Stimuli Responsive Self-Healing Polycaprolactone Urethane
Coatings from Waste Palm Cooking Oil
Norazwani Muhammad Zain1*, Farizah Adliza Ghazali1, and Ernie Suzana Ali2
1Fabrication and Joining, Universiti Kuala Lumpur Malaysia France Institute, 43650 Bandar Baru Bangi, Selangor, Malaysia.
2Faculty of Science & Technology, Universiti Sains Islam Malaysia, 71800 Bandar Baru Nilai, Negeri Sembilan, Malaysia.

*Corresponding author E-mail: norazwani@unikl.edu.myAbstract
The development of responsive polymers has expanded significantly owing to the development of well-defined polymer architechture withpredictable structure-to-property behavior. In this study, new thermal responsive self-healing polycaprolactone urethane coatings were prepared from waste palm cooking oil. Nine different formulations of coatings with various percentages of ε-caprolactone (ε-CL) were successfully developed and the effect of polycaprolactone network in the polyurethane coating on the self-healing behavior. The self-healingability of the coatings was studied by using thermal analysis and Atomic Force Microscopy (AFM). The PCLU coating with the highest
content of PCL network nearly complete healed at 90ºC after 2 hours. The results were confirmed by scanning electron microscopy before
and after healing. The mechanism of scratch closure by thermal stimuli responsive self-healing was also discussed.
Keywords: Waste palm cooking oil, polycaprolactone, polyurethane, coatings, self-healing.

1. Introduction
The self-healing polymers are a new classification of smart materials that have the ability to repair themselves when damaged without
requiring for detection or repair manually in any form. The demand
for smart materials is increasing in the era of Industry 4.0 with improve performance in challenging applications which drive the needfor materials with extended lifetimes.
Self-healing technology is designed to provide the ability ofpolymer materials to seize the propagation of cracks in the earlystage and thus prevent catastrophic failures. Nowadays, the emerging responsive polymers has amplified significantly owing to theconsistent development of new chemistries and controlled polymerization methods with predictable structure-to-function behavior. In
recent years, researchers have shown their significant efforts by focusing on the development of materials with self-healing propertiesinspired by the self-healing of wounds in biological systems [1, 2].
Other possibilities that have been focused by the researchers are todevelop self-healing materials to facilitate remotely controlled selfhealing properties upon damage or fracture that fully restore themechanical properties of pristine material and to repair the micronsize cracks or cuts with minimum external intervention [3, 4]. There
are two popular strategies to synthesize self-healing materials, i.e.
(i) through the impregranation of catalysts in the form of capsules
or fibres [5, 6] and (ii) through the incorporation of dynamic
reversible bonds in the polymer matrix either in the form ofcrosslink points or as (hetero-)bifunctional building blocks for
instance; monomers, oligomers and etc.
Moreover, shape memory polymers (SMPs) are also widely explored in the development of self-healing materials. Due to thermal

mismatch between two different segments, this type of polymers
tends to form phase separation i.e. soft and hard segments. The soft
segments subsidizes to the elastic properties of materials due to themolecular motion in a rubbery state and influences material performances such as rigidity, strength and modulus at low temperatures.
Meanwhile, the hard segments construct physical crosslinks between urethane networks because of the secondary bonding (hydrogen bonding or polar interaction) or crystallization in the hard domains [7]. Besides hydrogen bonding between urethane networks,
the conformation of phase separation is also reliant on types of polyol used, molecular weight as well as processing. Almost all existing SMPs fall into three major categories depending to the type ofstimulus applied to trigger the shape memory effect i.e. thermo-responsive, photo-responsive and chemo-responsive SMP. Thermoresponsive SMP is normally triggered by means of heating. Meanwhile, photo-responsive SMP is induced by light with different
wavelengths without any heat involved. Chemo-responsive SMP
involves chemicals such as water, ethanol and etc. [8, 9].
Rodriguez et al. [10] also used the shape memory concept intheir study. They have introduced the term ‘shape memory assisted
self-healing’ (SMASH). They reported that the damage of polymer
was healed by heating the polymer above the melting temperatureof the linear PCL. Rivero et al. [11] managed to heal the polyurethane at below than melting temperature of PCL by introducing the‘Diels-Alder based shape memory assisted self-healing’(DASMASH). However, almost all researches on self-healing polymer based on shape memory concept were used a commercial PCL
diol.
Therefore, in this work, we report on the development of thermal stimuli responsive self-healing polycaprolactone urethane2International Journal of Engineering & Technology(PCLU) coatings based on waste palm cooking oil with the ε-caprolactone (ε-CL) as a self-healing agent. The effects of different ratiosof ε-CL to WPCO based polyesteramide (PEA) on the self-healing
properties of PCLU coatings were investigated. The mechanism of
self-healing of PCLU coatings was also investigated and discussed.

melting temperature, Tm of all PCLU coatings produced by a dynamic scan with temperature range of -70 – 300°C and heating rateof 10 °C min-1 using a DSC823 Mettler Toledo calorimeter. A sample of pure PCL was also scanned for a reference.

2.5. Self-Healing Test

2. Materials and Methods
2.1. Materials
ε-caprolactone (high purity), 2-(dimethyamino)-ethanol (DMEA),
1,8-diazabicyclo 5.4.0 undec-7-ene (DBU), isophorone diisocyanate (IPDI) and diethylene glycol (DEG) were purchased from
Terra Scientific (M) Sdn. Bhd. Waste palm cooking oil (WPCO)
was collected from restaurants in Klang valley, Malaysia.

2.2. Synthesis of Waste Palm Cooking Oil (WPCO) based
Polycaprolactone Polyol
There are two-steps involved in the synthesis of WPCO based polyol. First step, WPCO based polyol was synthesized by using transesterification process by mixing a polyhydric solution with WPCO
and 1,8-diazabicyclo 5.4.0 undec-7-ene (DBU) as a catalyst. The
transesterification was performed in a chemical reactor at a temperature of 90°C for about 1 hour under vacuum and inert condition.
In the second step, the synthesis was continued by using the ring
opening polymerization by adding the ɛ-caprolactone (CL) into themixture and it was continuously stirred and maintained at the same
temperature for another 2 hours. The characteristic of PCL polyol
was discussed in our previous report [12].

2.3. Polycaprolactone urethane coatings preparation
For the preparation of polycaprolactone urethane (PCLU) coatings,
PCL polyol based on WPCO was reacted with isophorone diisocyanate (IPDI) with isocyanate to polyol (NCO:OH) ratio is 1:1. Nine
types of PCL-U coatings were prepared, labeled as PCLU-10,
PUCL-20, PCLU-30, PCLU-40, PCLU-50, PCLU-60, PCLU-70,
PCLU-80, PCLU-90. The details of the samples is described inTable 1. Prior to the coatings application, AA 6061 which was used
as a substrate was abraded with abrasive paper grits 600. The PCLU coating was applied on the metal surface by using nylon brushand the samples were left at room temperature for complete curing(72 hours) before testing their performance properties.
Table 1: PCLU Coating Samples

Self-healing ability of the coatings was characterized by making a
2 mm through-coating scratch with a sharp razor blade on the coating surface with a dimension of 10 x 10 mm2. The damage coatings
were heated in an oven at a temperature of 90°C for 2 hours to observe the self-healing behaviour of PCLU coatings. The height profile of PCLU coatings before and after heating, were measured using Atomic Force Microscopy (AFM). In this study, the maximum
peak to valley height roughness of both (before, Rtb and after heating, Rta) samples were used to calculate the shape recovery, Rt (%)
of PCLU coatings by using a formula in Equation 1.
𝑅 (%) =𝑅−𝑅𝑅(1)

× 100%The morphology before and after the healing process were also observed using an optical microscope with 50X and 100X magnifications.

3. Results and Discussion
3.1. Thermal Analyses
Glass transition temperatures, Tg and melting temperatures, Tm ofPCLU coatings are shown in Table 2. The results reveal that percentage of CL in polyol is significant to the reduction on Tg ofPCLU coating. The Tm was also lowered as the percentage of CL
increased. The reduction of Tg and Tm values might be due to thelong linear chain of PCL in the PU network. This linear chain iseasier to move and break compared to the branched chains fromwaste cooking oil which is more complex. In coatings with lower
CL content, the branched structures hindered the segmental mobility of the PCLU coating which led to higher Tg and Tm. Based on
this results, the heating condition was set to study the self-healing
behaviour of PCLU coatings. The shape memory effect is expected
to be triggered under this heating condition and the PCL chain are
expected to melt, diffuse into coating scratch and initiate the closureof the scratch.
Table 2: Glass transition and melting temperatures of PCLU coatings
SampleTg (°C)Tm (°C)PCLU-1035.27

208.15

SamplePolyols Used

Percentage of CL (%)
in Polyol

PCLU-10

PCL1010

PCLU-2034.04

200.0520

PCLU-30

33.57

198.86

PCLU-40

32.94

191.26

PCLU-20

PCL20

PCLU-30

PCL3030

PCLU-40

PCL4040

PCLU-5031.56

186.34

PCLU-60

30.05

172.11

PCLU-70

29.57

167.97

PCLU-80

27.23

162.96

PCLU-90

25.93

153.92

PCLU-50PCL50

50

PCLU-60

PCL60

60

PCLU-70

PCL70

70

PCLU-80

PCL80

80

PCLU-90PCL90

902.4. Differential Scanning Calorimetry

3.2. Self-Healing Behaviour

DSC test is one of the methods for thermal analyses. This test wascarried out to determine the glass transition temperature, Tg andFig.1 signifies the effect of CL content on shape recovery of PCLU
coatings. It is clearly shown that CL content gives a significanceInternational Journal of Engineering & Technology

effect on the self-healing behavior on PCLU coating. As the CLcontent increased the shape recovery is also improved. PCLU10
with 10% CL shows the lowest result on the shape recovery whichis 8.4% only. The percentage of shape recovery were slightly higher
as the CL content is higher, which reflects the improvement of theself-healing efficiencies of PCLU coating. The scratch healing process is more efficient for the samples with higher CL content. It
shows that the polymer network which contain the highest CL revealed nearly complete recovery due to the reconstruction of thepolymer network that was triggered at a temperature of 90ºC for 2hours. This might be due to the longer PCL network in the polymer
coating as the PCL contributes to soft segment.

3crack/scratch surfaces in spatial proximity to facilitate crack/scratchrebonding by the molten PCL.

Huang et al. [5] claimed that heating above the melting temperature (Tm) of PCL softened the PCL segments and induced theshape memory eﬀect. As a result, damage to the coating, such asmechanical scratches, could be physically closed to partially restore
the barrier properties of the coating [13]. Indeed, healing was only
effective in PCLU coating with higher CL content. It is worth noting that no appreciable differences was detected in PCLU coatingwith a low CL content, in accordance with the observed PCL recovery in the PCLU coating.

Fig. 2: Micrographs of self-healing behavior of PCLU coatings before andafter thermal stimulating at 90ºC , (a) PCLU10, (b) PCLU50, and (c)PCLU90.

3.3. Mechanism of Scratch Closure by Self-Healing

Fig. 1: Effect of CL content on shape recovery of PCLU coating

To confirm that self-healing effect indeed resulted from PCL content in the PU network, the PCLU coatings were further characterized using optical microscope. The micrographs of self-healing behavior on PCLU coatings before and after thermal stimulating at
90ºC can be observed in Fig.2. The original scratches in the figure
showed an effect as a consequence of plastic deformation inducedby the blade. The damage to the coating contains two forms i.e.
plastic/permanent deformation, indicated as the shaded area surrounding the crack tip; and cracks involving the creation of free surfaces. In severe cases, some portions of the material may even bepermanently removed from the coating, leaving voided space [14].
No significant different was found for PCLU10 sample before andafter 2 hours heating which is shown in Fig.2 (a). A low percentageof shape recovery might be the reason for this case. Fig.2 (b) shows
the behavior of PCLU50 sample. The image shows that the melted
PCL molecules have diffused and filled the gap but it was still notcomplete, ascertained the shape recovery result which is only
63.7%. Meanwhile, Fig.2 (c) shows the healing process melted PCL
molecules in PCLU90 samples was almost complete, supports theresult obtained which was recorded 98% recovery.
The results presented so far have revealed encouraging selfhealing of the PCLU coating. This eﬀectively brings the separated

Self-healing is initiated by heating the damaged coating to a temperature higher than both the liquefying temperature of the ﬁbers(in this case, melting temperature or Tm) as well as the transitiontemperature (glass transition temperature or Tg) of the SMP matrix.
The mechanism of scratch closure can be well explained by theschematic self-healing process of PCLU coating with a macroscratch shown in Fig.3. The healing process consists of two-steps.
In the first step, the damaged sample is heated to a temperature
above the melting temperature of PCL to trigger the thermal-induced shape memory behavior of PCL. The PCL molecules meltand diffuse into the narrowed crack by capillary force [1]. In thesecond step, the diffused PCL forms physical entanglement in thePU matrix and bridge the scratch with a solid wedge when the sample is cooled down. Therefore, the higher CL content will improvethe efficiencies of self-healing process.
During the repairing mechanism, the dissociation of the crosslinks temporarily enhances the mobility of the polymer chains andthe diﬀusion near the damaged area, following the formation of new
bonds that results in the repair of the network and recovery of themechanical properties [15]. Thus, the optimization of free volume
is an important factor to achieve the desirable mobility of the chains,which can also be controlled by external stimuli.

Fig. 3: Schematic depiction of thermal responsive self-healing mechanism of PCLU coatings.

4. Conclusion
A thermal stimuli responsive self-healing polycaprolactone urethane coating was developed from waste palm cooking oil. Basedon the results, it is concluded that the knowledge on the structure –property relationships is the key to improve the material design forenhancing the functional performance. It is believed that PCL structure in the PCL polyol based on waste palm cooking oil signified
the main contribution in the development of self-healing polyurethane coating which is contributed up to 98% of shape recovery bythermal stimulating. The shape memory effect favors the crack closure at temperature above the melting point of PCL. However, toverify the functional performance of PCLU coating, further studies
should be made especially on the recovery from the mechanical aspects.

Acknowledgement
We would like to express our sincere thanks to International Polyurethane Technology Foundation, Japan for the financial support.
We also express our gratitude to Universiti Kuala Lumpur and Islamic Science University of Malaysia (USIM) for the technical supports and facilities.

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