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Citation: Norambuena-Contreras, J.;
Concha, J.L.; Arteaga-Pérez, L.E.;
Gonzalez-Torre, I. Synthesis and
Characterisation of Alginate-Based
Capsules Containing Waste Cooking
Oil for Asphalt Self-Healing. Appl.
Sci. 2022,12, 2739. https://doi.org/
10.3390/app12052739
Academic Editor: Roberto Zivieri
Received: 12 January 2022
Accepted: 3 March 2022
Published: 7 March 2022
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4.0/).
applied
sciences
Article
Synthesis and Characterisation of Alginate-Based Capsules
Containing Waste Cooking Oil for Asphalt Self-Healing †
Jose Norambuena-Contreras 1, * , Jose L. Concha 1, Luis E. Arteaga-Pérez 2and Irene Gonzalez-Torre 1
1LabMAT, Department of Civil and Environmental Engineering, University of Bío-Bío,
Concepción 4051381, Chile; jlconcha@ubiobio.cl (J.L.C.); irenegon@ubiobio.cl (I.G.-T.)
2LPTC, Department of Wood Engineering, University of Bío-Bío, Concepción 4051381, Chile;
larteaga@ubiobio.cl
*Correspondence: jnorambuena@ubiobio.cl
† This paper is an extended version of the awarded best paper published in the Resilient Materials 4 Life
International Conference (RM4L2020), Cambridge, UK, 20–23 September 2021.
Featured Application: This work is a preliminary study for the potential application of biopoly-
meric polynuclear capsules containing waste cooking oil as a promising encapsulated rejuvenator
for microcrack self-healing in long-term aged bitumen.
Abstract:
This paper presents the synthesis and characterisation of biopolymeric capsules for asphalt
self-healing. A sodium alginate biopolymer extracted from the cell wall of brown algae was used
as the encapsulating material to contain Waste Cooking Oil (WCO) as a potential encapsulated
rejuvenating agent for aged bitumen. Polynuclear capsules were synthesised by ionic gelation.
The size, surface aspect and internal structure of the WCO capsules were evaluated using Optical
and Scanning Electron Microscopy. The physical-chemical properties and thermal stability of the
WCO capsules and their components were also evaluated. Moreover, the diffusion process and
self-healing capability of the released WCO on cracked bitumen test samples were determined by
image analysis through fluorescence microscopy. The main results of this study showed that the
WCO capsules presented a suitable morphology to be mixed in asphalt mixtures. WCO capsules
and their components presented mechanical and thermal stability and physical-chemical properties
which suggest their feasibility for self-healing applications. It was proven that the encapsulated WCO
can diffuse in the aged bitumen, reducing its viscosity and promoting the self-healing of microcracks.
Keywords: waste cooking oil; polynuclear capsule; asphalt rejuvenator; ageing; self-healing
1. Introduction
Self-healing by the action of encapsulated rejuvenating agents has been considered as
a revolutionary technology for autonomous crack-healing of asphalt materials [
1
]. Asphalt
materials are complex viscoelastic composites mainly used for asphalt pavement construc-
tion. Cracking of asphalt materials mainly occurs due to the oxidation of the hydrocarbons
in the bitumen by the action of operation and environmental agents [
2
]. When damage
occurs in an asphalt material containing embedded encapsulated rejuvenators, cracks
appear and eventually propagate until they reach and break or deform a capsule, releasing
the contained rejuvenating agent. Rejuvenating agents consist of lubricating and extender
oils with high proportions of maltene constituents, which restore the asphaltenes/maltenes
ratio in the aged bitumen during healing [
3
]. When the rejuvenating agent is released from
inside the capsule (i.e., activation of polynuclear or core-shell capsules by deformation or
break), the molecules of the released rejuvenator diffuse into the asphalt matrix and soften
the aged bitumen, allowing the rejuvenated bitumen to flow through the open microcracks,
thus facilitating the crack self-healing process [
4
]. Figure 1shows the concept of self-healing
asphalt by the action of encapsulated rejuvenators.
Appl. Sci. 2022,12, 2739. https://doi.org/10.3390/app12052739 https://www.mdpi.com/journal/applsci
Appl. Sci. 2022,12, 2739 2 of 10
Appl. Sci. 2022, 11, x FOR PEER REVIEW 2 of 10
the open microcracks, thus facilitating the crack self-healing process [4]. Figure 1 shows
the concept of self-healing asphalt by the action of encapsulated rejuvenators.
Recent studies have successfully proven the efficacy of numerous oils as promising,
more sustainable alternatives for the design of encapsulated rejuvenators for aged asphalt,
such as dense aromatic oil [5], waste cooking oil [6], mineral engine oil [7], sunflower oil
[8] and, recently, bio-oil from liquefied agricultural biomass waste [9] and pyrolytic oil
from waste tyres pyrolysis [10]. However, the polymers currently used for the encapsula-
tion process, for example melamine-formaldehyde [6], can give rise to a potentially high
environmental risk from the leaching of hazardous chemical compounds. Therefore, more
sustainable capsules based on biopolymers are necessary to make this technology suitable
for use in a new generation of asphalt pavements [11].
Sodium alginate is a biopolymer water-soluble polysaccharide extracted from the cell
wall of various species of brown algae. Aspects such as great availability, low cost [12],
high capacity to form a gel at low concentrations, nontoxicity, biocompatibility [13], and
long-term stability [14] make sodium alginate a promising encapsulating material, reduc-
ing the negative environmental impact of typical polymers. Currently, the use of alginate
as an encapsulating material of rejuvenators for asphalt has been explored by many au-
thors. Experimental work was carried out by Norambuena-Contreras et al. in stone mastic
asphalt [15] and a dense mixture [16]; the authors concluded that the addition of alginate-
based capsules with sunflower oil and incorporated in a concentration of 0.5% per total
weight of mixture significantly improved the healing capability of the respective mixtures.
More currently, Ruiz-Riancho et al. [17] characterised alginate-biopolymer polynuclear
capsules with virgin sunflower oil as a rejuvenator, proving that the strength of the cap-
sules was influenced by the pore size of the calcium-alginate structure, and that the cap-
sules could resist the temperature that is reached during asphalt mixing and compaction.
Figure 1. Concept of self-healing in asphalt using encapsulated rejuvenators. (a) Polynuclear cap-
sules are spherical particles with the encapsulated material distributed throughout a polymeric po-
rous matrix structure. (b) Propagation of microcracks in an asphalt mixture incorporating polynu-
clear capsules. (c) Microcracks reach a polynuclear capsule resulting in their superficial rupture, and
so, releasing the encapsulated healing agent (i.e., WCO). (d) The healing agent diffuses into the
cracked zone with a softening effect on the aged bitumen, sealing the damaged area.
Despite the urgent need to develop more sustainable and resilient construction ma-
terials, and the extensive diversity of renewable resource-based polymers for these pur-
poses, the use of biomaterials in the civil engineering industry remains limited. Studies
on the control and design variables for the correct synthesis of alginate-based capsules for
asphalt self-healing purposes are still very limited in number. This study aims to synthe-
sise and characterise polynuclear biopolymeric capsules for asphalt self-healing, where a
sodium alginate biopolymer from the cell wall of brown algae Laminaria hyperborea was
used as the encapsulating material. Waste Cooking Oil was encapsulated in biopolymer
as a potential rejuvenating agent to design more sustainable asphalt roads, because its
content of light oil components is similar to that of bitumen. As an innovative approach,
aspects such as 1) the adequate hydration of the biopolymer before encapsulation, 2) the
Figure 1.
Concept of self-healing in asphalt using encapsulated rejuvenators. (
a
) Polynuclear capsules
are spherical particles with the encapsulated material distributed throughout a polymeric porous
matrix structure. (
b
) Propagation of microcracks in an asphalt mixture incorporating polynuclear
capsules. (
c
) Microcracks reach a polynuclear capsule resulting in their superficial rupture, and so,
releasing the encapsulated healing agent (i.e., WCO). (
d
) The healing agent diffuses into the cracked
zone with a softening effect on the aged bitumen, sealing the damaged area.
Recent studies have successfully proven the efficacy of numerous oils as promising,
more sustainable alternatives for the design of encapsulated rejuvenators for aged asphalt,
such as dense aromatic oil [
5
], waste cooking oil [
6
], mineral engine oil [
7
], sunflower oil [
8
]
and, recently, bio-oil from liquefied agricultural biomass waste [
9
] and pyrolytic oil from
waste tyres pyrolysis [
10
]. However, the polymers currently used for the encapsulation
process, for example melamine-formaldehyde [
6
], can give rise to a potentially high en-
vironmental risk from the leaching of hazardous chemical compounds. Therefore, more
sustainable capsules based on biopolymers are necessary to make this technology suitable
for use in a new generation of asphalt pavements [11].
Sodium alginate is a biopolymer water-soluble polysaccharide extracted from the cell
wall of various species of brown algae. Aspects such as great availability, low cost [
12
],
high capacity to form a gel at low concentrations, nontoxicity, biocompatibility [
13
], and
long-term stability [
14
] make sodium alginate a promising encapsulating material, reducing
the negative environmental impact of typical polymers. Currently, the use of alginate as
an encapsulating material of rejuvenators for asphalt has been explored by many authors.
Experimental work was carried out by Norambuena-Contreras et al. in stone mastic
asphalt [
15
] and a dense mixture [
16
]; the authors concluded that the addition of alginate-
based capsules with sunflower oil and incorporated in a concentration of 0.5% per total
weight of mixture significantly improved the healing capability of the respective mixtures.
More currently, Ruiz-Riancho et al. [
17
] characterised alginate-biopolymer polynuclear
capsules with virgin sunflower oil as a rejuvenator, proving that the strength of the capsules
was influenced by the pore size of the calcium-alginate structure, and that the capsules
could resist the temperature that is reached during asphalt mixing and compaction.
Despite the urgent need to develop more sustainable and resilient construction materi-
als, and the extensive diversity of renewable resource-based polymers for these purposes,
the use of biomaterials in the civil engineering industry remains limited. Studies on the
control and design variables for the correct synthesis of alginate-based capsules for asphalt
self-healing purposes are still very limited in number. This study aims to synthesise and
characterise polynuclear biopolymeric capsules for asphalt self-healing, where a sodium
alginate biopolymer from the cell wall of brown algae Laminaria hyperborea was used as the
encapsulating material. Waste Cooking Oil was encapsulated in biopolymer as a potential
rejuvenating agent to design more sustainable asphalt roads, because its content of light oil
components is similar to that of bitumen. As an innovative approach, aspects such as (1) the
adequate hydration of the biopolymer before encapsulation, (2) the control of the physical
stability of the oil-in-water (O/W) emulsion, and (3) adequate height between the outgoing
emulsion from the needle tip and the hardening solution are reported in this study.
Appl. Sci. 2022,12, 2739 3 of 10
2. Materials and Methods
2.1. Materials
Biopolymeric capsules containing Waste Cooking Oil for asphalt self-healing were
prepared in this study. The polymeric structure of the capsules was prepared from low-
viscosity grade sodium alginate (viscosity @20
◦
C 200–300 cP for 2% w/vsolution) provided
by Buchi (Flawil, Switzerland), and calcium-chloride dihydrate (CaCl
2·
2H
2
O) with 70%
purity provided by Winkler (Concepción, Chile). Waste Cooking Oil coming from recycled
sunflower oil after one-cycle frying at 180
◦
C (density 0.85 g/cm
3
, viscosity @20
◦
C 89 cP
and pH @25
◦
C 4.4–4.6) was used as a sustainable rejuvenating agent. Additionally, a virgin
bitumen with density 1.04 g/cm
3
and penetration grade 50/70 (penetration 60
×
10
−1
mm
@25
◦
C and softening point 50
◦
C) was long-term aged by standard Pressure Air Vessel
(PAV) tests [
18
] to quantify the self-healing efficiency of the Waste Cooking Oil in aged
asphalt. The PAV test was carried out at 100
◦
C for 20 h, simulating the bitumen’s long-term
ageing during its service life [11].
2.2. Synthesis of WCO Capsules by Dripping Method
WCO capsules were synthesised in the laboratory through the cross-linking of sodium
alginate in the presence of calcium ions (Ca
2+
) by ionic gelation, using the microfluidic
pressure pump method. This method is based on the procedure described by Norambuena-
Contreras et al. [
9
]. Briefly, an aqueous solution of sodium-alginate with a concentration
of 2% of weight by volume of water was used to produce capsules. The alginate solution
was maintained in constant agitation for 24 h using a magnetic stirrer (Scilogex, Model
SCI550-S, Rocky Hill, CO, USA) to properly hydrate the biopolymer, based on the recom-
mendation of Norambuena-Contreras et al. [
10
]. The solution was then mixed with WCO to
generate a WCO-in-alginate emulsion, which is then pumped (2 mL/min) via an automatic
microfluidic device (New Era NE-1010, Farmingdale, NY, USA) through a hollow metal
needle (ID: 1.2 mm) into a calcium-chloride (CaCl
2
) solution acting as a hardener, which
was in constant agitation using a magnetic stirrer at 250 rpm. The height of separation
between the needle tip and the hardening solution was settled to 350 mm since a higher
height could result in the break of the droplets when dropped in the solution, as reported
by Martins et al. [
13
]. A graphical representation of this encapsulation procedure is shown
in Figure 2. Finally, produced WCO capsules (approximately 50 g in total) were placed into
a container and stored in a freezer at
−
18
◦
C to avoid the oxidation of the encapsulated
rejuvenating agent.
2.3. Experimental Characterisation of the WCO Capsules and Their Components
The size, surface aspect and internal microstructure of the WCO capsules synthetised
according to Figure 2were characterised by Optical (Leica EZ4, Wetzlar, Germany) and
Scanning Electron Microscopy (Hitachi SU 3500, Chiyoda, Tokyo, Japan), respectively.
Additionally, the presence of chemical elements in the surface of polynuclear WCO capsules
was evaluated by Scanning Electron Microscope (SEM) coupled to energy dispersive X-
ray spectroscopy (EDS, Bruker Quantax 100, Billerica, MA, USA) for semi-quantitative
determinations. Encapsulation efficiency of the WCO capsules was also quantified by a
chemical procedure based on Guadarrama-Lezama et al. [
19
]. Bulk density and uniaxial
compressive strength (@20
◦
C and loading rate 0.2 mm/min using a Universal Testing
Machine, Test Resources, Shakopee, MN, USA) of the capsules were measured by the test
method B of ASTM D792-13 [20] and ASTM D695-02a [21], respectively.
Appl. Sci. 2022,12, 2739 4 of 10
Appl. Sci. 2022, 11, x FOR PEER REVIEW 4 of 10
evaluated through the creaming index, by measuring the separation of the WCO oil drop-
lets from the alginate solution as proposed by McClements [22], and recently used by
Concha et al. [23]. Fourier transform infrared spectrometer in mode of attenuated total
reflection (FTIR-ATR) of the WCO and its comparison with a Virgin Cooking Oil (VCO)
was recorded in a Perkin Elmer Spectrum Two spectrometer (Waltham, MA, USA) be-
tween 400 and 4000 cm
−1
(20 Scans at 2 cm
−1
). Additionally, thermogravimetric analysis
and derivative thermogravimetry (TGA-DTG) of WCO was carried out between ambient
and 600 °C at 10 °C /min in N
2
(10 mL/min) in a TA Tech Q50 thermobalance (New Castle,
DE, USA).
Finally, the healing capacity of WCO to seal a microcrack overtime on long-term age-
ing bitumen samples by Pressure Air Vessel (PAV-samples) was evaluated under the cur-
rent method proposed by Norambuena-Contreras et al. [9] and [10]. For that, a ∼2 mg
drop of WCO was dropped on a cracked-PAV bitumen sample with dimensions 20 × 20 ×
0.5 mm and an artificial microcrack of 200 µm-width along the sample. The µ-crack clo-
sure over time by the effect of WCO diffusion was recorded by images using an inverted
fluorescence microscope (ICOE IV 5100 FL, Ningbo, China). Crack-width was measured
at six positions using the image software ImageJ
®
(Fiji distribution, version 1.52p, National
Institutes of Health, Bethesda, MD, USA). The complete crack closing process was rec-
orded in a maximum time of 85 min [9]. Lastly, the healing efficiency of the WCO over
time was quantified as the relationship between the average crack-width at a specific time
measured in µm and the average initial crack-width measured in µm [10].
Figure 2. Representation of the procedure to synthetise polynuclear capsules containing WCO as
rejuvenator. (a) First, an alginate-based O/W emulsion containing WCO is extruded by using a sy-
ringe pressure pump. (b) The alginate biopolymer present in the emulsion possesses Guluronic-
Guluronic (G-G) molecular blocks structures with the capacity to crosslink in the presence of diva-
lent Ca
2+
ions. Thus, when the extruded O/W emulsion drop into a CaCl
2
bath (c) the ionic gelation
process take place by exchanging the Na
+
ions from carboxylic acids in the G-G blocks with Ca
2+
ions, resulting in (d) the well-known “egg-box” crosslinked complex. In the end, (e) capsules are
formed, where the Ca-alginate complex encapsulate the WCO in multiple cavities. Figure inspired
on Norambuena-Contreras et al. [9].
3. Results and discussion
3.1. Physical-Mechanical Properties of the Synthesised Capsules
Figure 3 presents the main results of experimental characterisation of the WCO cap-
sules. Statistical size analysis of 100 individual WCO capsules registered an average size
of 1.649 mm (SD = 0.145 mm), with a spherical and uniform geometry (Figure 3a). These
capsules presented an encapsulation efficiency of WCO close to 90%. SEM-EDS observa-
tions (Figure 3d) proved that WCO capsules reveal a polynuclear structure on their sur-
face (Figure 3b,c) with an elemental composition of Ca (73%) and Na (27%), which is
Figure 2.
Representation of the procedure to synthetise polynuclear capsules containing WCO as
rejuvenator. (
a
) First, an alginate-based O/W emulsion containing WCO is extruded by using a
syringe pressure pump. (
b
) The alginate biopolymer present in the emulsion possesses Guluronic-
Guluronic (G-G) molecular blocks structures with the capacity to crosslink in the presence of divalent
Ca
2+
ions. Thus, when the extruded O/W emulsion drop into a CaCl
2
bath (
c
) the ionic gelation
process take place by exchanging the Na
+
ions from carboxylic acids in the G-G blocks with Ca
2+
ions, resulting in (
d
) the well-known “egg-box” crosslinked complex. In the end, (
e
) capsules are
formed, where the Ca-alginate complex encapsulate the WCO in multiple cavities. Figure inspired on
Norambuena-Contreras et al. [9].
The individual constituents used for synthesising the WCO polynuclear capsules
(see Figure 2) were also characterised. The physical stability of the O/W emulsion was
evaluated through the creaming index, by measuring the separation of the WCO oil droplets
from the alginate solution as proposed by McClements [22], and recently used by Concha
et al. [
23
]. Fourier transform infrared spectrometer in mode of attenuated total reflection
(FTIR-ATR) of the WCO and its comparison with a Virgin Cooking Oil (VCO) was recorded
in a Perkin Elmer Spectrum Two spectrometer (Waltham, MA, USA) between 400 and
4000 cm
−1
(20 Scans at 2 cm
−1
). Additionally, thermogravimetric analysis and derivative
thermogravimetry (TGA-DTG) of WCO was carried out between ambient and 600
◦
C at
10 ◦C/min in N2(10 mL/min) in a TA Tech Q50 thermobalance (New Castle, DE, USA).
Finally, the healing capacity of WCO to seal a microcrack overtime on long-term ageing
bitumen samples by Pressure Air Vessel (PAV-samples) was evaluated under the current
method proposed by Norambuena-Contreras et al. [
9
,
10
]. For that, a
∼
2 mg drop of WCO
was dropped on a cracked-PAV bitumen sample with dimensions 20
×
20
×
0.5 mm and
an artificial microcrack of 200
µ
m-width along the sample. The
µ
-crack closure over time
by the effect of WCO diffusion was recorded by images using an inverted fluorescence
microscope (ICOE IV 5100 FL, Ningbo, China). Crack-width was measured at six positions
using the image software ImageJ
®
(Fiji distribution, version 1.52p, National Institutes
of Health, Bethesda, MD, USA). The complete crack closing process was recorded in a
maximum time of 85 min [
9
]. Lastly, the healing efficiency of the WCO over time was
quantified as the relationship between the average crack-width at a specific time measured
in µm and the average initial crack-width measured in µm [10].
3. Results and Discussion
3.1. Physical-Mechanical Properties of the Synthesised Capsules
Figure 3presents the main results of experimental characterisation of the WCO cap-
sules. Statistical size analysis of 100 individual WCO capsules registered an average size
of 1.649 mm (SD = 0.145 mm), with a spherical and uniform geometry (Figure 3a). These
capsules presented an encapsulation efficiency of WCO close to 90%. SEM-EDS obser-
vations (Figure 3d) proved that WCO capsules reveal a polynuclear structure on their
Appl. Sci. 2022,12, 2739 5 of 10
surface (Figure 3b,c) with an elemental composition of Ca (73%) and Na (27%), which
is consistent with the materials used for the synthesis of the porous Ca-alginate matrix
structure in the presence of divalent calcium cations (Ca
2+
) allowing that the oil-in-alginate
can be cross-linked.
Appl. Sci. 2022, 11, x FOR PEER REVIEW 5 of 10
consistent with the materials used for the synthesis of the porous Ca-alginate matrix struc-
ture in the presence of divalent calcium cations (Ca
2+
) allowing that the oil-in-alginate can
be cross-linked.
The size distribution of WCO capsules is shown by the frequency histogram in Figure
3e, proving that the capsule size can be fitted to the normal probability distribution (P-
value 0.405 given by A–D test). Conversely, Figure 3f shows the pore size distribution of
the internal multicavity of the WCO capsules. The frequency histogram proves that the
capsule pore size can be fitted to the log-normal probability distribution function (P-value
0.159 given by A–D test). This Figure revealed that the internal structure of the capsules
was characterised by micropores with areas <120 µm
2
and with an occurrence probability
of 75%. Thus, the WCO was majorly encapsulated into small internal pores, contributing
to homogeneously distribute the oil into the overall volume of the capsule.
Additionally, Figure 3h and 3i show SEM images of an individual capsule and the
fracture type on a capsule broken by effect of a compression test, respectively. This result
proves that WCO capsules can break and partially release the encapsulated WCO by effect
of an external trigger. Biopolymeric matrix presented an elastic-plastic mechanical behav-
iour with breakage in plastic deformation around the propagating cracks, see Figure 3i
and 3j. WCO capsules registered an average maximum compressive force of 11.6 N (SD =
2.3 N) at an average maximum deformation of 0.7 mm (SD = 0.1 mm), Figure 3k. This
result suggests that polynuclear WCO capsules can resist the asphalt manufacturing pro-
cess (i.e., mixing and compaction) based on the results published by Garcia et al. [4] and
Norambuena-Contreras et al. [8] (minimum compressive force of 10 N); hence, WCO cap-
sules can be used as a resistant encapsulated rejuvenator for asphalt mixture self-healing.
Figure 3. Experimental characterisation of WCO capsules: (a) Optical image of capsule; (b) SEM
image of capsule; (c) SEM detail image of polynuclear surface structure of capsule; (d) SEM-EDS
observation on the capsule surface; (e) Frequency histogram of the size of capsules with Normal
fitting; (f) Frequency histogram of the pore size of internal structure of capsules with Log-normal
fitting; (g) SEM image of the multicavity (egg-box) structure into capsules; (h) SEM image of an
individual broken capsule by uniaxial compression; (i) Detail by SEM image of the fracture type in
the broken capsule; (j) Force and deformation average curves of the compression tests; and (k) Av-
erage values of the max. compression force and deformation registered by the capsules tested by a
load cell of 1 kN at a speed rate of 0.2 mm/min.
3.2. Physical Stability of Components of the Capsules
Furthermore, the characterisation of the capsule’s components was also evaluated.
Physical separation of the O/W components (i.e., WCO and alginate biopolymer solution)
is described by the creaming process in Figure 4a. Figure 4b shows that the average cream-
ing index results increased with time, and after 30 h the creaming index was 83.02%
Figure 3.
Experimental characterisation of WCO capsules: (
a
) Optical image of capsule; (
b
) SEM
image of capsule; (
c
) SEM detail image of polynuclear surface structure of capsule; (
d
) SEM-EDS
observation on the capsule surface; (
e
) Frequency histogram of the size of capsules with Normal
fitting; (
f
) Frequency histogram of the pore size of internal structure of capsules with Log-normal
fitting; (
g
) SEM image of the multicavity (egg-box) structure into capsules; (
h
) SEM image of an
individual broken capsule by uniaxial compression; (
i
) Detail by SEM image of the fracture type
in the broken capsule; (
j
) Force and deformation average curves of the compression tests; and (
k
)
Average values of the max. compression force and deformation registered by the capsules tested by a
load cell of 1 kN at a speed rate of 0.2 mm/min.
The size distribution of WCO capsules is shown by the frequency histogram in
Figure 3e
, proving that the capsule size can be fitted to the normal probability distribution
(p-value 0.405 given by A–D test). Conversely, Figure 3f shows the pore size distribution of
the internal multicavity of the WCO capsules. The frequency histogram proves that the
capsule pore size can be fitted to the log-normal probability distribution function (p-value
0.159 given by A–D test). This Figure revealed that the internal structure of the capsules
was characterised by micropores with areas < 120
µ
m
2
and with an occurrence probability
of 75%. Thus, the WCO was majorly encapsulated into small internal pores, contributing to
homogeneously distribute the oil into the overall volume of the capsule.
Additionally, Figure 3h,i show SEM images of an individual capsule and the fracture
type on a capsule broken by effect of a compression test, respectively. This result proves
that WCO capsules can break and partially release the encapsulated WCO by effect of an
external trigger. Biopolymeric matrix presented an elastic-plastic mechanical behaviour
with breakage in plastic deformation around the propagating cracks, see Figure 3i,j. WCO
capsules registered an average maximum compressive force of 11.6 N (SD = 2.3 N) at an
average maximum deformation of 0.7 mm (SD = 0.1 mm), Figure 3k. This result suggests
that polynuclear WCO capsules can resist the asphalt manufacturing process (i.e., mixing
and compaction) based on the results published by Garcia et al. [
4
] and Norambuena-
Contreras et al. [
8
] (minimum compressive force of 10 N); hence, WCO capsules can be
used as a resistant encapsulated rejuvenator for asphalt mixture self-healing.
3.2. Physical Stability of Components of the Capsules
Furthermore, the characterisation of the capsule’s components was also evaluated.
Physical separation of the O/W components (i.e., WCO and alginate biopolymer solution)
Appl. Sci. 2022,12, 2739 6 of 10
is described by the creaming process in Figure 4a. Figure 4b shows that the average
creaming index results increased with time, and after 30 h the creaming index was 83.02%
corresponding to the total separation of the O/W emulsion. This phenomenon was mainly
attributed to the increase in size of the WCO droplets with time by means of coalescence,
favouring the ascension of the oil droplets to form a creamed layer. Evidence of this
aggregation phenomenon between the oil droplets is shown by the microscopic fluorescence
images in Figure 4c–e, and further quantified by the droplet size frequency histograms
in Figure 4f–h. For the successful synthesis of the WCO capsules, physically stable O/W
emulsion must be used during the encapsulation process, and so, the creaming effect
should be controlled. Thus, based on this analysis, the encapsulation of the O/W emulsion
should take place for a time no longer that 2 h since the emulsion is fabricated and kept in
response at ambient temperature.
Appl. Sci. 2022, 11, x FOR PEER REVIEW 6 of 10
corresponding to the total separation of the O/W emulsion. This phenomenon was mainly
attributed to the increase in size of the WCO droplets with time by means of coalescence,
favouring the ascension of the oil droplets to form a creamed layer. Evidence of this ag-
gregation phenomenon between the oil droplets is shown by the microscopic fluorescence
images in Figure 4c–e, and further quantified by the droplet size frequency histograms in
Figure 4f–h. For the successful synthesis of the WCO capsules, physically stable O/W
emulsion must be used during the encapsulation process, and so, the creaming effect
should be controlled. Thus, based on this analysis, the encapsulation of the O/W emulsion
should take place for a time no longer that 2 h since the emulsion is fabricated and kept in
response at ambient temperature.
Figure 4. (a) Representation of the creaming evolution over time; (b) Results of creaming index
measurements over time; (c–e) Fluorescence microscopy images showing the droplet size evolution
over time (0 h, 3 h, and 30 h, respectively); and (f–h) frequency histograms of the droplet diameter
fitted to a log-normal distribution at 0 h, 3 h, and 30 h.
3.3. Thermal Stability of Capsule and Their Components
Moreover, cooking and frying activities involve the oxidation of oils with the conse-
quent loss of unsaturation through bond breaking, additions, substitution, and other well-
documented reactions [24]; thus, a significant degradation of the oil is expected after the
heating process. Despite this, the FTIR spectra recorded for both VCO and WCO were
very similar (see Figure 5a). The disappearance of the –OH stretching characteristic band,
at 3350 cm
−1
, in the WCO could be related to the absence of mono and diglycerides. Mean-
while, the band found at 725 cm
−1
is typical of –CH
2
rocking, while those at 1100 cm
−1
and
1250 cm
−1
correspond to the C–O stretching vibrations, commonly found in ethers [25].
The vibration of C–H bond in methyl groups is confirmed by the absorption bands at 1370
cm
-1
and 1450 cm
−1
, respectively. The strong signal at 1740 cm
−1
is typical of carbonyl
groups (C=O) in saturated aliphatic ethers, while the bands between 2850 and 3015 cm
−1
,
correspond to symmetrical and nonsymmetrical C–H stretching in methyl (–CH
3
) and
Figure 4.
(
a
) Representation of the creaming evolution over time; (
b
) Results of creaming index
measurements over time; (c–e) Fluorescence microscopy images showing the droplet size evolution
over time (0 h, 3 h, and 30 h, respectively); and (
f
–
h
) frequency histograms of the droplet diameter
fitted to a log-normal distribution at 0 h, 3 h, and 30 h.
3.3. Thermal Stability of Capsule and Their Components
Moreover, cooking and frying activities involve the oxidation of oils with the con-
sequent loss of unsaturation through bond breaking, additions, substitution, and other
well-documented reactions [
24
]; thus, a significant degradation of the oil is expected after
the heating process. Despite this, the FTIR spectra recorded for both VCO and WCO
were very similar (see Figure 5a). The disappearance of the –OH stretching characteristic
band, at 3350 cm
−1
, in the WCO could be related to the absence of mono and diglyc-
erides. Meanwhile, the band found at 725 cm
−1
is typical of –CH
2
rocking, while those at
1100 cm
−1
and 1250 cm
−1
correspond to the C–O stretching vibrations, commonly found
in ethers [
25
]. The vibration of C–H bond in methyl groups is confirmed by the absorption
bands at 1370 cm
−1
and 1450 cm
−1
, respectively. The strong signal at 1740 cm
−1
is typical
Appl. Sci. 2022,12, 2739 7 of 10
of carbonyl groups (C=O) in saturated aliphatic ethers, while the bands between 2850
and 3015 cm
−1
, correspond to symmetrical and nonsymmetrical C–H stretching in methyl
(–CH
3
) and methylene (–CH
3
) groups. The similarities in intensity, position and nature of
functional groups identified by their characteristic absorption bands in both spectra (VCO
and WCO), suggest that WCO is thermally stable, which supports its use as encapsulated
rejuvenating agent in asphalt materials usually manufactured at a temperature of 160 ◦C.
Appl. Sci. 2022, 11, x FOR PEER REVIEW 7 of 10
methylene (–CH
3
) groups. The similarities in intensity, position and nature of functional
groups identified by their characteristic absorption bands in both spectra (VCO and
WCO), suggest that WCO is thermally stable, which supports its use as encapsulated re-
juvenating agent in asphalt materials usually manufactured at a temperature of 160 °C.
Figure 5. Results of chemical and thermal characterisation of rejuvenating agents (WCO versus
VCO), WCO capsule and sodium-alginate biopolymer: (a) Normalised infrared spectra of waste and
virgin cooking oil; and (b) TGA results of biopolymer (BioPoly) and WCO capsules (WCO Cap).
Additionally, the thermal stability of the biopolymer (BioPoly) used for the prepara-
tion of the WCO capsules was confirmed by the TGA curve in Figure 5b. Indeed, decom-
position at 160 °C -temperature of asphalt mixture preparation-, was nearly 5% for the
polymer and remained unchanged when the WCO was encapsulated within the capsules.
Two major DTG peaks corresponding to 12% and 32% wt. loss were identified for the
BioPoly and, remained similar (with lower intensity) in the WCO Cap. The first peak at
183 °C is associated with structural dehydration reactions in the alginate, while the second
one, at 278 °C, corresponds to the degradation of CaCO
3
. The weight loss below 300 °C is
caused by the loss of hydroxyl groups in the alginate and above this temperature decar-
boxylation reactions take place forming CO
2
as main product [26].
The relative position of the TGA curve for the WCO capsule indicates a higher ther-
mal stability caused by the presence of the encapsulated WCO. In fact, the first decompo-
sition phase ends at 320 °C and is associated with the capsule. Above this temperature,
the oil decomposes in a two-stage process: the first stage (383 °C) corresponds to scissoring
and breakage of C–H and C–O bonds and, the second (ending at 500 °C) corresponds to
cross-linking and carbonisation.
Results of TGA are in line with that found in the FTIR and suggest that encapsulation
leads to a thermally stable material. Thus, when capsules are incorporated into a hot as-
phalt mixture, no significative thermal reaction between alginate matrix and asphalt
should be expected, maintaining the integrity of the capsule. In fact, Norambuena-Con-
treras el al. [16] proved that the alginate-based capsules present good thermal and me-
chanical stability, surviving the mixing and compaction processes showing a strong ad-
hesion to asphalt mastic by effect of a good interlocking with aggregates. They also con-
cluded that capsule content up to 0.5% wt. of total weight of mixture is adequate to not
affect the rheological properties of asphalt. With all these results in hand, capsules could
be effectively used for asphalt self-healing application.
3.4. Healing Ability of the Encapsulated Rejuvenator
The healing efficiency of WCO was quantified as a proof of concept in cracked long-
term ageing bitumen test samples, see Figure 6a. The crack closure by WCO diffusion (oil
Figure 5.
Results of chemical and thermal characterisation of rejuvenating agents (WCO versus VCO),
WCO capsule and sodium-alginate biopolymer: (
a
) Normalised infrared spectra of waste and virgin
cooking oil; and (b) TGA results of biopolymer (BioPoly) and WCO capsules (WCO Cap).
Additionally, the thermal stability of the biopolymer (BioPoly) used for the preparation
of the WCO capsules was confirmed by the TGA curve in Figure 5b. Indeed, decomposition
at 160
◦
C -temperature of asphalt mixture preparation-, was nearly 5% for the polymer and
remained unchanged when the WCO was encapsulated within the capsules.
Two major DTG peaks corresponding to 12% and 32% wt. loss were identified for
the BioPoly and, remained similar (with lower intensity) in the WCO Cap. The first peak
at 183
◦
C is associated with structural dehydration reactions in the alginate, while the
second one, at 278
◦
C, corresponds to the degradation of CaCO
3
. The weight loss below
300
◦
C is caused by the loss of hydroxyl groups in the alginate and above this temperature
decarboxylation reactions take place forming CO2as main product [26].
The relative position of the TGA curve for the WCO capsule indicates a higher thermal
stability caused by the presence of the encapsulated WCO. In fact, the first decomposition
phase ends at 320
◦
C and is associated with the capsule. Above this temperature, the
oil decomposes in a two-stage process: the first stage (383
◦
C) corresponds to scissoring
and breakage of C–H and C–O bonds and, the second (ending at 500
◦
C) corresponds to
cross-linking and carbonisation.
Results of TGA are in line with that found in the FTIR and suggest that encapsulation
leads to a thermally stable material. Thus, when capsules are incorporated into a hot asphalt
mixture, no significative thermal reaction between alginate matrix and asphalt should be
expected, maintaining the integrity of the capsule. In fact, Norambuena-Contreras el al. [
16
]
proved that the alginate-based capsules present good thermal and mechanical stability,
surviving the mixing and compaction processes showing a strong adhesion to asphalt
mastic by effect of a good interlocking with aggregates. They also concluded that capsule
content up to 0.5% wt. of total weight of mixture is adequate to not affect the rheological
properties of asphalt. With all these results in hand, capsules could be effectively used for
asphalt self-healing application.
Appl. Sci. 2022,12, 2739 8 of 10
3.4. Healing Ability of the Encapsulated Rejuvenator
The healing efficiency of WCO was quantified as a proof of concept in cracked long-
term ageing bitumen test samples, see Figure 6a. The crack closure by WCO diffusion (oil
amount 2 mg equivalent to the release of the 100% of WCO from one representative capsule)
over time was recorded taking microscopy images during 85 min. As example, Figure 6b–d
show fluorescence microscopy images at 0 min, 40 min, and 80 min, respectively. Healing
efficiency measured in percentage, was quantified as the relationship between the average
partial crack-width at a specific time and the average initial crack-width, both measured
in
µ
m. Fluorescence microscopy shows that WCO can be diffused in the long-term aged
bitumen samples reducing their viscosity and contributing to the self-healing of the artificial
microcracks. The quantification of the healing effect is presented in Figure 6e, showing that
the diffusion of the WCO into the cracked zone reached a maximum healing efficiency of
70% at 85 min, reducing the initial microcrack width to 60 µm.
Appl. Sci. 2022, 11, x FOR PEER REVIEW 8 of 10
amount 2 mg equivalent to the release of the 100% of WCO from one representative cap-
sule) over time was recorded taking microscopy images during 85 min. As example, Fig-
ure 6b–d show fluorescence microscopy images at 0 min, 40 min, and 80 min, respectively.
Healing efficiency measured in percentage, was quantified as the relationship between
the average partial crack-width at a specific time and the average initial crack-width, both
measured in µm. Fluorescence microscopy shows that WCO can be diffused in the long-
term aged bitumen samples reducing their viscosity and contributing to the self-healing
of the artificial microcracks. The quantification of the healing effect is presented in Figure
6e, showing that the diffusion of the WCO into the cracked zone reached a maximum
healing efficiency of 70% at 85 min, reducing the initial microcrack width to 60 µm.
Figure 6. (a) Experimental set-up to evaluate the crack-healing of WCO on a long-term aged bitumen
sample; (b–d) Fluorescence microscopy images showing the microcrack closure over time using
WCO as healing agent; (e) Results of healing efficiency and crack width measured on the long-term
aged bitumen sample.
4. Conclusions
This paper evaluated alginate-based polynuclear capsules containing Waste Cooking
Oil (WCO) as a promising encapsulated rejuvenator for microcrack self-healing in long-
term aged bitumen. Based on the results, the following conclusions have been obtained:
• The encapsulation process by simple extrusion-dripping yielded alginate-based
WCO capsules with an adequate encapsulation efficiency and multicavity morphol-
ogy for asphalt self-healing applications.
• WCO polynuclear capsules and their components presented a good thermal stability
and physical-chemical properties. Creaming results showed that encapsulation of the
O/W emulsion should be addressed during the first 2 h.
• TGA and FTIR tests suggested that the encapsulation process leads to a thermally
stable material with potential to be mixed with hot asphalt mixtures for asphalt heal-
ing purposes.
• Mechanical characterisation proved that alginate-based WCO capsules can break and
partially release the encapsulated WCO oil through an external force trigger effect.
• It was proven through transient oil-bitumen diffusion tests that the encapsulated re-
juvenating agent WCO can be diffused in long-term aged bitumen test samples, re-
ducing their viscosity, and hence, healing microcracks present in the asphalt matrix.
Future work of this research will include a comprehensive study to understand the
effect of rejuvenating oil type (including Virgin Cooking Oil, Waste Cooking Oil, and Vir-
gin Engine Oil) on the synthesis and properties of alginate-based polynuclear capsules for
asphalt self-healing. An evaluation of the chemical changes associated with their incorpo-
ration into aged bitumen samples (SARA fractions analysis) must be considered.
Figure 6.
(
a
) Experimental set-up to evaluate the crack-healing of WCO on a long-term aged bitumen
sample; (
b
–
d
) Fluorescence microscopy images showing the microcrack closure over time using WCO
as healing agent; (
e
) Results of healing efficiency and crack width measured on the long-term aged
bitumen sample.
4. Conclusions
This paper evaluated alginate-based polynuclear capsules containing Waste Cooking
Oil (WCO) as a promising encapsulated rejuvenator for microcrack self-healing in long-term
aged bitumen. Based on the results, the following conclusions have been obtained:
•
The encapsulation process by simple extrusion-dripping yielded alginate-based WCO
capsules with an adequate encapsulation efficiency and multicavity morphology for
asphalt self-healing applications.
•WCO polynuclear capsules and their components presented a good thermal stability
and physical-chemical properties. Creaming results showed that encapsulation of the
O/W emulsion should be addressed during the first 2 h.
•
TGA and FTIR tests suggested that the encapsulation process leads to a thermally
stable material with potential to be mixed with hot asphalt mixtures for asphalt
healing purposes.
•
Mechanical characterisation proved that alginate-based WCO capsules can break and
partially release the encapsulated WCO oil through an external force trigger effect.
•
It was proven through transient oil-bitumen diffusion tests that the encapsulated
rejuvenating agent WCO can be diffused in long-term aged bitumen test samples,
reducing their viscosity, and hence, healing microcracks present in the asphalt matrix.
Future work of this research will include a comprehensive study to understand the
effect of rejuvenating oil type (including Virgin Cooking Oil, Waste Cooking Oil, and
Virgin Engine Oil) on the synthesis and properties of alginate-based polynuclear capsules
Appl. Sci. 2022,12, 2739 9 of 10
for asphalt self-healing. An evaluation of the chemical changes associated with their
incorporation into aged bitumen samples (SARA fractions analysis) must be considered.
Author Contributions:
Conceptualisation, J.N.-C.; methodology, J.N-C., J.L.C. and I.G.-T.; software,
J.N.-C.; validation, J.N.-C. and I.G.-T.; formal analysis, J.N.-C., J.L.C. and L.E.A.-P.; investigation,
J.N.-C., J.L.C. and I.G.-T.; resources, J.N.-C. and L.E.A.-P.; data curation, J.N.-C., J.L.C. and I.G.-T.;
writing—original draft preparation, J.N.-C. and J.L.C.; writing—review and editing, J.N.-C., J.L.C.,
L.E.A.-P. and I.G.-T.; visualisation, J.N.-C. and J.L.C.; supervision, J.N.-C.; project administration,
J.N.-C.; funding acquisition, J.N.-C. All authors have read and agreed to the published version of
the manuscript.
Funding:
This research was funded by the National Research and Development Agency (ANID)
through the Research Projects FONDECYT 1190027, FONDEQUIP EQMI170077 and CONICYT
PIA/Apoyo CCTE 170007.
Data Availability Statement: Not applicable.
Acknowledgments:
Second author wishes to thank the financial support given by the University of
Bío-Bío for his internal PhD scholarship granted. The authors extend their gratitude to the former
student Felipe Muñoz from LabMAT and Rodrigo Briones from CIPA-CONICYT Regional, for their
technical support with some laboratory tests.
Conflicts of Interest: The authors declare no conflict of interest.
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