J. Biomedical Science and Engineering, 2009, 2, 245-253
doi: 10.4236/jbise.2009.24038 Published Online August 2009 (http://www.SciRP.org/journal/jbise/
JBiSE
).
Published Online August 2009 in SciRes. http://www.scirp.org/journal/jbise
Preparation and properties of cast polyurethane
elastomers with molecularly uniform hard segments
based on 2,4-toluene diisocyanate and
3,5-dimethyl-thioltoluenediamine
Xiao-Dong Chen*1,2, Nan-Qiao Zhou1, Hai Zhang2
1National Engineering Research Center of Novel Equipment for Polymer Processing, The Key Laboratory of Polymer Processing
Engineering Ministry of Education, South China University of Technology, Guangzhou, 510640, China; 2GuangZhou SCUT Bestry
Technology Joint-stock Co., Ltd., South China University of Technology, Guangzhou, 510640, China
Received 16 December 2008; revised 2 March 2009; accepted 5 March 2009.
ABSTRACT
A series of three cast polyurethane elastomers
were prepared from 2,4-toluene diisocyanate
(TDI) and 3,5-dimethyl-thioltoluenediamine (D
MTDA) chain extender, with polyethylene adi-
pate (PEA), polyoxytetramethylene glycol
(PTMG) and polycaprolactone (PCL) soft seg-
ments. The polyol molecular weights em-
ployed was 2000g/mol. The polyurethane
elastomers were characterized by an elec-
tronmechanical universal testing machine, an
Akron abrasion loss tester, a LX-A Shore du-
rometer, a rebound resilience equipment and a
Dynamic- Mechanical analyzer. In addition,
fractured surface of the polyurethane elas-
tomers was investigated by a field emission
scanning electron microscopy (SEM). The test
results showed the PCL based elastomer ex-
hibits the excellent tear and stress-strain
properties that polyester based elastomers
offer, while retaining superior compression set
and resilience similar to polyether based elas-
tomers. The static and dynamic properties of
the PCL based elastomer were more suitable
for dynamic applications. The SEM micro-
graphs of all polyurethane samples indicated
the existing of the microphase separation
structure. Particles of the dispersed phase
formed by the hard phase and crystalline part
of the soft phase grows bigger with the in-
creasing crystallinity of the soft segments. The
hard domains are irregular shapes and with
the sizes of a few micrometers.
Keywords: Soft Segment; Structure; Cast Polyure-
thane Elastomer; Properties
1. INTRODUCTION
In the recent decades, polyurethane elastomers have
been successfully employed in a growing variety of uses
and applications, due to their broad range of outstanding
properties [1,2,3,4,5,6,7,8,9,10,11,12]. The polyurethane
elastomers are composed of short, alternating polydis-
perse blocks of soft and hard segments. The soft seg-
ments with a low glass transition temperature are formed
generally from polyethers or polyesters, generally of mo-
lecular weight 400-5000. The rigid, polar hard segments
with a high glass transition temperature are based on
diisocyanates and low-molecular-weight chain extenders
[6,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27].
Because there exists a degree of thermodynamic im-
miscibility between the hard urethane segments and the
soft polyol segments, polyurethane elastomers exhibit
microphase separation, which could result in a structure
that can be considered as hard segment domains dis-
persed in a soft segment matrix [6,13,21,25,27,28,29,30,
31,32]. The resultant two-phase micro-domain structure
exhibited by polyurethane elastomers is responsible for
their superior mechanical properties. Usually, micro-
phase separation is incomplete and the hard and soft
segment phases still contain certain amounts of the other
segment. The mean domain size increases from 10 to 20
nm as the hard segment content increases and the shapes
of hard domains are in the form of spheres 5-20 nm, or
long needles 5 nm thick and 50-300 nm long [6,14].
The two-phase micro-domain structure is greatly in-
fluenced by the molecular structure of the diisocyanate,
polyol, and chain extender [13,14,33,34], by the ratio of
hard segment and soft segment components [35], by the
average segment length employed (including molecular
weight distribution) [13], by the crosslinking density
[18], and by the thermal history of the material [36].
Processing conditions, such as temperature, can also

246 X. D. Chen et al. / J. Biomedical Science and Engineering 2 (2009) 245-253
SciRes Copyright © 2009 JBiSE
change the domain structure significantly [37]. Some
researchers have employed many characterization tech-
niques to understand the relationship between the
chemical architecture, morphology, and chemical prop-
erties [37,38]. It is well known that the size, shape, and
structure of the hard-segment and soft-segment domains
play a crucial role in determination of macroscopic
properties [34,39]. Therefore, static and dynamic proper-
ties of polyurethane elastomers can be tailored by select-
ing different diisocyanates, polyols and chain extenders,
or by simply varying the processing temperature [40].
In this study, a series of polyurethane elastomers
based on polyethylene adipate (PEA), polyoxytetrame-
thylene glycol (PTMG) and polycaprolactone (PCL)
with molecular weight of 2000 as soft segments and hard
segments based on the combination of 2,4-toluene
diisocyanate and 3,5-dimethyl-thioltoluenediamine. In
addition to general mechanical properties, resistance to
thermal degradation, abrasion and dynamic properties
were investigated, and the micro-phase structure images
of samples were observed and captured by a Field Emis-
sion Scanning Electron Microscope (FE-SEM). The re-
lationship between micro-phase structure and macro-
scopic properties was discussed. These key engineering
properties are considered essential and the obtained re-
sults will provide foundation for the formula and struc-
ture design of the compounds for various applications,
especially for high-loading dynamic applications.
2. EXPERIMENTAL
2.1. Materials
PEA was obtained form JingXing Polyurethane Co.,
Ltd. (WuXi, China). PTMG was produced by Mitsubi-
shi Chemical Co., Ltd, Nippon. PCL was purchased
from Dow Chemical, USA. The three polyols should be
dehydrated in vacuum at 100~110°C for 2 hours before
use and their values were described in detail in Table 1.
2,4-toluene diisocyanate, purchased from Qingdao
Yutian Chemical Company, was imported in original
package and used as received. The chain extender,
3,5-dimethyl-thioltoluenediamine, was purchased Albe-
marle Company and should be purified by dehydrated
in vacuum at 80 for 1 night before use. Dib℃utyltin
dilaurate (DBTDL) was acquired from Atofina Chemi-
cals.
2.2. Preparation of Polyurethane Elastomers
Traditionally, polyurethane elastomers can be synthe-
sized via a “one-shot” process or prepolymer method.
While the one-shot process is the quickest and easiest of
the manufacturing techniques, preparation via the pre-
polymer method imparts greater control over the chem-
istry of the reaction, influencing the structure, mechani-
cal properties, reactivity and processability of the fin-
ished product [41,42]. In this study, the prepolymer
method was used. The first stage involves preparation of
a prepolymer from the polyol in excess diisocyanate to
produce an isocyanate-terminated molecule. Subsequent
reaction of the prepolymer with a diol or diamine chain
extender constitutes the second stage, which produces a
multi-block copolymer.
2.2.1. First Stage: Preparation of Prepolymer
2,4-toluene diisocyanate (0.83mol, 145g) was added
into a 4-necked round bottom-boiling flask equipped
with an overhead mechanical stirring unit, a ther-
mometer and a vacuum take-off/nitrogen inlet. A polyol
(0.37mol, 740g) was melted in an oven and added to
TDI with stirring and reacted at 80 °C for 2 h under a
nitrogen atmosphere to give a polyurethane prepolymer
as a viscous liquid. And the prepolymer was examined
for NCO content by using a standard method of n-butyl
amine titration.
Table 1. Specifications of the three polyols.
Polyols Molecular structure of the polyol Hydroxyl value,
mgKOH/g
Acid value,
mgKOH/g Molecular weight
PEA HO-[-CH2-CH2-OOC-(CH2)4-COO-)n-CH2-CH2-OH56 ≤0.5 2000
PTMG HO-(-CH2-CH2-CH2-CH2-O-)n-H 56 ≤0.02 2000
PCL HO-[-(CH2)5COO-]m-R-[-OOC(CH2)5-]n-OH 56 ≤0.05 2000
Table 2. Mechanical properties of elastomers based on different backbones.
Properties PEA-TDI PCL-TDI PTMG-TDI
100% Modulus, MPa 4.05 3.76 2.3
300% Modulus, MPa 12.15 10.43 3.7
Tensile Strength, MPa 51.15 44.09 29.86
Elongation at break,％ 468 438 380
Tear Strength, KN/m 69 61 52
Compression Set, % 5.6 4 4.4
Rebound, % 32 41 44
Hardness, Shore A 77 77 78

X. D. Chen et al. / J. Biomedical Science and Engineering 2 (2009) 245-253 247
SciRes Copyright © 2009 JBiSE
2.2.2. Second Stage: Synthesis of Polyurethane
The obtained prepolymer (200g) was heated to 80 ℃
under vacuum (<2 mm Hg). The chain extender (21g)
was added to the prepolymer. The resultant mixture was
stirred at high speed for 60 seconds. If time permitting,
the mixture should be degassed (for 1-2 min) to remove
the air introduced by stirring. Then the mixture was
poured into a pre-heated mold (110). The bubbles on ℃
the surface can be removed by sweeping it with a burner
flame or with a stream of hot air, what can make the
bubbles expanded and bursted. The mold was cured in a
vented oven at 110 for 30 minutes. The polymer ℃
sheets were demolded and post-cured at an elevated
temperature (for 12-16h at 110). The parts were stored ℃
at ambient temperature for 1 month. During this period,
secondary chemical reactions should be completely and
the microstructure would become established. This is
very important for testing the dynamic properties.
2.3. Characteristics
The tensile strength and elongation at break were deter-
mined with an electronmechanical universal testing ma-
chine (INSTRON Co. LTD, Model 5566, USA). The
abrasion resistance was performed with an Akron abra-
sion loss tester. The hardness was tested with a LX-A
Shore durometer according to standard method (ISO 48-
1984). The resilience was measured by a rebound resil-
ience equipment (CJ-6A, ShangHai fourth chemical
machine factory). The dynamic mechanical analysis was
carried out in an air atmosphere by means of a
NETZSCH Instrument, Dynamic-Mechanical Analyzer
DMA242, on samples of following sizes: 2.0×5.8×
10.0mm. The tests at 10 Hz frequencies, ±2N maximum
dynamic stress, ±40μm maximum deformation ampli-
tude and the temperature range of -100～150, with a ℃
heating rate of 5/min were accomplished. Fract℃o-
graphs were observed with a Field emission scanning
electron microscopy (FE-SEM, Philips XL30 ESEM-
FEG). Samples were prepared by tearing brittle samples
(0.5mm thick) at low temperature by immerging in liq-
uefied nitrogen. All samples were coated with a layer of
gold or platinum before characterization.
3. RESULTS AND DISCUSSION
3.1. Influence of the Polyol Structure on the
Mechanical Properties of Polyurethane
Elastomers
Table 2 reports a list of some general mechanical prop-
erties of a series of the polyurethane elastomers based on
PEA, PTMG and PCL as soft segments and hard seg-
ments based on the combination of 2,4-toluene diisocy-
anate and 3,5-dimethyl-thioltoluenediamine. The PEA
based elastomer had better tensile strength and elonga-
tion, and much better tear resistance compared to the
PTMG based elastomer. However, its compression set
and resilience were inferior to the PTMG based elas-
tomer. Interestingly, the PCL based elastomer offered
very competitive stress-strain properties and tear resis-
tance when compared with the PEA based elastomer,
while significantly improved compression set and resil-
ience over the PEA based elastomer. Its ability to retain
elastic properties after prolonged compressive stresses
was as good as the PTMG based elastomer, while its
resilience performance was close to that of the PTMG
based elastomer. From the testing data shown in Table 2,
it is clear that the PCL based elastomer possesses more
balanced properties. It exhibits the excellent tear and
stress-strain properties that polyester based elastomers
offer, while retaining superior compression set and resil-
ience similar to polyether based elastomers.
3.2. Influence of the Polyol Structure on the
Phase Morphology of Polyurethane
Elastomers
The SEM method was employed to investigate the mor-
phology of fractured surface of the polyurethane elastom-
ers. As shown in the presented images (Figures 1-3), mi-
crophase separation can be observed for all polyurethane
samples tested. A continuous phase is visible in SEM
micrographs, which is created by the amorphous part of
the soft phase and the intermediate phase, i.e., the
so-called matrix. Particles of the dispersed phase are
encapsulated in that matrix. They are formed by the hard
phase and the crystalline part of the soft phase. Three
principal thermodynamic factors contribute to the for-
mation of that phase structure: mobility of hard segments,
viscosity of the system, and interactions between hard
segments [14].
The hard phase and the crystalline part of the soft
phase form the so-called domains with irregular shapes
and with the sizes of a few micrometers. The size of the
domains is mainly dependent on its content of rigid
segments and the crystallinity of soft phase, which is
clearly visible for the three samples. The soft phase is
amorphous in some cases (Figure 2). A small number of
tiny particles composed of rigid segments can be ob-
served in the fractured surface only. Crystallization of
the soft phase causes the domains bigger in case of the
PEA based elastomer (Figure 1) and PCL based elas-
tomer (Figure 3). The particle size of the dispersed
phase grows bigger with the increasing crystallinity of
the soft segments. At the same time, the intermediate
phase interface of the PCL based elastomer is much
smoother than that of the PEA based elastomer due to its
lower degree of crystallinity and less regular arrange-
ment of soft segments. It should be noticed that there are
some scratches and cracks in the surface of the PCL
based elastomer (Figure 3). We obtained an explanation
hat the PCL based elastomer had been exposed under t

248 X. D. Chen et al. / J. Biomedical Science and Engineering 2 (2009) 245-253
SciRes Copyright © 2009 JBiSE
Figure 1. Scanning electron micrograph of the PEA based elastomer.
Figure 2. Scanning electron micrograph of the PTMG based elastomer

X. D. Chen et al. / J. Biomedical Science and Engineering 2 (2009) 245-253 249
SciRes Copyright © 2009 JBiSE
Figure 3. Scanning electron micrograph of the PCL based elastomer.
the electron beam bombardment for certain time after
discussed with the SEM operator. It sounds reasonable.
It also should be highlighted that the phase morphol-
ogy of polyurethane elastomers may be employed to
analyze the differences of the mechanical properties of
polyurethane elastomers. The differences in microstruc-
ture of three polyurethane elastomers could result in the
different mechanical properties and abrasion resistances.
The hard domains dispersed in that matrix act as the re-
inforcing carbon black in rubber, so that the polyure-
thane elastomer with higher microphase separation de-
gree performed better mechanical properties and less
abrasion loss than the sample with lower microphase
separation degree.
3.3. Lnfluence of the Polyol Structure on the
Thermal Stability of Polyurethane
Elastomers
While a good combination of properties normally sug-
gests toughness of the material, it does not ensure that
parts made from such material will survive harsh condi-
tions in the real application environment. As mentioned
earlier, polyester based elastomers are generally consid-
ered much tougher than polyether based elastomers,
however, because the ester linkage is susceptible to hy-
drolytic cleavage, polyester based elastomers break
down rapidly in a hot environment. Therefore, besides
general mechanical properties, one may need to carefully
examine other factors, such as resistance to thermal
degradation and abrasion, when selecting compounds for
specific applications.
In Table 3, stress-strain and tear properties of the
three polyurethane elastomers before and after aging in
air at 120°C for168 hours are listed. It is evident from
the data that PCL and PEA based elastomers retained
their original stress-strain and tear properties after expo-
sure to high temperature and oxidation, while the PTMG
based elastomer lost 40% to 80% of its original proper-
ties under the same conditions. The results imply that
PEA and PCL based elastomers are much more resistant
to thermal degradation as compared to the PTMG based
elastomer. However, there is no clear evidence that the
PEA based elastomer are better than the PCL based
elastomer in terms of resistance to thermal degradation,
and vice versa.
3.4. Lnfluence of the Polyol Structure on the
Abrasion Resistance of Polyurethane
Elastomers
The abrasion resistance is highly important in applica-
tions such as rubber pads for tank track, conveyor belts,
mining, pipeline pigs, squeegees, and industry wheels
and tires [7,43,44]. Table 4 shows Akron abrasion resis-
tance of the three polyurethane elastomers. The data

250 X. D. Chen et al. / J. Biomedical Science and Engineering 2 (2009) 245-253
SciRes Copyright © 2009 JBiSE
Table 3. Tensile Strength and tear properties of elastomers aged at 120℃ for 168 hours.
Tensile Strength, Mpa Elongation at break, ％ Tear Strength, KN/m
Elastomers Before After Before After Before After
PEA-TDI 41.38 51.38 780 946 103.25 105.26
PCL-TDI 40.69 52.41 640 730 107.63 108.55
PTMG-TDI 37.24 7.07 570 200 98.16 56.88
Table 4. Abrasion resistance of polyurethane elastomers based
on different backbones.
Elastomers PEA-TDIPCL-TDI PTMG-TDI
abrasion loss, mg 17.9 16.8 66.8
indicated that elastomers based on PCL and PEA had
similar abrasive resistance, and they are much better
than that of the PTMG based elastomer.
3.5. Lnfluence of the Polyol Structure on the
Dynamic Properties of Polyurethane
Elastomers
One important application for polyurethane elastomers is
tank track pads, road wheel & loading wheel flange,
many sorts of tires, wheels, rollers and vibration- ab-
sorptive materials. In this application, polyurethane
elastomers are constantly running at high-speed and un-
der high-load. It is the intrinsic nature of virtually any
material to build-up heat while running at high- speed
and bearing high-load. Wheels, tires and rollers made
from polyurethane elastomers generate heat when they
are operating. The buildup of heat can cause failure of
urethane parts by melting, tearing, or debonding. De-
pending on how much heat is generated and how fast the
heat is dissipated to the environment, different polyure-
thane elastomers have different service lifetimes. To
improve the service life of polyurethane elastomers in a
dynamic environment, we need to improve the dissipa-
tion of the heat and select elastomers with improved
dynamic properties that can generate less heat, thus run
cool for extended time. While the former can be ad-
dressed by engineering design of wheels, tires and roll-
ers, the latter has to be resolved from a formulation
standpoint.
Dynamic properties of polyurethane elastomers can be
analyzed using a Dynamic Mechanical Analyzer. The
storage modulus and tanδ curves of the polyurethane
elastomers based on different backbones are shown in
Figure 4 and 5. A good compound for dynamic applica-
tions is generally represented by low tanδ values and
constant modulus values over the working temperature
range in which the parts will be utilized. As tanδ= E″/E′,
where E″ is the loss modulus and E′ is the storage
modulus, a lower tanδ value means that energy trans-
ferred to heat is much lower than energy stored. There-
fore, lower heat buildup occurs in high-speed, high-load
bearing applications. Figure 5 shows tanδ value for the
-50050100 150
0
500
1000
1500
2000
2500
3000
3500
Temperature,?
E', MPa
PCL-CPU
PTMG-CPU
PEA-CPU
Figure 4. Storage modulus of the polyurethane elastom-
ers based on different backbones.
-50050100 150
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
tanδ
Temperature，℃
PCL-PU
PTMG-PU
PEA-PU
Figure 5. Tanδcurves of the polyurethane elastomers
based on different backbones at temperature range from
-80℃ to 160℃.
three polyurethane elastomers in a temperature range
from –80 to 180. The tanδ curves show peaks at low ℃
temperature, which is attributed to the glass transition
temperatures of the polyurethane elastomers. The tanδ
peak of the PEA based elastomer locates in the higher
temperature, this reason maybe the hindered cooperative
motion of the polymer chains. The PCL based elastomer
had higher tanδ value than the PTMG based elastomer at
low temperature, but its tanδ value was the lowest one
among the three polyurethane elastomers at higher tem-
perature (Figure 6). This may stem from the micro-
structure of the PCL based elastomer. The dispersed
phase formed by the hard phase and crystalline part of

X. D. Chen et al. / J. Biomedical Science and Engineering 2 (2009) 245-253 251
SciRes Copyright © 2009 JBiSE
50 60 70 80 90100
0.06
0.09
0.12
0.15
0.18
tanδ
Temperature，℃
PCL-PU
PTMG-PU
PEA-PU
Figure 6. Tan δ curves of the polyurethane elastomers
based on different backbones at operating temperature
zone.
the soft phase reinforced the elastomer, and the strong
interactions caused by lower degree of soft segment
crystallinity and hydrogen bonds enable the motion of
the polymer chains more synchronous with the load,
which makes the heat build up lower at operating tem-
perature. It is very important for dynamic application
accompanied by heat build up that could possibly
weaken materials, thus causing failure. Obviously, the
PEA based elastomer might not be the best choice for
dynamic applications if similar grades of PTMG and
PCL based elastomers are readily available. As for PCL
and PTMG based elastomers, though the PCL based
elastomer has higher tanδ value at low temperature,
some engineers believe that it is the tanδ value at higher
temperature that really matters. The higher tanδ value at
low temperature implies that a wheel made from the
PCL based elastomer will build up heat faster than a
wheel made from the PTMG based elastomer when the
wheel is cold. However, as the temperature increases,
tanδ value decreases. During use, the temperature of the
wheel will stabilize at the temperature where heat gener-
ated is equal to the heat dissipated, and that will be the
operating temperature of the wheel most of the time.
This temperature for the PCL based elastomer wheel
might be slightly higher than that of the PTMG based
elastomer, depending on the engineering design of the
wheels. On the other hand, the storage modulus of the
PCL based elastomer is the highest one among the three
elastomers, this is very helpful for high load application.
However, considering the enhanced mechanical strength
and resistance to thermal degradation of the PCL based
elastomer over that of the PTMG based elastomer, the
PCL based elastomer will perform better than the PTMG
based elastomer in the field.
4. CONCLUSIONS
Three polyurethane elastomers based on different soft
segments were prepared and their properties were com-
pared side by side.
The PCL based elastomer exhibits the excellent tear
and stress-strain properties that polyester based elastom-
ers offer, while retaining superior compression set and
resilience similar to polyether based elastomers.
The SEM results of all polyurethane samples showed
the existing of the microphase separation structure. Par-
ticles of the dispersed phase formed by the hard phase
and crystalline part of the soft phase grows bigger with
the increasing crystallinity of the soft segments. The
hard domains are irregular shapes and with the sizes of a
few micrometers.
As polyester based polyurethane elastomers, PEA and
PCL based elastomers are much more resistant to ther-
mal degradation as compared to the PTMG based elas-
tomer.
Polyurethane elastomers based on PCL and PEA had
similar abrasive resistance, and they are much better
than that of the PTMG based elastomer.
The tanδ value at operating temperature zone of the
PCL based elastomer is lower than those of the PEA and
PTMG based elastomers. And the PCL based elastomer
had higher tanδ value than the PTMG based elastomer at
the temperature around zero centidegree. Based on the
time-temperature superposition principle, a conclusion
can be made that the PCL based elastomer exhibits good
wet skid resistance, low rolling resistance and out-
standing dynamic application properties[45,46,47].
In a word, The PCL based elastomer possesses more
balanced properties. It is a favorable choice for applica-
tions where a combination of engineering properties is
desired.
5. ACKNOWLEDGEMENTS
This research is funded by the Polyurethane Department
of GuangZhou SCUT Bestry Technology Joint-stock Co.
Ltd. The authors are grateful to the teacher of National
Engineering Research Center of Novel Equipment for
Polymer Processing for their helpful advice.
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