Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands G. Thatte et al.
Thatte G.
1
, Hoa S.V.
1
, Merle P.G.
2
, Haddad E.
3
, and Guntzburger Y.
2
1
Department of Mechanical and Industrial Engineering
2
Department of Chemistry and Biochemistry
Concordia University
3
MPB Communications Technologies Ltd.
Montreal, Quebec, Canada hoasuon@alcor.concordia.ca
Composite materials used in space applications are susceptible to damage due to space debris, thermal cycles, exposure to UVlight, radiations and atomic oxygen impact. In space, repair of the damage to structure is very difficult and in some cases impossible. As such if the micro damage can get healed automatically, it may extend life of space structures. This paper introduces a new monomer with wide liquid temperature range including room temperature for self-healing of materials in space applications. Microcapsules filled with healing agent and catalyst was used as self-healing agents. A new approach to determine the healing performance is suggested and different environmental conditioning tests were carried out. Preliminary thermal shock test showed healing of cracks in extreme temperature fluctuation environment.
Composite materials have been widely used in transportation vehicles, for defense and automobile industry, space exploration and other commercial applications due to their favorable properties such as strength and stiffness combined with lightness as compared to metals.
Space structures (shuttles and satellites etc.) are susceptible to damage due to various reasons, the primary of which is the impact with space micrometeoroids and debris. In four decades of space activities, there have been some 4000 launches, which have generated a large number of space debris [1]. The space debris population is estimated to be about 2-4 x108 objects with diameter between 1mm and 1 cm for Low Earth Orbits (LEO) and Geo-Stationary orbits
(GEO) [2]. These space debris and meteoroids can cause severe damage to the spacecrafts. In addition to this space environment has special parameters such as thermal cycling, high vacuum, atomic oxygen, UV rays etc. Mainly thermal cycling is responsible for matrix expansion and contraction, which leads to matrix cracking [3]. Atomic oxygen in upper atmosphere is responsible for the deterioration of spacecraft material and coatings. Plasma and radiation environments are also responsible for the damage to the space shuttle/satellite components [4].
1 © Springer 2007

Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands G. Thatte et al.
Investigation of damage to the spacecraft structures is very difficult at the surface and within the body. This necessitates non-destructive testing such as Ultrasonic scanning and X-ray analysis in order to locate and quantify the extent of damage. In space, where inspection and repairing or replacement of a structure is very difficult, expensive and in some cases just impossible, the material, which heals itself, can become the key technology to repair damages in spacecrafts/satellites.
In the majority of structures a microcrack is the main reason for structural damage. At microcrack, stress concentration occurs and microcrack grows to a macrolevel, which ultimately leads to destruction of the structure. As such if there is some technique that can cure microcrack automatically and stop crack propagation to macrolevel, it may avoid further damage to the structure. In this sense self-healing materials have a wide scope in the future for space applications.
In recent work White et al. [5] have developed self-healing system for materials incorporating microcapsules containing healing agent and catalyst. The working principle of the self-healing system is shown in Fig.1. When there is microcrack in the material, it breaks the microcapsules in its path and the healing material in the microcapsules flow into the crack plane due to capillary action and becomes solid due to polymerization reaction with catalyst.
The group used Dicyclopentadiene (DCPD) monomer and Grubbs’ catalyst for healing system [6].
Figure 1: Self-healing concept with microcapsules and catalyst
Though the group demonstrated the new self-healing concept successfully, not much work has been carried out to check the feasibility of this self-healing approach and the material used for space applications.
In this paper, we focused on the application of this self-healing approach and material for space applications. Considering this fact a new monomer with wide liquid temperature range was found out.
2 © Springer 2007

Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands G. Thatte et al.
Mechanical tests with different environmental conditioning were done and healing performance was calculated using a new approach. Thermal shock test was carried out and effect of extreme temperature fluctuation on healing process was examined.
For self-healing of crack, the healing agent must be in a liquid state so that it will flow into the crack plane to heal the crack. DCPD monomer used in the previous work obtained from
Acros Organics® (Geel, Belgium) had liquid temperature range of 11 - 170 °C [7]. In this work a few microencapsulation processes were done with DCPD monomer obtained from
Sigma-Aldrich®. The liquid temperature range for this monomer was 32.5 – 170 °C. As such at room temperature and below DCPD is in solid state.
For space applications it is common to consider a temperature fluctuation between -100 °C and + 100 °C for LEO satellites and between –150 °C and + 150 °C for GEO satellites ,
DCPD may not be good choice as healing agent for space applications.
Four different compounds were compared with on the basis of liquid temperature range, polymerization reaction with catalyst, toxicity and cost. Among all, 5ethylidene 2norbornene
(5E2N) monomer with liquid temperature range -80 to 146 °C was found suitable for further work. Turn Over Number (TON) describes the productivity (Productivity = grams of polymer formed per gram of catalyst) and Turn Over Frequency (TOF) describes the activity (Activity
= grams of polymer formed per gram of catalyst per unit of time) of this new monomer was determined from experimentation. The activity of the 5E2N with catalyst was determined with
1 % w/w Grubbs’ catalyst and neat 5E2N at 500 rpm stirring at ambient atmosphere and at -
16, 0, 20, 45 °C.
Figure 2(a) shows that at 45 °C the polymerization duration is around 12 seconds and activity increases to 320 Kg polymer/Kg catalyst/h, whereas at -16 °C the polymerization duration is 2 hours and 26 minutes. Thus the temperature showed a strong influence on the system activity and curing kinetics. The TON and TOF as function of M/I (monomer/initiator) ratio was studied to determine optimal M/I ratio and reduce catalyst loading in the system. The experiments were carried out at 20 °C in ambient atmosphere and 500 rpm stirring rate in neat conditions. From Figure 2(b) it is clear that the higher is the M/I ratio (the w/w% of catalyst in system is reduced), the better is the TON. The TOF curve shows that the activity of the system is improved as M/I ratio increased. This is because for all five M/I ratios in this experiment, the polymerization time is almost same; varying from 3.5 to 5.5 min. This is advantageous, as decrease in catalyst loading in the system did not show adverse effect on the activity of the system.
3 © Springer 2007

Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands G. Thatte et al.
Polymerization time to reach more than 90% of monomer conversion and activity as function of temperature
TON and TOF vs. M/I ratio
30000 300000
350 160
140 25000 250000
300
250
200
150
120
100
80
60
TOF (activity)
20000
15000
200000
150000
TON
TOF
100
50
40
20
Polymerization time (min)
10000
5000
100000
50000
0
-20
0
0 20 40 60
0 0
5000 10000 15000 20000 25000 30000
Tem perature o
C
(a) (b)
M/I ratio
Figure 2: a. polymerization time and TOF (activity) as a function of temperature b. TON and TOF vs. M/I ratio
3.1 Sample preparation and testing procedure
Samples were prepared by mixing EPON® 828 epoxy resin with 47 % Epicure 3046 curing agent. Two types of samples were prepared. Neat sample referred as sample containing no microcapsules and catalyst and another type was 5E2N samples. 5E2N samples contained microcapsules and catalyst.
For 5E2N samples, 10 wt% microcapsules with diameter range from 150-350 µm were used and 1 wt% loading of Grubbs’ catalyst was used. After proper mixing of all constituents, mixture was degassed and the mixture was poured into a silicone mold. The samples were cured at room temperature for 24 hours and post cured at 45 °C for 2 days. In this work tapered double cantilever beam (TDCB) geometry was used for sample preparation [6].
Two types of tests were conducted. First test called ‘unhealed test’ was done with neat and
5E2N samples without sharp crack. In this test, the blade notch was created into the molded starter notch in the samples just to initiate the crack. This test was carried out to determine the effect of addition of microcapsules and catalyst on the material properties. Second type of test called ‘healed test’ was carried out with neat and 5E2N samples with sharp pre-crack. A sharp pre-crack was created in each sample by gently tapping a blade into the molded starter notch.
Samples with blade notch in unhealed test were loaded on MTS machine using 2.5 KN load cell. These samples were loaded in tension with displacement control of 5 µm/s and allowed to fail in two pieces. In healed test, after creation of sharp crack in the samples, samples were allowed to heal for 24 hours. Different sets of sample were healed for different environmental conditions. A sharp pre-crack length in different samples were obtained from 22 to 32 mm.
After 24 hours of healing, these samples were loaded on MTS machine till it falls into two pieces. In both types of tests load vs. displacement graph were recorded for all samples.
4 © Springer 2007

Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands G. Thatte et al.
From load vs. displacement graphs of all samples critical fracture load and stiffness of the samples at initial portion of graph were determined.
P
1
, P
2
and S
1
, S
2
are the critical fracture load and stiffness of neat and 5E2N samples used in unhealed test respectively. P
3
, P
4
and S
3
,
S
4 are the critical fracture load and stiffness of neat and 5E2N samples used in healed test respectively. Addition of soft particles to neat sample reduced the critical fracture load from
P
1
to P
2
and stiffness from S
1
to S
2
. In case of healed test, neat sample does not have healing material in crack plane in contrast to 5E2N samples. As such stiffness of 5E2N samples ( S
4
) should be more than stiffness of neat samples ( S
3
) at initial portion of load-displacement graph of samples. Stiffness ratios S
3
/ S
1
and S
4
/ S
1
were compared. Also critical fracture load ratio P
4
/ P
3
was determined.
Three of each neat and 5E2N samples with blade notch were tested at room conditions. Figure
3 shows the load vs. displacement graph for these samples. The graph shows that addition of microcapsules and catalyst to the neat sample lowered the average critical fracture load by 16
% and stiffness by 15.8 %. Considering this fact, the neat sample was considered as reference sample instead of 5E2N sample to compare the healing performance.
Load vs. displacement graph for the neat and
5E2N samples without a sharp crack
250
200
150
100
50
N1-23-NC
N2-23-NC
N3-23-NC
S1-23-NC
S2-23-NC
S3-23-NC
0
0 0.2
0.4
0.6
0.8
1 1.2
-50
Displacement (mm)
Figure 3: Load vs. displacement graph for neat and 5E2N samples with blade notch
Healing performance was determined for different environmental conditioning such as different temperatures 23 °C, 45 °C, 60 °C, -20 °C and under vacuum of 10
−
6 mbar at 23 °C.
For each test 5 of each neat and 5E2N samples were tested. Figure 4(a) shows load vs. displacement graph for neat and 5E2N samples healed at 23 °C.
5 © Springer 2007

Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands G. Thatte et al.
Figure 4(b) shows enlarged view of the framed portion of Figure 4(a). This portion was considered to determine the stiffness of the samples.
Figure 4(b) show that 5E2N samples are stiffer than neat samples in the presence of a sharp pre-crack. The average stiffness of 5E2N samples is about 35 % more than the average stiffness of the neat samples. The ratio P
4
/ P
3 shows 99 % healing efficiency.
Table 1 shows the summary of the results for all the tests. From the stiffness ratio values
( S
3
/ S
1
and S
4
/ S
1
), the stiffness in case of 5E2N sample was increased due to healing of the crack. Though the addition of microcapsules lowered the critical fracture load in case of 5E2N samples, due to healing of crack the ratio P
4
/ P show improvement in the critical fracture
3 load of 5E2N samples.
Load vs. displacement graph for neat and 5E2N samples with a sharp crack, conditioned at 23 o
C
120
100
5E2N samples
(Black lines)
Neat samples
(Gray lines)
S8-23-WC
S9-23-WC
80
S7-23-WC
S4-23-WC
60
N8-23-WC
N9-23-WC
40
N4-23-WC
N5-23-WC
20
0
N6-23-WC
S6-23-WC
0 0.2
0.4
0.6
0.8
1 1.2
1.4
-20
Displacement (mm)
(b)
(a)
Figure 4: (a) Load vs. displacement for neat and 5E2N samples with a sharp crack, conditioned at 23 °C, (b)
Enlarged view of framed portion shown in (a)
Table 1: Summary of all test results
Sr.
No.
Different
Tests
(
5E2N sample
S
4 /
S
1 ) (
Neat sample
S
3 /
S
1
)
P
4
%
/
P
3 at 23ºC
Stiffness distribution of healed 5E2N and neat samples is shown in Figure 5. From the stiffness values from table 1, there is 35 % stiffness recovered in case of samples healed at 23 oC, 23 % stiffness recovery in case of 45 oC, 11 % in case of 60 oC, 31 % in case of -20 oC and 22 % in case of samples healed under vacuum at 23 oC.
6 © Springer 2007

Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands G. Thatte et al.
Stiffness variation between 5E2N and Neat samples conditioned at different conditions
200
180
160
140
120
100
80
60
40
20
0
5E
2N
-2
3
Ne at
-2
3
5E
2N
-4
5
Ne at
-4
5
5E
2N-
60
Ne at-
60
5E
2N
-(
-2
0)
N eat
-(
-2
0)
5E
2N-
V
23
Environmental conditions
Ne at-
V2
3
High
Low
Average
Figure 5: Bar chart showing stiffness distributions of 5E2N and neat samples for all tests
Microscopic picture of sharp pre-crack [Fig. 6(a)] showing empty microcapsules in crack propagation path reveals that the crack propagation broke the microcapsules in its path and healing material from microcapsule shell flowed into the crack plane. Figure 6(b) shows fractured crack surface of the 5E2N sample with polymerized film of healing material over the crack surface. The dividing line as shown in Figure 7(a) clearly shows healed crack with thin film of healing material and unhealed portion (portion ahead of the crack tip). Neat sample have smooth shiny crack surface without any healing material [Fig. 7(b)].
Broken microcapsules Healing material film
Unhealed portion
300µm
Healed crack
200µm
Healed portion
(a)
(b)
Figure 6: (a) 5E2N sample with sharp pre-crack, (b) Crack face with healed and unhealed portion
7 © Springer 2007

Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands G. Thatte et al.
Smooth crack surface
Healing material
150µm
150µm
(a) (b)
Figure 7: (a) 5E2N sample with healed material on crack surface, (b) Crack surface of neat sample
Each of neat and 5E2N sample was cooled using liquid nitrogen (boiling point = -196 °C) and then they were heated in an oven at 60 °C. Samples were kept for 5 min in liquid nitrogen and transferred to the oven temperature in less than one minute and left it for 5 minutes in oven.
Figure 8 shows the neat sample and 5E2N sample subjected to thermal shock test and Figure
9 shows healed cracks in 5E2N sample.
Figure 8 (a) and (b) show neat and 5E2N samples with many cracks. After 20 thermal cycles, the neat sample was broken into pieces while, inspite of having many cracks in 5E2N sample; the sample remained in one piece due to the healing of crack [Fig. 8 (c)].
Healed crack
(a) (b) (c)
Figure 8: (a) Neat sample, (b) 5E2N sample, (c) 5E2N sample with healed crack
300µm
8 © Springer 2007

Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands G. Thatte et al.
A new monomer (5E2N) with a suitable liquid temperature range was selected to replace
DCPD monomer. Its ROMP behavior was investigated. The new monomer was successfully encapsulated in poly UF microcapsules. Self-healing was achieved with the new monomer
5E2N without any external assistance for temperature range from -20 to 40 ºC and vacuum test at 23 ºC. Preliminary thermal shock test results and vacuum test results showed that the
5E2N monomer can be a potential candidate for space application and can be selected for future work. New approach using stiffness to determine healing performance is a good choice to compare healing efficiency among different types of samples.
ACKNOWLEDGEMETS
The authors wish to thank Canadian Space Agency (CSA) and MPB Technologies Inc. for financial support and co-operation during this research work. Also authors sincerely acknowledge the support of the Concordia
University. The authors would like to thank Dr. Ming Xie for his support during this work.
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9 © Springer 2007