R. YU et al.: EFFECT OF A SYNTHESIS FORMULATION ON THE THERMAL PROPERTIES OF POLYURETHANE
215–220
EFFECT OF A SYNTHESIS FORMULATION ON THE THERMAL
PROPERTIES OF POLYURETHANE
VPLIV OBLIKOV ANJA SINTEZE NA TERMI^NE LASTNOSTI
POLIURETANA
Ruien Yu
1,2,3*
, Yanfei Kou
1,3
, Lina Cai
2
, Changxin Fan
2
, Yanjun Shao
3
, Xiaoyan
Zhang
2
, Xijing Zhu
1,3
1
School of Mechanical Engineering, North University of China, No. 3 Xueyuan Road, Taiyuan 030051 Shanxi, P. R. China
2
Shanxi Transportation Technology Research & Development Co., Ltd, No. 27 Wuluo Road, Taiyuan 030032 Shanxi, P. R. China
3
Shanxi Key Laboratory of Advanced Manufacturing Technology, North University of China, Taiyuan 030051 Shanxi, P. R. China
Prejem rokopisa – received: 2019-04-30; sprejem za objavo – accepted for publication: 2019-12-23
doi:10.17222/mit. 2019.092
In this paper, the effect of a synthesis formulation on the thermal properties of polyurethane was studied. Based on the design of
the polyurethane hard-segment content and isocyanate index, 10 kinds of TPU were synthesized with the prepolymer method.
The molecular weight of TPU was determined with GPC, the thermal behavior of TPU was studied with differential scanning
calorimetry (DSC) and a thermal gravimetric analysis (TGA), and the molecular structure was studied with Fourier-transform
infrared spectroscopy (FTIR). The results showed that TPUs with different synthetic formulations exhibited different heat
absorptions during differential scanning calorimetry, and the heat resistance of the TPU with 20 % hard segment was better than
the TPU with 40 % hard segment.
Keywords: polyurethanes, synthesis, thermal properties, structure
V pri~ujo~em ~lanku avtorji opisujejo {tudijo vpliva oblikovanja sinteze na termi~ne lastnosti poliuretana. Na temelju izbire
vsebnosti trdega segmenta v poliuretanu in izocianatnega indeksa, so avtorji s prepolimerno metodo sintetizirali 10 razli~nih vrst
termoplasti~nega poliuretana (TPU). Molekularno maso TPU so dolo~ili z gelsko izklju~itveno kromatografijo (GPC, angl.: Gel
Permeation Chromatography), termi~ne lastnosti TPU so {tudirali z diferencialno vrsti~no kalorimetrijo (DSC, angl.:
Differential Scanning Calorimetry) in termogravimetri~no analizo (TGA). Molekularno strukturo TPU pa so {tudirali s
Fourierjevo transformacijsko infrarde~o spektroskopijo (FTIR). Rezultati {tudije so pokazali, da TPU, sintetiziran na razli~ne
na~ine, ka`e razli~no absorpcijo toplote in da je termi~na stabilnost TPU z ve~jim dele`em trdega segmenta (40 %), ve~ja kot
tista z manj{im dele`em trdega segmenta (20 %).
Klju~ne besede: poliuretani, sinteza, termi~ne lastnosti, struktura
1 INTRODUCTION
Polyurethane is a polymer compound which has a
repeating carbamate group (-NHCOO-) on its molecular
backbone.
1,2
Different synthesis substances and different
chain-extension reactions generate a wide range of
polyurethane.
3
So, it can be used to produce plastic,
4
hard and soft foam,
5
elastomers,
6
fibers,
7
adhesives,
8
biomedical materials
9
and coatings.
10
In the field of
packaging, polyurethane is mainly used for two or more
layers of plastic film between the bonding, especially dry
composite film, mainly for drugs,
11
food
12
and other
goods.
13
In addition, it can also be used for paper and
plastic composites,
14
mainly stickers advertising paper or
carton packaging.
In recent years, more and more scholars have devoted
themselves to the modification of polyurethane and
development of a waterborne polyurethane. S. K. Dogan
et al.
15
studied and obtained PLA/TPU composites with a
thermal response and shape-recovery properties. P. Alagi
et al.
16
transformed castor oil and soybean oil into
polyols to prepare TPU and obtain a modified TPU with
excellent toughness and transparency. W. Q. Lei et al.
17
found three kinds of representative chain extenders to
prepare polyurethane elastomers. The effects of chain
extenders on the morphology and thermal properties of
polyurethane were studied. Y . Xiao et al.
18
investigated
the effect of a microphase separation on the crystalli-
zation of the soft segment of green waterborne poly-
urethane. It was found that the phase separation hindered
the crystallization of the soft-segment phase and that the
high phase separation lowered the crystallization
capacity, crystallinity and the amount of crystals of the
soft segment.
Polyurethane prepolymer is produced by reacting
isocyanate and polyether polyol in the initial phase, and
finally converted to a polymer with polymer chains using
the chain extension and curing processes; thus, the
chemical structure and properties of polyurethane are
changed. Our group was engaged in the research of poly-
urethane-modified asphalt.
19,20
Thermoplastic polyure-
thane was synthesized with the prepolymer method to
modify the asphalt. The small-molecule polyurethane
prepolymer produced during the initial stage of the
Materiali in tehnologije / Materials and technology 54 (2020) 2, 215–220 215
UDK 620.1:539.4.016:678.664 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 54(2)215(2020)
*Corresponding author's e-mail:
yuruien@nuc.edu.cn (Ruien Yu)

reaction between isocyanate and polyester polyol can
form a uniform system and has good compatibility with
asphalt.
21
However, with the process of chain extension
and curing, polyurethane prepolymers gradually produce
polymers with high molecular chains through chemical
reactions.
22
The changed chemical structure and thermal
properties have different modification effects on the base
asphalt. In this study, the effect of the polyurethane-
synthesis formulation on the thermal properties was
studied by controlling the synthesis process.
2 EXPERIMENTAL PART
2.1 Materials
4,4’-diphenylmethane diisocyanate (a molecular
weight of 250), polytetramethylene ether glycol a
molecular weight of 2000), 1,4-butanediol (a molecular
weight of 90) were provided by Shanghai Aladdin
Biochemical Science & Technology Co., Ltd. The
structure of the three raw materials is shown in Table 1.
2.2 Experiment design and calculation
The hard-segment content in the polyurethane
chemistry is the mass fraction of the hard segment in the
TPU, and the mass of the hard segment is the sum of
diisocyanate and small molecular diol. The isocyanate
index is the ratio of the equivalent number of diiso-
cyanate to the sum of the equivalent number of oligo-
meric polyol and small molecule diol, namely the
NCO/OH ratio. The three raw materials are bifunctional
groups and the NCO/OH is also the molar ratio.
C
WW
WWW
h
id
idg
=
+
++
(1)
r
n
n
W
M
W
M
W
M
i
i
d
d
g
g
==
+
NCO
OH
(2)
Based on Equations (1) and (2), the following is ob-
tained:
C
r
W
M
M
rM
M
W
rM
M
W
r
h
g
g
i
i
d
d
i
d
d
=
⋅⋅++
⋅ ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ ⋅
⋅
+
⎛ ⎝ ⎜ ⎞ ⎠ ⎟⋅+
⋅
1
1
M
M
W
i
g
g
+
⎛ ⎝ ⎜ ⎞ ⎠ ⎟ ⋅ 1
(3)
In Equations (1–3), W
i
, W
d
, and W
g
are the weights of
MDI, BDO and PTMEG, respectively; M
i
, M
d
and M
g
are
the molecular weights of MDI, BDO and PTMEG,
respectively; C
h
is the hard-segment content of polyure-
thane; r is the isocyanate index.
In this study, the hard-segment content is 20 % and
40 % while isocyanate index NCO/OH is controlled at
0.95, 0.98, 1, 1.02 and 1.05. The mass values of BDO
and MDI are calculated on the basis of 100-g PTMEG.
The formulation of the 10 kinds of TPU to be syn-
thesized is shown in Table 2.
Table 2: Synthesis formulation of TPU
No. Ch rW
g/g Wi/g Wd/g
1
#
20 % 0.95 100 21.39 3.607
2
#
20 % 0.98 100 21.58 3.426
3
#
20 % 1 100 21.69 3.309
4
#
20 % 1.02 100 21.8 3.195
5
#
20 % 1.05 100 21.97 3.032
6
#
40 % 0.95 100 51.606 15.056
7
#
40 % 0.98 100 52.05 14.62
8
#
40 % 1 100 52.325 14.337
9
#
40 % 1.02 100 52.598 14.064
10
#
40 % 1.05 100 52.991 13.669
2.3 Preparation of polyurethane
TPU was synthesized with the prepolymer method.
The detailed operation is as follows: we put the weighed
PTMEG in a 500-mL, dry, three-necked flask, in a
110 °C oil bath (model DF-101S, Shaanxi Taikang Bio-
technology Co., Ltd.), heated it with a 20 mL/min flow
of dry nitrogen for protection, stirred it (R30, Shanghai
Fluko Technology Development Co., Ltd.) at a speed of
200 min
–1
and dehydrated it for 90 min. The temperature
of the system was then reduced to 80 °C. The MDI,
which had been weighed and dried in an oven at 80 °C,
was added to the system and kept at constant temperature
and speed for 2 h. When the viscosity of the system in-
creased, producing blue light, the polyurethane pre-
216 Materiali in tehnologije / Materials and technology 54 (2020) 2, 215–220
R. YU et al.: EFFECT OF A SYNTHESIS FORMULATION ON THE THERMAL PROPERTIES OF POLYURETHANE
Table 1: Raw materials of TPU and their molecular structures
Chemical name Abbreviation Structure
4,4’-diphenylmethane diisocyanate MDI
Polytetramethylene ether glycol PTMEG
1,4-butanediol BDO

polymer was prepared successfully. The temperature of
the system was then reduced to 50 °C; the BDO was
weighed; the stirrer’s speed was increased to 400 min
–1
;
the stirring continued for 10 min; the temperature of the
system was raised to 100 °C; the synthesized polymer
was poured into the container coated with Teflon; and,
finally, the required TPU samples were successfully
synthesized after the curing at 100 °C. In the case of high
hardness and a high isocyanate index, the viscosity of the
system was increased rapidly after adding BDO. In order
to facilitate the discharge, the mixture was quickly mixed
and the stirring time was appropriately adjusted accord-
ing to the viscosity.
2.4 Characterization
The molecular weight and molecular-weight distri-
bution of TPU were measured with Waters 515-2414 gel
permeation chromatography (Waters, USA), with tetra-
hydrofuran as the mobile phase, a flow rate of 1 mL/min,
a column temperature of 35 °C and a detector tempe-
rature of 35 °C. The samples were dissolved with
chromatographic tetrahydrofuran.
The endothermic, exothermic and crystalline be-
havior of the TPU was tested using a DSC 200F3
differential scanning calorimeter (NETZSCH, Germany).
The nitrogen protection was carried out at a flow of
60 mL/min, with the purge gas also being nitrogen, at a
flow of 30 mL/min, in a temperature range between
–100 °C and –300 °C and a heating rate of 10 k/min. A
40-μL standard alumina crucible was used and the
sample amount was 5–10 mg.
The loss of the TPU in the above temperature range
was tested using a TG 209F3 thermogravimetric analyzer
(NETZSCH, Germany). The nitrogen protection was
carried out at a flow of 20 mL/min, while nitrogen purg-
ing took place at a flow of 30 mL/min in a temperature
range of 100–800 °C and at a heating rate of 10 k/min.
The sample amount in the alumina crucible was
10–15 mg.
The molecular structure of the TPU was determined
with FTIR-8400S Fourier transform infrared spectros-
copy (Shimadzu, Japan) with a scanning range of
400–4000 cm
–1
, a scanning frequency of 25 and a reso-
lution of 16.0 cm
–1
.
3 RESULTS AND DISCUSSION
3.1 Molecular weight
TPU is a linear molecule; its molecular weight is the
key indicator of its properties. In the synthesis of TPU,
the NCO/OH ratio (the r value) plays an important role
in controlling the molecular weight. When r  1.0, the
molecular weight of TPU increases with the increase in
the r value. When r = 1, the TPU molecular weight
reaches the highest value. When r  1, and the r value
continues to increase, the TPU molecular weight
declines. Table 3 shows the molecular weight and
polydispersity index of the TPU. The molecular weight
of the TPU does not increase first and then decrease as
the r value increases, but the maximum molecular weight
of TPU is at r = 1.05. The purity of raw materials,
calculation and measurement error, mixing, environ-
mental humidity and many other factors very easily
make the actual r value deviate from the designed value;
so they are the reasons for the deviation of the actual
molecular weight and theoretical molecular weight.
23
The content of hard segment increased, the content of the
-NCO group increased while the TPU molecular weight
decreased. In addition, -NCO continued to react with
moisture, urea and carbamate to form urea, biuret and
allophanate, respectively. TPU branched or crosslinked
and could not be dissolved in THF, so we failed to
measure the molecular weight of TPU samples 6#, 8#
and 10#. The occurrence of side reactions in the process
of polymer synthesis was also a cause for the molecu-
lar-weight dispersion of the polymer. The larger the PDI,
the wider was the molecular-weight distribution.
Table 3: Molecular weight and the polydispersity index of TPU
samples
TPU Ch r ###
1
#
20 % 0.95 70,386 147,839 2.1
2
#
20 % 0.98 58,104 110,647 1.904
3
#
20 % 1 75,597 220,379 2.915
4
#
20 % 1.02 105,264 277,144 2.633
5
#
20 % 1.05 414,826 1,046,191 2.522
r6
#
40 % 0.95 – – –
7
#
40 % 0.98 51,669 281,597 5.45
8
#
4 0 %1–––
9
#
40 % 1.02 76,825 255,620 3.327
10
#
40 % 1.05 – – –
3.2 Thermal behavior
In terms of morphology, TPU molecules have a
two-phase structure: the soft segment belongs to the
soft-segment phase and the hard segment belongs to the
hard-segment phase.
24
When the hard content is 20 %,
the soft segment phase appears to be crystalline, while
the hard segment is amorphous. In Figure 1a,t h e
glass-transition temperature of the TPU occurs between
–63 °C and –60 °C. The endothermic peak appears at
10–20 °C, being the result of the proximity order and the
amorphous hard-segment melting; the melting peak
gradually decreases and the heat absorption decreases
with the increase in the molecular weight of the TPU. In
Figure 1b, the hard-segment content increases and
crystals appear in it. The endothermic melting peaks at
(170, 190 and 220) °C indicate the remote-ordered hard
segment and the crystalline-state hard-segment phase,
while the heat absorption of the melting peaks increases
with the increase in the isocyanate index. The formation
of urea and allophanate in the reaction process makes the
melting peaks in the hard-segment crystallization zone
R. YU et al.: EFFECT OF A SYNTHESIS FORMULATION ON THE THERMAL PROPERTIES OF POLYURETHANE
Materiali in tehnologije / Materials and technology 54 (2020) 2, 215–220 217

different. Compared with 20 % hard-segment content of
TPU samples 1#–5#, the content of the proximity-or-
dered hard segments is reduced and the amount of heat
absorption is gradually reduced, while the peak is moved
forward. The MDI/BDO aromatic-ring hard segment is
crystalline; the high-degree crystallization of the hard
segment is conducive to the purification of the soft-seg-
ment phase and the glass-transition temperature of the
TPU is reduced.
3.3 Thermal degradation
The heat resistance of the polymer is measured based
on its thermal-decomposition temperature. Figures 2 and
3 show the TGA and DTG curves for the TPU, respect-
ively. The TPU samples with two different hard seg-
ments are all subjected to two degradation processes.
The TPU with 20 % hard segment is in temperature
ranges of 260–375 °C and 375–460 °C, while 40 % hard
segment is in temperature ranges of 260–325 °C and
325–460 °C. The degradation of the first stage is the
decomposition of the carbamate in the hard segment. The
hard-segment content increases, the hard-segment phase
crystallizes, the degradation difference between this
stage and the second stage of 40 % hard-segment content
of the TPU is more obvious than that of 20 % hard-
segment content of the TPU, and the degradation rate
reaches the maximum at 307 °C. Urea, biuret and
allophanate ar produced during the preparation of the
TPU with a different isocyanate index. The decompo-
sition temperature of these groups is different from the
decomposition temperature of the carbamate group,
causing differences in the degradation temperature and
degradation rate. The second stage is the degradation of
oligomer diol; the highest degradation rates of the two
types of the TPU samples were all in the vicinity of
415 °C. Finally, the degradation included the carbon
218 Materiali in tehnologije / Materials and technology 54 (2020) 2, 215–220
R. YU et al.: EFFECT OF A SYNTHESIS FORMULATION ON THE THERMAL PROPERTIES OF POLYURETHANE
Figure 1: DSC curves of the PU samples
Figure 2: TGA curves of the PU samples

chain and aromatic ring; the benzene-ring content of
40 % hard content of the TPU is large and difficult to
degrade and the final residue is greater than that of 20 %
hard-segment content of the TPU. Considering (10, 20,
30 and 40) % TPU degradation, the heat resistance of
20 % hard-segment content of the TPU is better than that
of 40 % hard-segment content of the TPU, and it is
known from the TGA curve in Figure 3 that TPU sample
5# is the best.
3.4 FTIR analysis
Polyurethane molecules are often the existing struc-
tures such as urethane, hydrocarbyl, phenyl, ester, ether,
hydroxyl, isocyanate, ureido, amide, biuret and
allophanate. Figure 4 shows the infrared spectrum of the
TPU. The imino group has its absorption peak at
3250–3500 cm
–1
and the hydroxyl group also has its
absorption peak in the region of 3400–3500 cm
–1
.
However, the peak is rarely found for the hydroxyl group
of polyurethane. The peak at 3325 cm
–1
is mainly imino
(N-H) and another absorption peak of the imino group is
found at 1527 cm
–1
. The absorption peak at 2360 cm
–1
indicates that the TPU contains isocyanate groups
(-NCO), showing that the prepolymer is not completely
extended during the synthesis of the TPU. The broad
absorption peak near 2900 cm
–1
indicates the vibration of
the hydrocarbon group (-CH
2
and -CH
3
)a n da t
1300–1500 cm
–1
, there is also the absorption peak of the
hydrocarbon group. The absorption peaks at 1728 cm
–1
and 1604 cm
–1
are mainly due to the carbonyl (C=O)
vibrations in the urethane group, ureido group or amide
group, and they are characteristic of synthetic poly-
urethane. At 1226 cm
–1
, there is the absorption peak of
the C-O group from the ester group and the C-O group of
the hydroxyl group, connected with carbochain. At
1110 cm
–1
, there is the absorption peak of the ether group
(-O-) from the urethane group. The multiple absorption
peaks in the range of 500–900 cm
–1
indicate the
vibrations of the benzene ring from isocyanate.
4 CONCLUSIONS
Based on the design of a polyurethane hard segment
and isocyanate index, 10 kinds of TPU were synthesized
using the prepolymer method. With the effect of purity
of the raw materials and the environmental humidity in
R. YU et al.: EFFECT OF A SYNTHESIS FORMULATION ON THE THERMAL PROPERTIES OF POLYURETHANE
Materiali in tehnologije / Materials and technology 54 (2020) 2, 215–220 219
Figure 3: DTG curves of the PU samples
Figure 4: FTIR curve of TPU

the synthesis process of TPU, the molecular weight of
TPU deviates from the theoretical value of the isocyanate
index, and the side reaction of the NCO group makes
TPU branched and crosslinked so that the molecular
weight of some samples cannot be measured. The TPU
hard-segment content is different; the hard-segment and
soft-segment phases are different, resulting in the soft
and hard segments interacting with each other; the heat
absorption is also found to be different with DSC. From
the perspective of the TPU molecular weight, the heat
resistance of the TPU with 20 % hard-segment content is
better than that of 40 % hard segment of TPU.
Acknowledgment
The authors acknowledge the financial supports
provided by the National Natural Science Foundation of
China (Grant No. 51902294), Applied Basic Research
Project of Shanxi Province (Grant No. 201901D211206),
Project in Scientific Innovation of Colleges and
Universities of the Shanxi Province of China (Grant No.
201802073) and the Opening Foundation of the Shanxi
Key Laboratory of Advanced Manufacturing Technology
(Grant No. XJZZ201808).
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