celluloseca]c
% difference
22
12
10
43
53
i0
31
35
3
3:1 Lignin/G. Karayaobs
3:1 Lignin/G. Karayac.lc
% difference
26
24
2
36
35
1
32
38
6
a
Air flow 50ml m i n - t ; heating rate 5 ° C m i n 1. Data represent average of three trials.
G.C. Galletti et al. [ J. Anal Appl. Pyrolysis, 24 (1992) 147-161
155
showing that there is no additive relationship between lignin and cellulose
under these reaction conditions.
Wiedmann et al. [6] have shown that the pyrolytic decomposition of a
sample can be studied independently from loss of moisture, decomposition
and combustion by using a two-step heating procedure. Subsequent to
moisture removal, the milkweed floss is subjected to two thermal processes
in the different atmospheres. Milkweed was heated from 25 to 325°C,
maintained at 325°C for 30 min, then cooled to 200°C in nitrogen. Nitrogen
is used to achieve complete volatilization [7]. By changing from nitrogen to
air, and heating from 200 to 600°C, the milkweed is combusted. Using this
method, the observed data for whole milkweed, extracted milkweed and the
commercial samples for the two processes were collected. Using the thermal
behavior of a wide variety of commercial samples in the sequential process,
we calculated a theoretical percentage composition value for the milkweed.
Table 4 compares the observed pyrolysis and combustion data with the
theoretical weight loss of a sample consisting of the analytical composition
of milkweed. When the analytical composition of milkweed is used to
compute the expected weight loss for the pyrolysis and combustion reactions, there is good agreement with the actual observed weight loss. This
indicates that under the conditions which separate the thermal reaction into
a simpler system, additivity of the thermal reactions is observed.
Figure 8 shows the FT-IR spectra for the pyrolysis of milkweed at 300°C
[8]. The main degradation products were H20 and CO2. The other organic
volatile products were identified as formic acid, acetic acid and methanol.
The evolution of gas from milkweed with time was shown in Fig. 9 as
TABLE 4
Synergistic relationship of milkweedcomponentswith pyrolysis and combustion a
Sample
Milkweed (whole)obs
Milkweed (extracted)obs
Celluloseobs
Lignin ATob S
Gum Karayaobs
[55% cellulose
18% lignin AT
24% hemicellulose
(as gum Karaya)]catc
% wt. loss
Pyrolysis
Combustion
68
70
89
24
52
23
23
l0
71
27
66
25
a Reactioncarried out using sequentialheating/hold and cool in nitrogen, then heated in air.
Data represent average of three trials.
156
P. Gu et al. / J. Anal. Appl. Pyrolysis, 24 (1992) 147-161
ZOO. 2 0 :iT
Y
,.tha.o,
~ Formic Acid
~t
Acetic Acid
C02
co. 30
i
i
i
2@00
2400
2200
I
2000
!
i
i
|
i
1BOO
laOO
1400
1200
1000
oil "~
Fig. 8. A typical FT-IR spectrum of evolved gases from milkweed in nitrogen (300°C).
~so~B~ce
C02~s~r~ance
0.0'15
0,08
0.015
0.01
0.04
0.005
0.02
0
10
lOOC -
20
325C
uUZl~2~o0/cml t
-z-
Formic AcidlllO6/cmJ
30,.,.
~
aml.u~.,
50
60
0
70
isothermal 30 mln.
~25C - 200C I
.-F- Acetic Acidlfl/~/cml
~
MethanolllO34/cm)
Fig. 9. A composite IR spectrum of gas absorbance vs. time from milkweed in nitrogen.
absorbance vs. time at: 2358cm -] (CO2); 1178cm -] (acetic acid);
l l 0 7 c m -] (formic acid); 1032cm -1 (methanol) [9]. The maximum CO2
evolution occurred in about 30 min, at 325°C. After about 35 min (325°C)
the other gases evolved. These data seem to suggest that the milkweed
degraded in about 20 min (230°C) as CO2 evolution approached its maximum; the other volatile products evolved later and almost simultaneously.
157
G.C. Galletti et al. / J. Anal. Appl. Pyrolysis, 24 (1992) 147-161
MILKWEED
CELLULOSE
HEHICELLUL~E
LIGNIN
7
i
2400
i
2004]
i
i
l
l
1BOO
Ie~O0
1400
1200
l
~n "i
10~0
Fig. 10. A composite FT-IR spectrum of gases evolved from milkweed, Cellulose, hemicellulose and lignin in nitrogen (300°C).
Figure 10 is a composite of the typical FT-IR spectra of milkweed,
cellulose, gum Karaya hemicellulose and lignin AT. All the samples exhibit
CO2 formation, but formic acid formation is characteristic of cellulose,
acetic acid formation of hemicellulose, and methanol formation of lignin.
The most likely source of CO2 at low temperature is the decarboxylation of
uranic acids in hemicellulose [ 10]. One source of acetic acid is hydrolysis of
acetyl groups of hemicellulose [10]. The possible source for methanol is the
degradation of syringyl and guaiacol units in lignin [10]. The general
conclusions from the analysis of the organic volatile products of milkweed
(in nitrogen, 300°C) are summarized below:
milkweed
{
~ formic acid ( , from cellulose
~ acetic a c i d , , hydrolysis of hemicellulose
methanol ( , from lignin
Figure 11 shows the pyrolysis portion of the sequential thermolysis of
milkweed floss. Cellulose, hemicellulose and lignin all evolved CO2 by a
simple reaction mechanism. From the area under the absorbance curve,
milkweed evolves large amounts of CO2, while lignin and cellulose evolve
small amounts of CO2.
Figures 12-14 show that in nitrogen, the volatile organic products
evolved from commercial samples compared well with the profile of milkweed. The ratio of the absorbance area from milkweed/cellulose was 41%
158
P. Gu et al./ J. Anal. Appl. Pyrolysis, 24 (1992) 147-161
C02 t~number-2360/crn
Y2-MIIkweed, HemJcelluJoBe
Absorbance
Alosorbance
0.014
0.'1
0.012
9
0.08
0.01
o
0.008
0.06
0.006
0.04
0.004
0.002
0.02
0
'
0
I0
I
~
30__ ~,n,~__
lOOC - 3E~C /
Mill{~ea
~
-
=
50
3251: Iootlleeml 30 rain.
Cellulose - t - Hemieellulose
Fig. l 1. A composite IR spectrum of
hemicellulose and lignin in nitrogen.
CO 2
0
~
~
+
70
- ~C
Lignin
absorbance vs. time from milkweed, cellulose,
Formic Acid wBvenumbee-llO7/cm
Absorbance
0.0'12
0.0'1
0.008
0.006
0.004
0.002
0
:,T_-:_-:ZZZ?ZZZZZZ. ZZL.__.
0
I
t0
lOOC- 3 ~
20
I
~
a
....~- t,,,i..,,ZO_
t
50
3~5C Ioothoemol30 mln.
• Cellulose
•
60
70
~I~'5C- 200Cl
Milkweed
Fig. 12. A composite IR spectrum of formic acid absorbance vs. time from milkweed and
cellulose in nitrogen.
for formic acid, from milkweed/hemicellulose was 23.8% for acetic acid and
from milkweed/lignin was 22.4% for methanol. Compared with the chemical
composition analyzed by Natural Fiber Corp., these results are not in very
good agreement. However, these analyses did confirm the previously observed additive behavior. The thermal reactions of lignin and hemicellulose,
159
G.C. Galletti et al./ J. Anal. Appl. Pyrolysis, 24 (1992) 147-161
Acetic Acid vmvenumber-l175/cm
Absorbance
0.1
0.08
0.06
0.04
0.02
0
0
J , J , , =
1 T , l w v
a . , l
i , , ; , , ,
, , , , -
to
IOOC - 325C
I
- - -
20
so
I
•
325C Isothermal 30 rain.
Hemicellulose
so
70
3p-nSC - 200C I
= Milkweed
Fig. 13. A composite IR spectrum of acetic acid absorbance vs. time from milkweed and
hemicellulose in nitrogen.
Methanol wevenumbee-lO34/cm
Absorbance
0.007
0.006
0.005
0.004
0.003
0.002
O.OOi
0
j
I
0
I
tO
lOOC - 325C
20
I
'
.--~"
'
imlnu~i-~_,
325C isothermal 30 min.
•
Lignin
n
50
60
70
~25C - 200q
Milkweed
Fig. 14. A composite IR spectrum of methanol absorbance vs. time from milkweed and
lignin in nitrogen.
and cellulose and hemicellulose are additive, but the thermal reaction of
lignin and cellulose is not additive; that is, there are different thermal
reactions for the lignin and cellulose in the same thermal stage. F r o m the
absorbance data we calculated the percentages of hemicellulose, cellulose and
lignin. The hemicellulose value (23.4%) is very close to that obtained from
160
P. Gu et al. / J. Anal. AppI. Pyrolysis, 24 (1992) 147-161
C02 t~avenul~er-23EO/cm
Y2-Ugnln
~s0~f:e
~s0Nnce
0
2
.5 ~
0
2
.
2
1.5
0.15
1
0.1
0.05
05
0 cz=~z-_T
200
250
300
Milkweed
350
400
450
Temperature[CI
-+-. Cellulose
500
-.*- Hemicellulose
550
,"J
600
--a- Lignin
Fig. 15. A composite IR spectrum of CO2 absorbance vs. temperature from milkweed,
cellulose, hemicellulose and lignin in air.
Natural Fiber Corp. (24%), the percentage of cellulose (41.0%) is smaller,
and the lignin value (24.8%) is larger than the reported values (55% and
18%, respectively). This seems to suggest that cellulose evolves small
amounts of methanol which are not accounted for. Similarly, lignin may be
evolving formic acid, which is not accounted for in this study under the
experimental conditions. The above results were corroborated by the work
of Fenner and Lephardt [ 11], which was a study of the thermal decomposition of Kraft lignin.
The IR spectrum of the gases evolved during combustion of whole
milkweed floss showed that the major decomposition product is CO2.
Because all the volatile matter was evolved in the pyrolysis step, the
milkweed was completely carbonized in the combustion step. Evolution of
CO2 (Fig. 15) indicates that the mechanism of the combustion reaction was
different for each material. The milkweed floss undergoes at least two
reactions; one occurred at about 350°C and the other at 500°C. Cellulose
began to decompose at 330°C; it evolved CO2 between 350°C and 530'C.
This illustrates that at low temperatures (330-400°C) the weight loss of
milkweed is due to destruction of cellulose. The lignin and hemicellulose
only undergo one reaction evolving CO2, and degrade at about 410 and
510°C, respectively. This can be explained as the thermal reaction (410°C)
of milkweed derived from lignin, while at the high temperature (510°C) the
hemicellulose and cellulose in the milkweed decomposed simultaneously.
SUMMARY
This study has shown that the thermal stability of milkweed floss is of the
same order as its least stable component, hemicellulose. Cellulose has a
higher degradation temperature and degradation onset temperature, and
although lignin begins to degrade at a lower temperature than milkweed
G.C. Galletti et al. / J. Anal. Appl. Pyrolysis, 24 (1992) 147 161
161
floss, it degrades more slowly as the temperature rises. Gum Karaya hemicellulose was chosen as model hemicellulose in milkweed by the thermal
analysis results of the commercial samples. For the commercial samples,
gum Karaya hemicellulose and lignin, gum Karaya hemicellulose and
cellulose display an additive reactivity relationship, but lignin and cellulose
do not. These behaviors are also confirmed in the processes which form
organic volatile products measured using TG-FT-IR.
A sequential procedure proved to be a better way of carrying out the
milkweed analysis, because it separated the thermal reactions into two
simpler processes. By using FT-IR, the products for milkweed pyrolysis
were identified. The chemical composition of milkweed was correlated with
the various organic volatile products.
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