1. 1
SHEAR CHAR STRENGTH OF THERMOPLASTIC POLYURETHANE
ELASTOMER NANOCOMPOSITES
Jake A. Lewis, Manuel H. Jaramillo, and Joseph H. Koo*
The University of Texas at Austin, Dept. of Mechanical Engineering, 204 E. Dean Keeton St, C2200,
Austin, TX 78712, USA
*jkoo@mail.utexas.edu
ABSTRACT
Ablative materials, such as thermoplastic elastomer nanocomposites (TPUNs) are used as internal
insulation materials for solid rocket motors (SRMs). These TPUNs are thermoplastic elastomer
reinforced by nanoclays, multi-walled carbon nanotubes, and carbon nanofibers. When these
TPUN materials are exposed to an extreme heat flux, a char layer forms along the surface as it
ablates. This research includes the study of testing procedures for evaluating the shear strength of
this char layer, a characteristic that is important for aerospace and defense applications. The
designated testing procedure will incorporate aspects of previous projects that determined the shear
strengths of several types of TPUNs through the use of our Shear Strength Sensor. This sensor
contains a method for applying traverse loading on a sample, while measuring the reaction force
and the induced strain. This sensor will be used on several types of TPUN test specimens to collect
data until each sample ruptures. In order to determine which ablative exhibited the best
performance, the energy of destruction or the energy of dissipation will be evaluated. The
maximum force will also be accounted for as a secondary quantity for determining the char shear
strength. The proposed test method is fully automated to ensure that each test is repeatable. This
guarantees reliability when collecting the data and eliminates the potential for human error. The
char layer thickness varies depending on the material. Thus, a method for distinguishing the virgin
material from the char will be explored.
1. INTRODUCTION
1.1 Context of Research
Polymeric ablative materials (PAMs) are critical components in the aerospace industry. Ablative
materials are specifically designed for thermal protection against extreme heat fluxes and high
shear forces. The defense mechanism of the ablator stems from the formation of char when subject
to the aforementioned environment. This exposure causes the material to decompose, where it
releases pyrolysis gases and produces a burned carbonaceous residue. This residue is known as the
char layer. The char structure limits the total heat transfer to the remaining virgin material by heat
conduction. With only one mode of heat transfer, the thermal insulation property of the ablative
material significantly improves, which in turn protects the underlying substrate. Therefore, how
well an ablative material protects is directly correlated to the strength of the char layer.
The purpose of this research is to study the feasibility of potential ablative materials for use as
thermal insulation for solid rocket motors (SRMs). The state-of-the-art (SOAT) ablative used for
such application is ethylene propylene diene monomer rubber (EPDM) reinforced with Kevlar®.
With its chemical stability, low thermal conductivity, high ablation resistance, and high physical
adhesion, its performance at protecting against combustion gases is above other ablatives.

2. 2
However, previous research has shown potential for using thermoplastic polyurethane elastomer
nanocomposite (TPUN) as an alternative for SRM insulation [1-5]. Not only do these ablatives
form a thick char layer when exposed to a hyperthermal environment, they are also easily
fabricated. A proper method for assessing a material’s potential is evaluating the strength of the
char. Accordingly, samples of TPUN-5 wt% carbon nanofibers (CNF), TPUN-5, 7.5, and 10 wt%,
multi-walled carbon nanotube (MWNT), and EPDM reinforced with Kevlar were exposed to a
well-controlled lab device to simulate the extreme environments [6], then the shear strength tested
to evaluate each material’s char strength.
1.2 Oxy-acetylene Test Bed (OTB)
The OTB was utilized to produce an intense hyperthermal environment for testing ablatives [6].
The device has been used extensively to test ablatives for gathering data of their thermophysical
properties. It is capable of producing heat fluxes above 1,100 W/cm2
and temperatures well above
2000°C. Since the heat flux decreases with the distance from the torch, the OTB was calibrated
using a copper slug. In knowing the slug’s mass, dimension, and specific heat, a calibration curve
was created by testing the sample at multiple distances. The resulting calibration curve allows the
user to place a test sample at the location of the desired heat flux. As for testing TPUN material,
each sample was placed at a torch distance of 12.55mm and a heat flux of 600W/cm2
, an
environment quite similar to the conditions of the ablative liner inside of solid rocket motors
(SRMs). Each test was exposed to the flame for a duration of 25 seconds.
1.3 Advanced Diagnostic Equipment
As a way to measure the surface temperature of each sample during ablation testing, the research
team included a two color infra-red (IR) IMPAC ISQ 5 Pyrometer manufactured by LumaSense
Technologies, Germany (Figure 1). To guarantee that the instrument is in fact observing the
sample, a laser pointer affixed to the pyrometer points to the precise location of measurement.
Each measurement is converted into an electric signal, which is then processed and converted into
a temperature value.
Figure 1: (a) IMPAC ISQ 5 Pyrometer; and (b) DALSA Falcon series HD camera used.
To further take advantage of the capabilities of our Polymer Nanocomposite Technology Lab, the
high definition (HD) video camera was used to monitor the surface behavior of a sample during
ablation testing (Figure 1). By doing so, we can optically study the deformation of each sample.
At 15 frames per second, the DALSA Falcon series HD camera captures varies moments of an
ablation test at a pixel resolution of 2048x2048. To account for the brightness of the torch – which

3. 3
would oversaturate the image – the HD camera is equipped with an adjustable aperture and an
optical filter.
2. EXPERIMENTAL
2.1 Sample Preparation
This study involves the use of a previously built compressive strength sensor [5] and shear strength
sensor [7] at UT Austin. A sketch of the sensor is presented in figure 2. The majority of the body
of the sensor consists of a linear actuator, the driving force of the machine. The actuator’s motion
is regulated by an H-bridge with a LabVIEW VI, where the user can set Boolean signals, the duty
cycle, and the frequency sent to the H-bridge during testing. By having the speed and the power
of the actuator adjustable, the sensor can test a wide range of ablatives of varying strengths.
Figure 2: Illustration of the set up for the Shear Strength Sensor.
The tester functions by applying a traverse load on the char of the test specimen while the sample
is clamped still. As the probe tip pushes against the sample, the resulting deflection of the ablative
causes a reaction force on the sensor. Throughout this process, the sensor is sending voltage signals
to the labView VI where each signal is converted into a reaction force value (further discussed in
the next section). The reaction forces are measured until the sample fractures. The generated force
and distance values are stored in a text file in the folder of the user’s choice. These values are used
to plot a force versus distance graph, which is used to study the strength of the charred material.
2.2 Calibration of Shear Strength Sensor
In order to collect accurate data from using the sensor, the potentiometer and force sensor must be
calibrated. The potentiometer is calibrated to find the relationship between the actuator’s probe tip
position and the voltage across the potentiometer. This is accomplished by measuring the distance
the probe tip traveled via calipers and measuring the voltage across the potentiometer with a
multimeter. The force sensor - placed inside the probe tip - is calibrated to find the relationship
between the reaction force against the probe and the voltage signal from the sensor. This calibration
is performed with an apparatus consisting of four identical metal springs sandwiched in parallel
between two metal platforms. When the probe presses down on this apparatus, the springs

4. 4
compress, and the force sensor sends a voltage signal to the LabVIEW control program. By
knowing the equivalent spring constant of the four springs and the displaced distance of the pressed
metal platform, the reaction force from the springs and the voltage signal from the sensor are
known. This calibration data is used to develop a unique equation for converting each voltage
signal to a reaction force.
2.3 Analysis Method
With the Shear Strength Sensor, we can test and define the shear strength of the char of various
samples. This was accomplished by developing two separate methods for evaluating strength when
given force and distance data. During a shear test, the sample yields - or plastically deforms - when
nearing its breaking point. At this point, the sample’s resistance to the applied load increases to a
peak before significantly decreasing, indicating that the ultimate shear strength has been reached.
It is at this moment that the sample experiences its maximum allowable stress. We ranked the
strength of the different samples based on their ultimate stress as the first method of evaluation.
Even though each sample has the same dimensions, the distance needed to fracture the char varies
per sample. This is evident in the results shown later in this paper. Accordingly, it is necessary to
evaluate the total energy needed to fracture each sample. For example, if two different materials
showed the same maximum reaction force, the sample that prevented fracture at the furthest
distance has the greater shear strength. As a result, the work needed for the probe to fracture a
sample was the second method used for determining shear strength. Work is defined by the formula
displayed below:
W = F*D [1]
Work can be calculated after each force-distance measurement. After the test, the area under the
force versus distance plot represents the energy needed to fracture the sample. The area can be
calculated by summing up the values of each incremental force multiplied by its respective
distance.
3. RESULTS AND DISCUSSION
The TPUN and Kevlar EPDM samples were cut into 10x10x50 mm and burned using the OTB to
generate char. Three samples of each formulation were tested to ensure and verify correct data. All
the materials were exposed to a heat flux of 600W/cm2
to simulate an SRM combustion chamber
environment.
3.1 Surface Temperature Measurements
Surface temperatures were measured for all the TPUN nanocomposites and the Kevlar-filled
EPDM (control materials) in this study. Figures 3 to 8 show the surface temperatures of the three
samples of TPUNs with 5wt% MMT (nanoclays), TPUNs with 5wt% CNF (carbon nanofibers),
TPUNs with 5wt% MWNT (multi-walled carbon nanotubes), 7.5wt% MWNT, 10wt% MWNT,
and Kevlar-filled EPDM, respectively.
Within each formulation group, the surface temperature of the samples for the most part are
comparable with one another. Examining the surface temperature plots above, the starting surface
temperature for each sample is 1,000 °C. This is because the pyrometer has a set lower limit,
meaning it cannot measure values under the limit. After the test, the samples were pulled away
from the flame and out of sight of the pyrometer, causing the measurements to end immediately.

5. 5
Note that in some tests the surface temperature rate decreased - and in some cases became negative
- before leveling out.
For the most part, the surface temperatures of the formulations resulted very similarly. The 5 and
7.5 wt% MWNT and the Kevlar filled EPDM maxed at 2000 °C. The results for these three
formulations were the least noisy and the most trust worthy due to the matched values within each
formulation. Interestingly, the 10 wt% MWNT did have an average max surface temperature that
matched the 5 and 7.5 wt% MWNT formulations. However, the temperature data of samples did
not agree as the samples’ maximum values were above and below 2000°C. With samples of
varying data, more tests are needed to conclude its actual maximum surface temperature during
these conditions. This applies to the 5 wt% MMT formulation as well. It had an average of 1865°C,
but the surface temperature pattern did not repeat between samples. Oddly, the surface
temperatures of the 5 wt% CNF samples were irregularly collected. The pyrometer’s measuring
system failed to continuously measure the surface temperature largely due to the lack of a flat
distinguishable char layer on the CNF surface. Instead, it deformed in such a way that was not
uniform and rough, causing trouble for the pyrometer that needs an even surface to measure the
radiation. Its rough post surface structure can viewed and compared to that of Kevlar EPDM in
Figure 10.
Figure 3: Surface temperatures of 5wt% montmorillonite nanoclay (MMT) thermoplastic
elastomer nanocomposite at an exposure of 600W/cm2
.
1000
1200
1400
1600
1800
2000
0 5 10 15 20 25 30
Temperature(°C)
Time (s)
5%MMT1
5%MMT2
5%MMT3

6. 6
Figure 4: Surface temperatures of 5wt% carbon nanofibers (CNF) thermoplastic elastomer
nanocomposite at an exposure of 600W/cm2
.
Figure 5: Surface temperatures of 5wt% multi-walled carbon nanotubes (MWNT) thermoplastic
elastomer nanocomposite at an exposure of 600W/cm2
.
Figure 6: Surface temperatures of 7.5wt% multi-walled carbon nanotubes (MWNT) thermoplastic
elastomer nanocomposite.
0
500
1000
1500
2000
2500
0 5 10 15 20 25 30
Temperature(°C)
Time (s)
5%MWNT1
5%MWNT2
5%MWNT3
0
500
1000
1500
2000
2500
0 5 10 15 20 25 30
Temperature(°C)
Time (s)
5% CNF 1
5% CNF 2
5% CNF 3
0
500
1000
1500
2000
2500
0 5 10 15 20 25 30
Temperature(°C)
Time (s)
7.5%MWNT1
7.5%MWNT2
7.5%MWNT3

7. 7
Figure 7: Surface temperatures of 10wt% multi-walled carbon nanotubes (MWNT) thermoplastic
elastomer nanocomposite at an exposure of 600W/cm2
.
Figure 8: Surface temperatures of Kevlar-filled ethylene propylene diene monomer (EPDM) at an
exposure of 600W/cm2
.
Figure 9: (a) Post test samples of 5 wt% CNF; and (b) Kevlar-filled ethylene propylene diene
monomer.
0
500
1000
1500
2000
2500
0 5 10 15 20 25 30
Temperature(°C)
Time (s)
EPDM-K 1
EPDM-K 2
EPDM-K 3
1000
1200
1400
1600
1800
2000
2200
0 5 10 15 20 25 30
Temperature(°C)
Time (s)
10%MWNT1
10%MWNT2
10%MWNT3

8. 8
3.2 High Definition Video Camera Analysis
The HD images (Figures 10 and 11) presented below detail the surface structure of each sample
during ablation testing. Each picture was taken around five seconds of the test.
Figure 10: Samples during ablation testing: (a) 5%MWNT, (b) 5%MMT, and (c) 5%CNF during
ablation testing.
Figure 11: Samples during ablation testing: (a) 7.5w%MWNT; (b) 10%MWNT; and (c) Kevlar
EPDM.
As stated earlier, the 5 wt% CNF formulation deformed unlike the other samples. This is further
supported by observing the HD pictures. The HD picture of CNF is much darker than the others.
This could be due to a number of factors, such as the emissivity and the brightness of the flame as
it touches the surface. Note that in the picture there are several bright spots spotted in the center.
This surface behavior requires further research.

9. 9
3.3 Char Shear Strength Results
The proposed method for testing the shear strength was used with the posttest TPUNs and Kevlar
filled EPDM samples. Figures 12 to 17 show the plotted force history of the tested three samples
per formulation.
Figure 12: Shear char force of 5wt% montmorillonite nanoclay (MMT) thermoplastic elastomer
nanocomposite.
Figure 13: Shear char force of 5wt% carbon nanofibers (CNF) thermoplastic elastomer
nanocomposite.
0
1
2
3
4
5
0 2 4 6 8 10 12 14 16
Force(N)
Distance (mm)
5%MMT1
5%MMT3
5%MMT2
0
10
20
30
40
50
0 2 4 6 8 10 12 14 16
Force(N)
Distance (mm)
5%CNF1
5%CNF2
5%CNF3

10. 10
Figure 14: Shear char force of 5wt% multi-walled carbon nanotubes (MWNT) thermoplastic
elastomer nanocomposite.
Figure 15: Shear char force of 7.5wt% multi-walled carbon nanotubes (MWNT) thermoplastic
elastomer nanocomposite.
Figure 16: Shear char force of 10wt% multi-walled carbon nanotubes (MWNT) thermoplastic
elastomer nanocomposite
-1
0
1
2
3
4
5
6
0 5 10 15 20
Force(N)
Distance (mm)
Charred 7.5 wt% MWNT1
Charred 7.5 wt% MWNT2
Charred 7.5 wt% MWNT3
-2
0
2
4
6
8
0 5 10 15 20 25
Force(N)
Distance (mm)
10%MWNT1
10%MWNT2
10%MWNT3
0
1
2
3
4
5
6
0 2 4 6 8 10 12 14 16 18
Force(N)
DIstance (mm)
5%MWNT1
5%MWNT2
5%MWNT3

11. 11
Figure 17: Shear char force of Kevlar-filled ethylene propylene diene monomer (EPDM).
3.4 Discussion of Results
The first method for evaluating the shear strengths of the formulations was determining the
maximum force required to fracture the char layer off the specimens. With three samples per
formulation, the maximum char force values were averaged for ease of study. Figure 19 displays
these averages for all of the tested formulations.
Figure 18: Average maximum force of each type of thermoplastic elastomer nanocomposite and
Kevlar-filled EPDM.
After studying the char force data, all formulations except 5wt% CNF exhibited a maximum char
force under 5N. Surprisingly, the weakest formulation was the Kevlar-filled EPDM – the current
material used as insulation for SRMs. Although the experiments resulted in 10 wt% MWNT having
Sample
5 wt% MMT 4.06
5 wt% CNF 37.52
5 wt% MWNT 3.94
7.5 wt% MWNT 3.83
10 wt% MWNT 4.46
Kevlar EPDM 2.67
0.00
5.00
10.00
15.00
20.00
25.00
30.00
35.00
40.00
45.00
50.00
MaxForce(N)
Average Max Force (N)
-2
-1
0
1
2
3
4
0 5 10 15 20 25
Force(N)
Distance mm)
Charred Kevlar EPDM1
Charred Kevlar EPDM2
Charred Kevlar EPDM3

12. 12
the strongest char force, it cannot be determined which formulation is the strongest due to the
overlapping standard deviations of all formulations. Overall, these formulations exhibited a very
weak maximum char force.
As stated before, the CNF samples did not generate a distinguishable char layer. Thus, when the
samples were sheared, the sensor was shearing the virgin material, a much stronger and more
ductile material than a char layer. As such, the maximum reaction force of the CNF samples
averaged to be 37.52N which roughly occurred around 8 seconds. Without a char layer, the CNF
formulation values are not comparable with the other formulations.
For the most part, the char layers sheared off before the sensor reached 5mm. The data after these
peaked values are when the sensor is both pushing the already sheared char layer off the sample
and rubbing against the virgin material. For some tests, such as the second test with 5 wt% MWNT,
the contact between the sensor and the top of the virgin material produce large forces after fracture.
This needs to be noted when evaluating the energy dissipated as these values do not contribute to
these calculations.
The second method for evaluating the shear strength of char is by determining the energy required
to fracture the char layer. This was accomplished by calculating the area under the force versus
distance plots. Since the char layer of each formulation fractured mid test, the values after fracture
were not used. Figures 19 and 20 display the average energy dissipated for all of the formulations.
Figure 19: Average energy dissipated of 5wt% MMT, 5, 7.5, and 10wt% MWNT, and Kevlar
EPDM.
Sample
5 wt% MMT 3.80
5 wt% MWNT 4.72
7.5 wt% MWNT 2.04
10 wt% MWNT 20.72
Kevlar EPDM 1.83
0.00
5.00
10.00
15.00
20.00
25.00
30.00
EnergyDissipated(N·mm)
Average Energy Dissipated

13. 13
Figure 20: Average energy dissipated of 5wt% CNF.
The experimental data display that 5wt% CNF experienced the greatest energy dissipated.
However, since this material did not generate a testable char layer, its results of the virgin material
vary significantly from the other formulations and, thus, were displayed in a separate graph. Within
the formulations worth comparing, 10 wt% MWNT performed well above the other formulations
with an average energy dissipation of 20.72 N·mm. This is mainly due to the fact that it withstood
a much larger strain than the rest of the formulations. It was the only formulation to break in parts
as opposed to the char layer fracturing intact. The next strongest formulation is 5wt% MWNT
(4.72 N·mm), followed by 5wt% MMT (3.80 N·mm), 7.5wt% MWNT (2.04 N·mm), and Kevlar-
filled EPDM (1.83 N·mm).
It can be concluded that the TPUN formulations performed comparably to the current insulator for
SRMs, Kevlar-filled EPDM. With the two methods to evaluate the shear strength, it can be
concluded that the ablator with the greatest strength is 10wt% MWNT.
3.4 Scanning Electron Microscopy (SEM) Analysis
To better understand the nature of the char layer of the TPUN formulations, we performed a
scanning electron microscopy (SEM) analysis on the surfaces of the burnt compositions. The SEM
images displayed on the left are at a magnification of 100X, while the SEM images on the right
are of the pictures on the left displayed at a magnification of 10,000X.
Sample
5 wt% CNF 254.90
0.00
50.00
100.00
150.00
200.00
250.00
300.00
350.00
EnergyDissipated(N·mm) Average Energy Dissipated
a b

14. 14
c d
e f
g
h
i j
k l

15. 15
Figure 21: Scanning electron microscopy (SEM) results of the char layers: (a, b) 5 wt% carbon
nanofibers (CNF), (c, d) 5 wt% montmorillonite nanoclay (MMT), (e, f) 5 wt% multi-walled
carbon nanotubes (MWNT), (g, h) 7.5 wt% MWNT, (i, j) 10 wt% MWNT, and (k, l) Kelvar-filled
EPDM.
Figure 21a displays a visual image of the appearance of the surface of a burnt CNF formulation.
On the micro-scale, it can be seen that there exists a large arrangement of interlocking CNFs. By
examining Figure 21b, we can conclude that the fibers stay entangled due to re-positioning from
the intense exposure. While a char layer may not be visible by the naked eye, it can be confirmed
that CNF does produce char, however, at a much lesser extent than the other formulations.
The char of MMT is a hybrid organic/inorganic material and can be viewed in Figure 21c.
Examining the char layer at a higher magnification (Fig. 21d) shows a highly compacted wall of
nanoclay platelets. Such characteristics of a porous char provide good mechanical properties.
Figures 21e, g, and i, display the appearances of char on the surface MWNT compositions. At a
higher magnification (Figs. 21f, h, and j), the compactness of the char is clearly visible. Also, just
as the MMT formulation, the only remaining materials are the nanoparticles. What remains is a
network of protective burnt MWNT, which was reassembled during the ablation test.
The low magnification images of the charred surface of Kevlar-filled EPDM is displayed in Figure
21k. It is clear that the char layer is much smoother and more intact than the other char layers.
When examining at a higher magnification (Fig. 2l), a field of compact spheres and fibers are
visible. Here, unlike the other formulations, the resin remained and helped protect the composition,
highlighting a key reason for its superior protective performance.
After studying the char of each formulation at the microscopic level, it can be concluded that
when the combustion gases penetrate into the porous material, the compositions act in a similar
way. The TPUNs protected in such a way that the polymer burnt at a faster rate than the
nanoparticles, which in turn were the main source of defense against the hypothermal
environment. This was not the case for the Kevlar-filled EPDM, as both the polymer matrix and
the micron-sized Kelvar fibers were protecting against the intense heat flux.
4. CONCLUSION
This study involved an innovative instrument designed to study the unique behaviors of charred
ablators. By using the previously designed shear strength tester, we were able to evaluate the
shear strengths of the char layers from ablated formulations of TPUNs and Kevlar-filled EPDM.
However, it was found that the shear strengths of these insulating materials were very small with
standard deviation values overlapping one another. As for the CNF composition, the tester was
unable to test its minimal char layer. Thus, in order to achieve an effective shear strength test, the
probe of the sensor would need to be remade with a smaller diameter. Although the obtained data
portrays the reliability of the used tester, more tests of these formulations would better verify
their respective shear strengths and minimize the relatively large standard deviations. In doing
so, the shear strengths of these ablators can be more accurately characterized. This shear char
strength sensor and testing method will be adapted to more burnt polymeric ablatives and
nanostructured ablatives developing in our lab.

16. 16
5. ACKNOWLEDGEMENT
The authors would like to acknowledge financial support of KAI, LLC for this project.
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