The Synthetization and Analysis of Dicyclopentadiene and Ethylidene-Norbornene Microcapsule Systems

polymers

Article The Synthetization and Analysis of Dicyclopentadiene and Ethylidene-Norbornene Microcapsule Systems

Ionut Sebastian Vintila 1,2,*, Horia Iovu 2, Andreea Alcea 1, Andreia Cucuruz 3, Andrei Cristian Mandoc 1 and Bogdan Stefan Vasile 4

1 National Research and Development Institute for Gas Turbines COMOTI, 061126 Bucharest, Romania; [email protected] (A.A.); [email protected] (A.C.M.) 2 Department of Bioresources and Polymer Science, Faculty of Applied Chemistry and Materials Science, University Politehnica of Bucharest, 011061 Bucharest, Romania; [email protected] 3 Department of Science and Engineering of Oxide Materials and Nanomaterials, Faculty of Applied Chemistry and Materials Science, University Politehnica of Bucharest, 011061 Bucharest, Romania; [email protected] 4 National Research Centre for Micro and Nanomaterials, Faculty of Applied Chemistry and Materials Science, University POLITEHNICA of Bucharest, 060042 Bucharest, Romania; [email protected] * Correspondence: [email protected]; Tel.: +40-726998218

Received: 7 April 2020; Accepted: 25 April 2020; Published: 4 May 2020

Abstract: The activities of this paper were focused on an in-situ fabrication process for producing two self-healing systems containing dicyclopentadiene and 5-ethylidene-2-norbornene monomers encapsulated in a urea-formaldehyde shell and integration methods applied in the epoxy matrix to analyse and compare the influences of their integration into the neat epoxy matrix. The self-healing systems were first synthesized according to a literature review, and subsequently, an optimization process was conducted for the fabrication process. Neat epoxy specimens were fabricated as reference specimens and subjected to flexural tests. Several integration methods for incorporating the self-healing systems into the epoxy resin were investigated. The optimal method presenting the best dispersion of the healing system was achieved by reducing the viscosity of the epoxy matrix with 10 vol % acetone solution, the addition of a microcapsule in the matrix, and homogenization at 60 ◦C at 100 rpm. Thermal analysis was performed in order to observe the mass loss obtained with an increasing temperature and phase changes for both poly-urea-formaldehyde (PUF)-dicyclopentadiene (DCPD) and melamine-urea-formaldehyde (MUF)-5-ethylidene-2-norbornene (ENB) systems. The thermogravimetric analysis performed for the PUF-DCPD system indicates a total loss of mass in the range of 30–500 ◦C of 72.604% and for the MUF-ENB system, indicates a total mass loss in the range of 30–500 ◦C of 74.093%. Three-point bending tests showed higher mechanical properties for PUF-DCPD (80%) than MUF-ENB (40%) compared to the neat epoxy systems. Numerical simulations were performed to obtain a better understanding of the microcapsule behavior when embedded in an epoxy matrix.

Keywords: polymer composites; self-healing; thermal stability; FEM analysis; dicyclopentadiene; 5-ethylidene-2-norbornene

1. Introduction As thermoset polymers possess good physical and mechanical properties (good wettability, corrosion resistance, and high strength), are easy to process, and have a low cost of production, they are extensively used in the manufacturing process of ﬁber reinforced polymer (FRP) composite structures.

Polymers 2020, 12, 1052; doi:10.3390/polym12051052 www.mdpi.com/journal/polymers Polymers 2020, 12, 1052 2 of 27

During their service life, these polymer composite materials are exposed to several mechanical loads that cause distinctive failure mechanisms with respect to their morphology, when compared to metals. As crack formation appears as a result of mechanical loads, the necessity of repairing these materials became an important aspect in the development of such materials. Therefore, several self-healing techniques have emerged and been investigated in order to overcome this limitation [1,2]. Currently, self-healing techniques are rapidly growing, comprising a wide range of self-healing polymer systems. Drawing inspiration from natural healing systems, researchers have designed and developed materials that can cure themselves after a damage event, thus extending their lifetime. Over the last years, distinct, nature-inspired concepts for delivering an autonomous ﬂuid system to the damaged area have been developed [3–6], in order to completely repair the FRP composites. With respect to traditional materials, self-healing materials possess the ability to heal themselves when subjected to failure through fracture or fatigue. To date, the majority of studies in the self-healing area have been concentrated on polymers and polymer composites as they are widely used in many important industrial applications (aeronautic, aerospace, marine, transport, military, medicine, etc.). To be considered a self-healing material, a specimen has to possess the following properties:

- The ability to heal the material automatically; - The ability to heal the damage multiple times; - The ability to heal defects of any size; - A low maintenance cost; - Present at least equal mechanical properties compared to traditional materials; - Present an economic advantage over traditional materials.

Major industries are using epoxy resin systems as matrices in the fabrication of multiple composite structures due to their excellent chemical, physical, and mechanical properties. Epoxies are more effective as healing agents when compared to other healing compounds in terms of cost and healing capabilities. In order for the damage recovery to be effective in large-scale composites, it must present a high restoration ability. Considering this, different studies [4–6] have successfully synthesized microcapsules embedding epoxy healing agents. Based on the self-healing chemistry of polymers and polymer composites, state-of-the-art approaches have categorized these materials into two classes: autonomic (or extrinsic) and non-autonomic (intrinsic). These classes are presented in Figure1[7,8]. Starting from the inspiring paper on liquid encapsulation by White et al. [9] for self-healing polymers, the amount of research based on autonomous (extrinsic) healing methods has highly advanced with respect to the following:

- Improvement in encapsulation techniques for obtaining a higher efficiency, stronger shell walls, capsule homogeneity, capsule stability, etc. [10–14]; - Selection of healing agents and catalyst pairs suitable for different matrices and encapsulating shell materials for the use of efficient, less expensive catalysts; use of healing agents adapted to the media of use; and use of new encapsulated chemistries, such as azide/alkene click reactions [15]; - Development of healing agents that do not require cross-linker- or catalyst-like solvents, or water- and surface-reactive agents, such as silyl esters and oils [16–20]; - Implementation of the encapsulation concept in more application-oriented research.

The occurrence of self-healing in microcapsule-based polymer and polymer composite systems has been demonstrated using fracture experiments [9,16–22], which have exhibited a healing efficiency as high as 75% and even reached values of up to 90%. Although a variety of alternative microencapsulation techniques are available, they are not all necessarily suited to self-healing, and the encapsulation method is often only suitable for specific types of core materials [23]. Therefore, Table1 provides an overview of different materials that can be used as healing agents, particularly within a capsule-based system. Additionally, it can be seen that only a few healing agents have been used in fiber reinforced composite self-healing studies [24–32]. Of the healing systems presented in Table1, dicyclopentadiene (DCPD) Polymers 2020, 12, 1052 3 of 27

and 5-ethylidene-2-norbornene (ENB) are those which have been the most extensively investigated Polymersand 2020 are, also12, x theFOR object PEER REVIEW of this study. 3 of 27

FigureFigure 1. Polymer 1. Polymer material material classiﬁcationclassification with with respect respect to their to healingtheir healing chemistry: chemistry: autonomic autonomic self-healing self- healingsystems systems and non-autonomicand non-autonomic systems systems (adapted (adapted from [7]). from [7]).

Table 1. Overview of materials that can be used as healing agents and their respective delivery system. Starting from the inspiring paper on liquid encapsulation by White et al. [9] for self-healing polymers, the amount of research basedDelivery on auto Systemnomous (extrinsic)Catalyst healing methodsUsed in FRP has highly Healing Agent advanced with respect to the following:Microcapsule HGF Microvascular Required Composites − DCPD/ENB X (UF) X X X ImprovementSiloxanes in encapsulation X (UF /techniquesPU) for obtaining a higher efficiency, X stronger shell walls, capsule homogeneity,Epoxy capsule X (UF) stability, etc. [10–14]; X X − SelectionAmine-Epoxy of healing agents and X (UF) catalyst pairs X suitable X for different matrices and X encapsulating Thiol-epoxy X (MF) X X shell materialsThiol-ene for the use of X efficient, (UF) less expensive catalysts; use X of healing agents adapted to the mediaThiol-isocyanate of use; and use of X (MF new/PU) encapsulated chemistries, such as azide/alkene click reactions [15]; Azide-alkyne X (UF) X Methacrylates X X − Development of healing agents that do not require cross-linker- or catalyst-like solvents, or GMA X (MF) water- andMelaimides surface-reactive Xagents, (UF) such as silyl esters and oils [16–20]; X − ImplementationIsocyanates of the encapsulation X (PU) concep X t in more application-oriented research. Cyanoacrylates X X X The occurrenceVinyl ester of self-healing X in microcapsule-based polymer and X polymer composite X systems has beenUnsaturated demonstrated polyester using fracture X experiments [9,16–22], which X have exhibited X a healing efficiency as high as 75% and even reached values of up to 90%. Although a variety of alternative microencapsulation techniques are available, they are not all necessarily suited to self-healing, and the encapsulation method is often only suitable for specific types of core materials [23]. Therefore, Table 1 provides an overview of different materials that can be used as healing agents, particularly within a capsule-based system. Additionally, it can be seen that only a few healing agents have been used in fiber reinforced composite self-healing studies [24–32]. Of the healing systems presented in Table 1, dicyclopentadiene (DCPD) and 5-ethylidene-2-norbornene (ENB) are those which have been the most extensively investigated and are also the object of this study.

Table 1. Overview of materials that can be used as healing agents and their respective delivery system.

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2. Materials and Methods

2.1. Manufacturing of the Poly-Urea-Formaldehyde (PUF)-Dicyclopentadiene (DCPD) Healing System

2.1.1. Materials The materials used for the synthesis of poly-urea-formaldehyde (PUF) microcapsules containing dicyclopentadiene (DCPD) as healing agent are presented in Table2. All materials were purchased from Sigma-Aldrich (https://www.sigmaaldrich.com) and used as received.

Table 2. Chemical substances used in the production of poly-urea-formaldehyde (PUF) microcapsules with embedded dicyclopentadiene (DCPD).

Material Molecular Formula Physical Properties Role Crystalline, white powder. Melting Urea CH4N2O Formation of the capsule shell in the aqueous state point at 133–135 ◦C Resorcinol (1,3- benzenediol) C6H4(OH)2 White crystals. Melting point at 113 ◦C Blending resin with formaldehyde Formaldehyde CH2O Colorless aqueous solution Formation of capsule shell Solid state (gel-like state at room Dicyclopentadiene (DCPD) C10H12 Monomer-capsule core temperature). Melting point at 32.5 ◦C Maleic anhydride C2H2(CO)2O Solid state. White powder Emulsiﬁer Ammonium chloride NH4Cl Solid state. White powder Formaldehyde hardener Sodium hydroxide NaOH Solid state. White powder Rising solution pH Hydrochloric acid HCl Aqueous solution with strong odor Lowering solution pH

2.1.2. Synthesis of PUF-DCPD Microcapsules Microcapsules from the same batch, synthesized by in situ polymerization, were used, as presented in [33]. In a 600 mL tall-form glass beaker, two solutions were initially prepared: a solution containing 150 mL distilled water, 7 g urea, 0.5 g resorcinol, and 0.5 g ammonium chloride, and a solution containing 100 mL co-polymer 5 wt % maleic anhydride. The two solutions were mixed until homogenization on a magnetic stirrer with a hot plate (C-MAG HS 10) at 500 rpm for 10 min. The solution pH was adjusted to 3.5 using 10 vol % sodium hydroxide and 10 vol % hydrochloric acids and the temperature was increased from room temperature to 35 ◦C to prevent the DCPD monomer from crystalizing when added. In order to form healing agent spherulites in the pre-polymer solution, the DCPD monomer must be in a liquid state. Considering this, in a 200 mL glass beaker, 60 g of DCPD was weighted (OHAUS PA224 analytical balance) and heated at 35 ◦C (Figure2a). After the DCPD monomer became liqueﬁed, it was poured over the previous mixture solution, under continuous stirring for approximately 5 min, to form microspheres. A formaldehyde 37% solution (0.23 mL, 18.91 g) was made and introduced after microspheres began to form. The temperature was increased to 50 ◦C and the solution was left under continuous stirring at 500 rpm for 2 h. During this period, the initially formed healing agent (DCPD) microspheres were wrapped in a polymeric urea-formaldehyde layer. After 20 min, whitening of the DCPD microspheres could be observed, showing that the process of coating them with urea-formaldehyde polymer was carried out under good conditions, as presented in Figure2b. It was observed that some microcapsules remained suspended on the side of the beaker wall due to the evaporation of the water over time. To compensate for this, 200 mL distilled water was pre-heated at 50 ◦C and introduced in the solution. The distilled water was pre-heated to prevent crystallization of the unreacted monomer. After two hours, the solution was left to cool at room temperature while the synthesized microcapsules precipitated on the bottom. The microcapsule suspension was diluted with 200 mL distilled water and ﬁltered under vacuum, after which the microcapsules were washed three times with 500 mL distilled water to remove traces of uncoated monomer that adhered to the surface of the microcapsules at the time of recrystallization. The suspension was then left to dry for 24 h at room temperature (21 1 C), as presented in Figure2c. ± ◦ Polymers 2020, 12, x FOR PEER REVIEW 5 of 27

Microcapsules from the same batch, synthesized by in situ polymerization, were used, as presented in [33]. In a 600 mL tall-form glass beaker, two solutions were initially prepared: a solution containing 150 mL distilled water, 7 g urea, 0.5 g resorcinol, and 0.5 g ammonium chloride, and a solution containing 100 mL co-polymer 5 wt % maleic anhydride. The two solutions were mixed until homogenization on a magnetic stirrer with a hot plate (C-MAG HS 10) at 500 rpm for 10 min. The solution pH was adjusted to 3.5 using 10 vol % sodium hydroxide and 10 vol % hydrochloric acids and the temperature was increased from room temperature to 35 °C to prevent the DCPD monomer from crystalizing when added. In order to form healing agent spherulites in the pre-polymer solution, the DCPD monomer must be in a liquid state. Considering this, in a 200 mL glass beaker, 60 g of DCPD was weighted (OHAUS PA224 analytical balance) and heated at 35 °C (Figure 2a). After the DCPD monomer became liquefied, it was poured over the previous mixture solution, under continuous stirring for approximately 5 min, to form microspheres. A formaldehyde 37% solution (0.23 mL, 18.91 g) was made and introduced after microspheres began to form. The temperature was increased to 50 °C and the solution was left under continuous stirring at 500 rpm for 2 h. During this period, the initially formed healing agent (DCPD) microspheres were wrapped in a polymeric urea- formaldehyde layer. After 20 min, whitening of the DCPD microspheres could be observed, showing that the process of coating them with urea-formaldehyde polymer was carried out under good conditions, as presented in Figure 2b. It was observed that some microcapsules remained suspended on the side of the beaker wall due to the evaporation of the water over time. To compensate for this, 200 mL distilled water was pre-heated at 50 °C and introduced in the solution. The distilled water was pre-heated to prevent crystallization of the unreacted monomer. After two hours, the solution was left to cool at room temperature while the synthesized microcapsules precipitated on the bottom. The microcapsule suspension was diluted with 200 mL distilled water and filtered under vacuum, after which the microcapsules were washed three times with 500 mL distilled water to remove traces of uncoated monomer that adhered to the surface of the microcapsules at the time of recrystallization.

The Polymerssuspension2020, 12 was, 1052 then left to dry for 24 h at room temperature (21 ± 1 °C), as presented 5in of Figure 27 2c.

Figure 2. Synthetization process of PUF-DCPD microcapsules showing the (a) weighting of DCPD Figure 2. Synthetization process of PUF-DCPD microcapsules showing the (a) weighting of DCPD monomer at room temperature before phase transformation at 35 ◦C, (b) microcapsule formation at monomer at room temperature before phase transformation at 35 °C, (b) microcapsule formation at Polymers 2020, 1250, ◦xC, FOR and PEER (c) microcapsule REVIEW suspension left to dry at room temperature for 24 h. 6 of 27 50 °C, and (c) microcapsule suspension left to dry at room temperature for 24 h. Figure3 presents a schematic representation of the work procedure for poly-urea-formaldehyde stirring, the microcapsules were separated. A quantity of 17.3 g of microcapsules was obtained from microcapsuleFigure 3 presents synthesis a schematic with dicyclopentadiene representation asof the healing work agent. procedure During for the poly-urea-formaldehyde synthesis of DCPD this process. microcapsuleembedded synthesis in PUF microcapsules, with dicyclopentadiene at about 30 min afteras healing the addition agent. of theDuring monomer, the asynthesis small specimen of DCPD embeddedFigurewas 5a collected illustratesin PUF to controlmicrocapsules, the thepresence synthesis at of about processuncoated 30 (Figure min mo 4afternomer). It wasthe on observedaddition the microcapsule that of microcapsulesthe monomer, surface, were a evensmall after thoroughspecimenalready cleansing. was formed collected andThe wrappedto micrcontrolocapsules in the the synthesis polymeric obtained process layer. were(Figure Subsequent analyzed 4). It to was microcapsule observedwith the that synthetization, Olympusmicrocapsules optical it was seen that, after cooling, an amount of DCPD monomer was not encapsulated, but crystallized microscopewere already GX and formed measured and with wrapped a 1 cm inruler, the having polymeric the unitlayer. of 100Subsequent µm (Figure to 5b).microcapsule SEM analysis on the surface. This can be attributed to the diﬀerent stirring regime employed compared to those (FEIsynthetization, Inspect F50) produced it was seen a that,better after image cooling, of the an uncoatedamount of monomer DCPD monomer on the wasmicrocapsule not encapsulated, surface, as presented in literature reports, where blends of over 500 rpm were reported in the synthesis process. wellbut as Afterthecrystallized size. drying, As theon can microcapsulethe be surface. seen surfacein This Figure can presented be6, theattr small ibutedmicrocapsule lumps, to butthe with different dimensions mild stirring, stirring are the regime microcapsulesequivalent employed to those reportedcomparedwere in the separated. to literature, those presented A quantity varying in of literature 17.3 between g of microcapsules reports, 100 and where 300 was µm,blends obtained with of fromover a mean this500 process. rpmvalue were that reported does not in theexceed 250 µm.synthesis process. After drying, the microcapsule surface presented small lumps, but with mild

FigureFigure 3. Process 3. Process of of PUF-DCPD PUF-DCPD microcapsule microcapsule synthesis. synthesis.

Figure 4. Sample taken during microcapsule synthesis process (optical image, 25× zoom).

(a) (b)

Polymers 2020, 12, x FOR PEER REVIEW 6 of 27 stirring, the microcapsules were separated. A quantity of 17.3 g of microcapsules was obtained from this process. Figure 5a illustrates the presence of uncoated monomer on the microcapsule surface, even after thorough cleansing. The microcapsules obtained were analyzed with the Olympus optical microscope GX and measured with a 1 cm ruler, having the unit of 100 µm (Figure 5b). SEM analysis (FEI Inspect F50) produced a better image of the uncoated monomer on the microcapsule surface, as well as the size. As can be seen in Figure 6, the microcapsule dimensions are equivalent to those reported in the literature, varying between 100 and 300 µm, with a mean value that does not exceed 250 µm.

Polymers 2020, 12, x FOR PEER REVIEW 6 of 27

stirring, the microcapsules were separated. A quantity of 17.3 g of microcapsules was obtained from this process. Figure 5a illustrates the presence of uncoated monomer on the microcapsule surface, even after thorough cleansing. The microcapsules obtained were analyzed with the Olympus optical microscope GX and measured with a 1 cm ruler, having the unit of 100 µm (Figure 5b). SEM analysis (FEI Inspect F50) produced a better image of the uncoated monomer on the microcapsule surface, as well as the size. As can be seen in Figure 6, the microcapsule dimensions are equivalent to those reported in the literature, varying between 100 and 300 µm, with a mean value that does not exceed 250 µm.

Polymers 2020, 12, 1052 6 of 27 Figure 3. Process of PUF-DCPD microcapsule synthesis.

Figure 3. Process of PUF-DCPD microcapsule synthesis.

Figure 4. Sample taken during microcapsule synthesis process (optical image, 25 zoom). Figure 4. Sample taken during microcapsule synthesis process (optical image,× 25× zoom). Figure5a illustrates the presence of uncoated monomer on the microcapsule surface, even after thorough cleansing. The microcapsules obtained were analyzed with the Olympus optical microscope GX and measured with a 1 cm ruler, having the unit of 100 µm (Figure5b). SEM analysis (FEI Inspect F50) produced a better image of the uncoated monomer on the microcapsule surface, as well as the size. As can be seen in Figure6, the microcapsule dimensions are equivalent to those reported in the µ µ literature,Figure varying 4. Sample between taken 100 during and 300 microcapsulem, with a synthe meansis value process that (optical does notimage, exceed 25× zoom). 250 m.

(a) (b)

Polymers 2020, 12, x FOR PEER REVIEW 7 of 27 (a) (b)

Figure 5.5. OpticalOptical imagesimages illustrating illustrating (a )(a microcapsules) microcapsules after after 24 h 24 drying h drying at room at room temperature—25 temperature—25×zoom × zoomand (b and) microcapsule (b) microcapsule dimensions—45 dimensions—45×zoom zoom (one division (one division represents represents 100 µm). 100 µm). ×

(a) (b) (c)

Figure 6. 6. SEMSEM images images of ( ofa) (microcapsulea) microcapsule agglomeration, agglomeration, (b) a (250b) aµm 250 microcapsule,µm microcapsule, and (c) anda broken (c) a microcapsule.broken microcapsule.

2.2. Manufacturing of the Melamine-urea-formaldehyde (MUF)-5-ethylidene-2-norbornene (ENB) Self- Healing System 5-ethylidene-2-norbornene is an organic colorless liquid compound formed from an ethylene group bound to the norbornene molecule. The ethylene norbornene molecule consists of two unsaturated groups, one of which participates in the copolymerization process (norbornene), with the other (the ethylidene group) preceding the “vulcanization” phenomenon. The cross-linked agent (CL) is the isomer of the compound (exo-endo-isomer and exo-exo-isomer). DCPD healing agent was replaced by Lee et al. [34] with ENB, which is known for its higher reactivity. The norbornene molecule consists of a cyclohexene ring with a methylene bridge between carbon atoms 1 and 4. This monomer has a double bond that generates a greater ring resistivity and reactivity, and thus a better reactivity of ENB monomer compared to DCPD monomer. Higher healing efficiencies were reported when ENB healing agent and a 1st generation Hoveyda–Grubbs catalyst were used together. With 5 wt % of Grubbs catalyst and 10 or 20 wt % of 760 nm MUF/EMA (ethylene maleic anhydride) nanocapsules containing ENB, 97% and 123% of the fracture toughness could be recovered, respectively. Moreover, the epoxy systems used in this concept were first pre-cured during low temperature cycles, followed by a high temperature curing cycle at 170 or 180 °C [34–38].

2.2.1. Materials Materials used for the synthesis of melamine-urea-formaldehyde (MUF) microcapsules containing 5-ethylidene-2-norbornene (ENB) as healing agent are presented in Table 3. All materials were purchased from Sigma-Aldrich (https://www.sigmaaldrich.com) and used as received.

Table 3. Chemical substances used in the production of melamine-urea-formaldehyde (MUF) microcapsules with embedded 5-ethylidene-2-norbornene (ENB).

Material Molecular Formula Physical Properties Role White powder. Melting Formation of capsule Melamine C3H6N6 point at 345 °C shell Crystalline white Formation of the Urea CH4N2O powder. Melting point capsule shell in the at 133–135 °C aqueous state Colorless aqueous Formation of capsule Formaldehyde CH2O solution shell Sodium lauryl CH3(CH2)10CH2 Emulsifier—oil Powder soluble in water sulfate (SLS) (OCH2CH2)nOSO3Na solidification

Polymers 2020, 12, 1052 7 of 27

2.2. Manufacturing of the Melamine-Urea-Formaldehyde (MUF)-5-ethylidene-2-norbornene (ENB) Self-Healing System 5-ethylidene-2-norbornene is an organic colorless liquid compound formed from an ethylene group bound to the norbornene molecule. The ethylene norbornene molecule consists of two unsaturated groups, one of which participates in the copolymerization process (norbornene), with the other (the ethylidene group) preceding the “vulcanization” phenomenon. The cross-linked agent (CL) is the isomer of the compound (exo-endo-isomer and exo-exo-isomer). DCPD healing agent was replaced by Lee et al. [34] with ENB, which is known for its higher reactivity. The norbornene molecule consists of a cyclohexene ring with a methylene bridge between carbon atoms 1 and 4. This monomer has a double bond that generates a greater ring resistivity and reactivity, and thus a better reactivity of ENB monomer compared to DCPD monomer. Higher healing eﬃciencies were reported when ENB healing agent and a 1st generation Hoveyda–Grubbs catalyst were used together. With 5 wt % of Grubbs catalyst and 10 or 20 wt % of 760 nm MUF/EMA (ethylene maleic anhydride) nanocapsules containing ENB, 97% and 123% of the fracture toughness could be recovered, respectively. Moreover, the epoxy systems used in this concept were ﬁrst pre-cured during low temperature cycles, followed by a high temperature curing cycle at 170 or 180 ◦C[34–38].

2.2.1. Materials Materials used for the synthesis of melamine-urea-formaldehyde (MUF) microcapsules containing 5-ethylidene-2-norbornene (ENB) as healing agent are presented in Table3. All materials were purchased from Sigma-Aldrich (https://www.sigmaaldrich.com) and used as received.

Table 3. Chemical substances used in the production of melamine-urea-formaldehyde (MUF) microcapsules with embedded 5-ethylidene-2-norbornene (ENB).

Material Molecular Formula Physical Properties Role White powder. Melting Melamine C3H6N6 Formation of capsule shell point at 345 ◦C Crystalline white powder. Formation of the capsule Urea CH4N2O Melting point at 133–135 ◦C shell in the aqueous state Formaldehyde CH2O Colorless aqueous solution Formation of capsule shell CH (CH ) CH Sodium lauryl sulfate (SLS) 3 2 10 2 Powder soluble in water Emulsifier—oil solidification (OCH2CH2)nOSO3Na Polyvinyl alcohol (PVA) (C2H4O)x Melting point at 200 ◦C Separation film

Polyvinyl alcohol (C2H4O)x Melting point at 200 °C Separation film (PVA)

2.2.2. Synthesis of MUF-ENB Microcapsules Microcapsules from the same batch, synthesized by in situ polymerization, were used, as presented in [33]. The fabrication of MUF-ENB microcapsules required four solutions to be prepared. For the first solution, 0.61 g urea was introduced in 30 mL of distilled water under continuous stirring until homogenization (approximately 10 min). The second solution comprised the melamine- formaldehyde pre-polymer. In 70 mL of distilled water, 3.81 g of melamine and 6.89 g of formaldehyde (37%) were introduced and left for 25 min at 70 °C to react, and were subsequently cooled down to room temperature. The third solution was formed of 30 mL 0.5 wt % sodium lauryl sulfate (SLS) and left for 20 min at 70 °C for homogenization. Preparation of the fourth solution included 30 mL 6.3 wt % polyvinyl alcohol being left for 2 h under continuous stirring at room temperature. After the preparation of urea solution, the stirring rate was increased to 300 rpm. SLS and polyvinyl alcohol (PVA) solutions were added to the admixture and the stirring rate was increased again to 500 rpm. After homogenization (10–15 min), 30 mL of ENB was slowly poured into the admixture. As ENB is not miscible with water, under continuous stirring, it forms spherules. The stirring rate was kept constant for 10 min at room temperature, after which the temperature was increased to 86 °C within 40 min. The process of coating the microspheres obtained in the melamine- urea-formaldehyde polymer solution took place at 86 °C with continuous stirring at 500 rpm for 320 min, as shownPolymers in2020 Figure, 12, 1052 7. After the reaction was completed, the hot plate was turned8 of 27 off and the solution was left under continuous stirring until it reached room temperature, after which the reaction vesselreaction was was moved completed, and the left hot for plate 30 was min turned unti offland microcapsule the solution was deposition left under continuous. Microcapsule stirring emulsion was vacuumuntil filtered it reached and room rinsed temperature, three times after whichwith 300 the reaction mL distilled vessel was water. moved The and microcapsules left for 30 min were left to dry at roomuntil microcapsuletemperature deposition. and 21 Microcapsule± 1 °C for emulsion12 and was24 vacuumh, respectively. filtered and rinsedFigure three 8 shows times that after with 300 mL distilled water. The microcapsules were left to dry at room temperature and 21 1 C drying, there was a tendency of microcapsule agglomeration in the form of lumps,± which◦ was the for 12 and 24 h, respectively. Figure8 shows that after drying, there was a tendency of microcapsule same as in theagglomeration case of DCPD, in the form but of lumps,these whichlumps was were the same easily as in separated the case of DCPD, with but mild these stirring. lumps were The fact that the obtainedeasily microcapsules separated with were mild stirring. easily Theseparated fact that the by obtained this mild microcapsules stirring shows were easily that separated the respective by batch had been successfullythis mild stirring prepared. shows that The the respectiveblue shade batch of had the been filtrate successfully was due prepared. to the The us bluee of shade PVA. of In Figure 9, the filtrate was due to the use of PVA. In Figure9, a schematic representation of the work procedure a schematicused representation for melamine-urea-formaldehyde of the work microcapsule procedure synthesis used with for 5-ethylidene-2-norbornene melamine-urea-formaldehyde as microcapsulehealing synthesis agent is with presented. 5-ethylidene-2-norbornene as healing agent is presented.

Figure 7. TheFigure reaction 7. The reactionmixture mixture employed employed for for obtaining obtaining MUF MUF microcapsules microcapsules with embedded with embedded ENB ENB Polymers 2020, 12, xhealing FOR PEER agent. REVIEW 9 of 27 healing agent.

(a) (b)

Figure 8.Figure Microcapsules 8. Microcapsules after after a a ( (aa) 1212 h h and and (b) ( 24b) h 24 drying h drying period. period.

Figure 9. Schematic illustration of melamine-urea-formaldehyde microcapsule synthesis with embedded 5-ethylidene-2-norbornene as repair agent.

During the synthesis, at about 30 min after the addition of the monomer, a sample was collected to control the synthesis process. After 12 h at room temperature, small traces of water were still visible, which eventually dried out after 24 h. The red highlighted areas in Figure 10 a represent microcapsules that were broken to observe their behavior. It was observed that when the microcapsules were broken, the healing agent evaporated and the specific area tended to darken. Therefore, the process proceeded in optimal conditions and the MUF microcapsule synthesis was performed successfully. After 24 h at room temperature for drying, a quantity of 22 g of microcapsules was obtained. In the microstructural analysis, a major difference in the size of the microcapsules was observed between the two processes. It could be easily observed that most of the obtained microcapsules were smaller than 100 µm for the MUF-ENB system. This represents a favorable outcome, as the MUF- ENB system will be easier to integrate in the matrix and composite material.

Polymers 2020, 12, x FOR PEER REVIEW 9 of 27

(a) (b) Polymers 2020, 12, 1052 9 of 27 Figure 8. Microcapsules after a (a) 12 h and (b) 24 h drying period.

FigureFigure 9.9. SchematicSchematic illustration of melamine-urea-formaldehydemelamine-urea-formaldehyde microcapsule synthesissynthesis withwith embeddedembedded 5-ethylidene-2-norbornene5-ethylidene-2-norbornene asas repairrepair agent.agent.

DuringDuring thethe synthesis,synthesis, atat aboutabout 3030 minmin afterafter thethe additionaddition ofof thethe monomer,monomer, aa samplesample waswas collectedcollected toto controlcontrol the the synthesis synthesis process. process. After After 12 h12 at h room at room temperature, temperature, small small traces traces of water of werewater still were visible, still whichvisible, eventually which eventually dried out dried after 24out h. after The red24 highlightedh. The red highlighted areas in Figure areas 10a in represent Figure 10 microcapsules a represent thatmicrocapsules were broken that to observe were theirbroken behavior. to observe It was th observedeir behavior. that when It was the microcapsulesobserved that were when broken, the themicrocapsules healing agent were evaporated broken, andthe thehealing speciﬁc agent area evaporated tended to darken.and the Therefore, specific area the processtended proceededto darken. inTherefore, optimal conditionsthe process and proceeded the MUF in microcapsule optimal cond synthesisitions and was the performed MUF microcapsule successfully. synthesis After 24 hwas at room temperature for drying, a quantity of 22 g of microcapsules was obtained. Polymersperformed 2020, 12successfully., x FOR PEER REVIEW After 24 h at room temperature for drying, a quantity of 2210 gof 27of microcapsules was obtained. In the microstructural analysis, a major difference in the size of the microcapsules was observed between the two processes. It could be easily observed that most of the obtained microcapsules were smaller than 100 µm for the MUF-ENB system. This represents a favorable outcome, as the MUF- ENB system will be easier to integrate in the matrix and composite material.

(a) (b)

Figure 10. OpticalOptical image image illustrating illustrating a a ( (aa)) sample sample taken taken 30 30 min min after after the the addition addition of of ENB ENB and and a a ( (bb)) sample after 12 h drying at room temperaturetemperature (one division representsrepresents 100100 µµm).m).

2.3. FourierIn the microstructuralTransform Infrared analysis, Spectroscopy a major Analysis diﬀerence in the size of the microcapsules was observed between the two processes. It could be easily observed that most of the obtained microcapsules were In order to verify that the PUF-DCPD and MUF-ENB microcapsules were successfully synthesized, FT-IR analysis was performed. The synthesized microcapsules were characterized by FT-IR using a Nicolet iS50FT-IR (Nicolet, MA, USA) spectrometer equipped with a DTGS detector, providing information with a high sensitivity in the range of 400 to 4000 cm−1 at a resolution of 4 cm−1. Each spectrum was obtained by co-adding 32 scans. The analysis is presented in Figure 11, where the constituent shell and core elements can be observed. The spectra conﬁrm that the shell material of the capsules contains urea-formaldehyde (UF) polymer. Furthermore, the FT-IR spectra of capsules entirely contain the core and shell peaks, proving the successful encapsulation of DCPD and ENB monomers in the UF/MUF shell.

(a)

Polymers 2020, 12, x FOR PEER REVIEW 10 of 27

Polymers 2020, 12, 1052 10 of 27

(a) (b) smallerFigure than 10. 100 Opticalµm for image the MUF-ENBillustrating system.a (a) sample This taken represents 30 mina after favorable the addition outcome, of ENB as the and MUF-ENB a (b) systemsample will after be easier 12 h drying to integrate at room in temper the matrixature and(one compositedivision represents material. 100 µm).

2.3. Fourier Transform Infrared SpectroscopySpectroscopy AnalysisAnalysis In orderorder to to verify verify that that the PUF-DCPD the PUF-DCPD and MUF-ENB and MUF-ENB microcapsules microcapsules were successfully were successfully synthesized, FT-IRsynthesized, analysis FT-IR was analysis performed. was Theperformed. synthesized The microcapsulessynthesized microcapsules were characterized were characterized by FT-IR using by aFT-IR Nicolet using iS50FT-IR a Nicolet (Nicolet, iS50FT-IR MA, (Nicolet, USA) spectrometerMA, USA) spectrometer equipped with equipped a DTGS with detector, a DTGS providing detector, 1 1 informationproviding information with a high with sensitivity a high sensitivity in the range in the ofrange 400 of to 400 4000 to 4000 cm− cmat−1 aat resolutiona resolution of of 4 4 cmcm−−1.. Each spectrum was obtained by co-adding 3232 scans. The analysis is presented in Figure 1111,, where the constituent shell and and core core elements elements can can be be observed. observed. The The spectra spectra con conﬁrmﬁrm that that the the shell shell material material of the of thecapsules capsules contains contains urea-formaldeh urea-formaldehydeyde (UF) (UF) polymer. polymer. Furthermore, Furthermore, the the FT-IR FT-IR spectra spectra of capsulescapsules entirely contain the core andand shellshell peaks,peaks, provingproving thethe successfulsuccessful encapsulationencapsulation of DCPD and ENB monomers in the UFUF/MUF/MUF shell.shell.

Polymers 2020, 12, x FOR PEER REVIEW (a) 11 of 27

(b)

Figure 11. FT-IRFT-IR spectra spectra of the main ( a)) PUF-DCPD PUF-DCPD and ( b) MUF-ENB microcapsu microcapsulele constituents.

2.4. Thermogravimetric Analysis Thermal analyses, consisting of thermogravimetric analysis (TGA) and differential thermal analysis (DTA), were conducted simultaneously on Shimadzu DTG-TA-50H equipment at a heating rate of 10 °C. Figure 12 and Figure 13 present the TGA/DTA analysis. The microcapsule samples were scanned in air atmosphere, at a ramp rate of 10 °C/min from ambient temperature to 500 °C, while the mass loss trace was simultaneously recorded with the equipment software. This analysis was performed in order to observe the mass loss obtained with an increasing temperature and phase changes for both PUF-DCPD and MUF-ENB systems. The thermogravimetric analysis performed for the PUF-DCPD system indicates a total loss of mass in the range of 30–500 °C of 72.604%. The PUF-DCPD system is thermally stable up to 250 °C and thermal degradation can then be observed, which takes place in two stages up to 500 °C. It is possible that the first step, between 250 and 350 °C, is an associated process of degradation of the PUF shell, and the second stage of the interval, between 350 and 500 °C, is an associated process of degradation of the core. The thermogravimetric analysis performed for the MUF-ENB system indicates a total mass loss in the range of 30–500 °C of 74.093%. The MUF-ENB system is thermally stable up to about 250 °C and a continuous thermal degradation process can then be observed up to 500 °C.

Polymers 2020, 12, x FOR PEER REVIEW 11 of 27

(b)

Figure 11. FT-IR spectra of the main (a) PUF-DCPD and (b) MUF-ENB microcapsule constituents.

2.4. Thermogravimetric Analysis Thermal analyses, consisting of thermogravimetric analysis (TGA) and differential thermal analysis (DTA), were conducted simultaneously on Shimadzu DTG-TA-50H equipment at a heating rate of 10 °C. Figure 12 and Figure 13 present the TGA/DTA analysis. The microcapsule samples were scanned in air atmosphere, at a ramp rate of 10 °C/min from ambient temperature to 500 °C, while the mass loss trace was simultaneously recorded with the equipment software. This analysis was performed in order to observe the mass loss obtained with an increasing temperature and phase changes for both PUF-DCPD and MUF-ENB systems. The thermogravimetric analysis performed for the PUF-DCPD system indicates a total loss of mass in the range of 30–500 °C of 72.604%. The PUF-DCPD system is thermally stable up to 250 °C andPolymers thermal2020, 12 degradation, 1052 can then be observed, which takes place in two stages up to 500 °C.11 It of is 27 possible that the first step, between 250 and 350 °C, is an associated process of degradation of the PUF shell, and the second stage of the interval, between 350 and 500 °C, is an associated process of 2.4. Thermogravimetric Analysis degradation of the core. TheThermal thermogravimetric analyses, consisting analysis of performed thermogravimetric for the MUF-ENB analysis system (TGA) indicates and diﬀ aerential total mass thermal loss inanalysis the range (DTA), of 30–500 were conducted °C of 74.093%. simultaneously The MUF-ENB on Shimadzu system is DTG-TA-50H thermally stable equipment up to about at a heating250 °C andrate a of continuous 10 ◦C. Figures thermal 12 and degradation 13 present process the TGA can/DTA then analysis. be observed up to 500 °C.

Polymers 2020, 12, x FOR PEER REVIEW 12 of 27

Figure 12. Thermogravimetric analysis (TGA)/differential thermal analysis (DTA) analysis of the Figure 12. Thermogravimetric analysis (TGA)/diﬀerential thermal analysis (DTA) analysis of the PUF-DCPD system. PUF-DCPD system.

Figure 13. TGA/DTATGA/DTA analysis of the MUF-ENB system.

2.5. TransmissionThe microcapsule Electron samples Microscopy were Analysis scanned in air atmosphere, at a ramp rate of 10 ◦C/min from ambient temperature to 500 ◦C, while the mass loss trace was simultaneously recorded with the equipmentThe core-shell software. structure This analysis of both was microcapsule performed types in order was to confirmed observe the by massthe TEM loss images obtained presented with an increasingin Figure 14. temperature TEM analysis and phasewas performed changes for on both a Titan PUF-DCPD Themis and80–200 MUF-ENB (Thermo systems. Fisher (former FEI) Hillsboro,The thermogravimetric OR, USA) high-resolution analysis performed transmission for theelectron PUF-DCPD microscope, system at indicates 200 kV, awith total a loss field of emission gun (FEG), courtesy of the National Research Centre for Food Safety [39]. The bright field mass in the range of 30–500 ◦C of 72.604%. The PUF-DCPD system is thermally stable up to 250 ◦C and images presented in Figure 14 present clear and spherical particles of the capsules. The size of the thermal degradation can then be observed, which takes place in two stages up to 500 ◦C. It is possible capsules varies from 0.2 to 2.5 µm. From the detailed images, we can see that the shell is that the ﬁrst step, between 250 and 350 ◦C, is an associated process of degradation of the PUF shell, approximately 150 nm thick. and the second stage of the interval, between 350 and 500 ◦C, is an associated process of degradation of the core.

(a) (b)

Figure 14. TEM images presenting the core-shell structure of synthetized (a) PUF-DCPD and (b) MUF- ENB microcapsules.

2.6. Fabrication of Reference Specimens Prior to the integration of self-healing systems in the epoxy matrix and evaluation of their mechanical properties, an investigation was conducted to find the optimal matrix curing cycle. Different curing cycles were investigated for the epoxy system Resoltech 1050 and Hardener 1055S, in order to achieve the best mechanical properties; firstly, according to the technical data parameters

Polymers 2020, 12, x FOR PEER REVIEW 12 of 27

Figure 12. Thermogravimetric analysis (TGA)/differential thermal analysis (DTA) analysis of the PUF-DCPD system.

Polymers 2020, 12, 1052 12 of 27

The thermogravimetric analysis performed for the MUF-ENB system indicates a total mass loss in the range of 30–500 ◦C of 74.093%. The MUF-ENB system is thermally stable up to about 250 ◦C and a continuous thermal degradationFigure 13. process TGA/DTA can analysis then be of observed the MUF-ENB up to system. 500 ◦C.

2.5. Transmission Transmission Electron Microscopy Analysis The core-shell structure of both microcapsule types was confirmed confirmed by the TEM images presented presented in Figure 1414.. TEM TEM analysis analysis was was performed performed on on a a Titan Titan Themis 80–200 (Thermo (Thermo Fisher Fisher (former (former FEI) FEI) Hillsboro, OR,OR, USA) USA) high-resolution high-resolution transmission transmission electron electron microscope, microscope, at 200 at kV, 200 with kV, a fieldwith emission a field emissiongun (FEG), gun courtesy (FEG), courtesy of the National of the National Research Research Centre for Centre Food for Safety Food [39 Safety]. The [39]. bright The field bright images field presentedimages presented in Figure in 14 Figure present 14 clearpresent and clear spherical and sphe particlesrical particles of the capsules. of the capsules. The size The of the size capsules of the variescapsules from varies 0.2 tofrom 2.5 µ0.2m. to From 2.5 theµm. detailedFrom the images, detailed we canimages, see thatwe thecan shellsee that is approximately the shell is 150approximately nm thick. 150 nm thick.

(a) (b)

Figure 14. 14. TEMTEM images images presenting presenting the core-s the core-shellhell structure structure of synthetized of synthetized (a) PUF-DCPD (a) PUF-DCPD and (b) MUF- and (ENBb) MUF-ENB microcapsules. microcapsules.

2.6. Fabrication Fabrication of Reference Specimens Prior to the integration of self-healing systems in the epoxy matrix and evaluation of their mechanical properties, an an investigation investigation was was conduc conductedted to to find find the the optimal optimal matrix matrix curing curing cycle. cycle. DifferentDifferent curing cycles were investigated for th thee epoxy system Resoltech 1050 and Hardener 1055S, 1055S, in order to achieve the best mechanical mechanical properties; properties; firstly, firstly, according to the technical data parameters (16 h at 60 ◦C) and secondly, by increasing the temperature to 80 ◦C and reducing the curing time to 2 h. Curing was done in a POL-EKO 240 SLN oven. Neat epoxy reference specimens were fabricated according to SR EN ISO 14125:2003 (Class IV) and were compared with specimens containing PUF-DPCD and MUF-ENB self-healing systems. Any specimen with a measured thickness (h) exceeding 2% tolerance was eliminated. Additionally, ± the width (b) and thickness (h) were measured with a 1% accuracy. The length (L) was adjusted by exactly 1% of the calculated value to match the distance between the support grips and average specimen thickness (L/h), according to the standard.

(16 h at 60 °C) and secondly, by increasing the temperature to 80 °C and reducing the curing time to 2 h. Curing was done in a POL-EKO 240 SLN oven. Neat epoxy reference specimens were fabricated according to SR EN ISO 14125:2003 (Class IV) and were compared with specimens containing PUF-DPCD and MUF-ENB self-healing systems. Any specimen with a measured thickness (h) exceeding ±2% tolerance was eliminated. Additionally, the width (b) and thickness (h) were measured with a 1% accuracy. The length (L) was adjusted by exactly 1% of the calculated value to match the distance between the support grips and average specimen thickness (L/h), according to the standard.

2.7. Finite Element Method Analysis Detailed structural analysis regarding the mechanical behavior of composite materials has gained a lot of interest in recent years, especially with their increasing usage in almost all major industries. This type of analysis has become much faster and cheaper to perform compared to experimental methods that are expensive and time-consuming. The numerical approach by means of the finitePolymers element2020, 12method, 1052 (FEM) offers the advantage of a faster, cheaper, and more detailed13 of 27 analysis for an infinite number of material configurations, especially composite materials [40]. Therefore,element methoda numerical (FEM) oanalysisffers the advantageapproach of was a faster, taken cheaper, into andconsideration more detailed for analysis this paper, for an in order to obtaininfinite a better number understanding of material configurations, of the microcapsu especiallyle composite behavior materials when embedded [40]. in an epoxy matrix. This will furtherTherefore, help a numericalin the investigation analysis approach and wasanalysis taken intoof the consideration mechanical for thisbehavior paper, in of order self-healing to obtain a better understanding of the microcapsule behavior when embedded in an epoxy matrix. compositesThis when will further subjected help in to the different investigation loads and and analysis will be of the mechanicalsubject of behavioranother ofpaper. self-healing Accordingcomposites to when elasticity subjected theory, to di fftherente presence loads and of will a microcapsule be the subject of within another a paper. given material represents an inclusionAccording and after to elasticity its rupture, theory, the the presence broken of a microcapsule microcapsule within creates a given materiala stress represents concentration phenomenon.an inclusion For andthe afteranalysis its rupture, of this the stress broken concentration, microcapsule creates a cube-cell a stress concentration with a side phenomenon. of 0.2 mm and a 20 µ µm microcapsuleFor the analysis embedded of this stress was concentration, designed, aas cube-cell shown with in Figure a side of 15a. 0.2 mm The and material a 20 m chosen microcapsule for the cube- embedded was designed, as shown in Figure 15a. The material chosen for the cube-cell was the same cell wasResoltech the same epoxy Resoltech resin used forepoxy the microcapsule resin used integration for the in microcapsule Section3. Tetrahedron integration elements in with Section 3. Tetrahedron10 nods elements were used with for the 10 analysis, nods havingwere aused 2nd degree for the order analysis, of interpolation, having which a 2nd increases degree the order of interpolation,analysis which accuracy. increases This model the is analysis presented accuracy. in Figure 15 Thisb. model is presented in Figure 15b.

(a) (b)

Figure 15.Figure Illustration 15. Illustration of the ( ofa) thecube-cell (a) cube-cell element element and ( andb) finite (b) finite element element model model with with an anembedded microcapsule.embedded microcapsule. The cube-cell was tensile stressed until it broke at 125 MPa. To model the healing agent, an elastic Thematerial cube-cell with was a very tensile low Young’s stressed Modulus until it was broke chosen at and125 aMPa. 0.495 To Poisson model coe ffithecient healing was employed agent, an elastic materialto with represent a very the low liquid Young’s incompressibility. Modulus The was Von chosen Mises stressand a distribution 0.495 Poisson is presented coefficient in Figure was 16 employed. to representFor the the liquid filled microcapsule,incompressibility. the maximum The Von stress Mises was found stress at 150 distribution MPa, with a stressis presented concentration hereafter. factor (Kc) of 1.2. After microcapsule rupture, the maximum stress was found at 242 MPa, with a stress concentration factor of 1.94. The analysis performed for the 20 µm microcapsule is presented in Figure 17 The maximum stress caused by the spherical inclusion can be found at 624 MPa, corresponding to a stress concentration factor of 1.49. Another aspect of this numerical analysis was the effect on the matrix mechanical loads for the relative position of two microcapsules, while embedded within the matrix. The first case was the arrangement of two microcapsules at a distance of 0.085 mm at an angle of 45 degrees. In this case, the maximum stress was 795 MPa and the stress concentration factor was 1.87, as illustrated in Figure 18a. The second case was characterized by the disposition of the two microcapsules in the median plane, disposed in the normal direction of the tensile force. The stress distribution is shown in Figure 18b. In this case, the maximum stress was 818 MPa and the stress concentration was 1.92. The third case was characterized by the microcapsule position overlapping on the plane perpendicular to the direction Polymers 2020, 12, x FOR PEER REVIEW 14 of 27

Polymers 2020, 12, 1052 14 of 27

of the tensile force.(a) It was found that the maximum stress was 786 MPa and the stress(b) concentration factor was 1.85, as presented in Figure 18c. Polymers 2020, 12, x FOR PEER REVIEW 14 of 27

(a) (b)

Figure 16. Von Mises illustration of the (a) cube-cell cross-section view, (b) cube-cell quarter view, (c) filled microcapsule, and (d) base cube-cell with an empty microcapsule.

For the filled microcapsule, the maximum stress was found at 150 MPa, with a stress concentration factor (Kc) of 1.2. After microcapsule rupture, the maximum stress was found at 242 MPa, with a stress concentration factor of 1.94. The analysis performed for the 20 µm microcapsule is (c) (d) presented in Figure 17 The maximum stress caused by the spherical inclusion can be found at 624 Figure 16. VonVon Mises Mises illustration illustration of of the the (a ()a cube-cell) cube-cell cross-section cross-section view, view, (b) (cube-cellb) cube-cell quarter quarter view, view, (c) MPa, correspondingfilled(c) ﬁlled tomicrocapsule, microcapsule,a stress and concentration and (d) (based) base cube-cell cube-cell with factor with an anempty emptyof 1.49.microcapsule. microcapsule.

Figure 17. Von Mises distribution on the 20 µm microcapsule. Figure 17.Figure Von 17. Mises Von Mises distribution distribution on on the the 20 µm 20 microcapsule. µm microcapsule.

Another aspectAnother of this aspect numerical of this numerical analysis analysis was was the the effect effect on the on matrix the mechanicalmatrix mechanical loads for the loads for the relative position of two microcapsules, while embedded within the matrix. The first case was the relative positionarrangement of two of microcapsules, two microcapsules at while a distance embedded of 0.085 mm atwithin an angle the of 45 matrix. degrees. InThe this case,first case was the arrangement ofthe two maximum microcapsules stress was 795 MPa at aand distance the stress concentrationof 0.085 mm factor at was an 1.87, angle as illustrated of 45 indegrees. Figure In this case, 18a. The second case was characterized by the disposition of the two microcapsules in the median the maximum plane,stress disposed was 795 in the MPa normal and direct theion stress of the tensile concentration force. The stress factor distribution was is1.87, shown as in illustrated Figure in Figure 18a. The second case was characterized by the disposition of the two microcapsules in the median plane, disposed in the normal direction of the tensile force. The stress distribution is shown in Figure

Polymers 2020, 12, x FOR PEER REVIEW 15 of 27

18b. In this case, the maximum stress was 818 MPa and the stress concentration was 1.92. The third case was characterized by the microcapsule position overlapping on the plane perpendicular to the Polymersdirection2020 of, 12 the, 1052 tensile force. It was found that the maximum stress was 786 MPa and the 15stress of 27 concentration factor was 1.85, as presented in Figure 18c.

(a)

(b)

(c)

Figure 18. Von MisesMises stress stress distribution distribution with with respect respect to sphericalto spherical inclusions inclusions (a) placed(a) placed at 45 at◦ at45° 0.085 at 0.085 mm, (mm,b) placed (b) placed in the medianin the median plane at plane 0.05 mm, at and0.05 (mm,c) in anand overlapping (c) in an positionoverlapping in the position perpendicular in the planeperpendicular at 0.05 mm. plane at 0.05 mm.

The presence of two microcapsules caused the concentration coeﬃcient to increase by 29% in the most unfavorable case, corresponding to a decrease of the breaking force of 23%. Moreover, increasing the spherical inclusion diameter by two times induced an increase of the concentration factor from 1.2 to 1.49 (25%). This represents a 20% decrease in the breaking force with respect to classical failure theories. Polymers 2020, 12, 1052 16 of 27

The maximum tensile strength of the cube-cell without the spherical inclusion is deﬁned by Equations (1)–(4): F σ = r . (1) r A In the presence of the inclusion, the tensile strength is

Fc F σc = k r = r . (2) r A A The expression of the breaking force in the case of an inclusion is as follows:

F 1 Fc = r = F = 0.8F . (3) r k 1.25 r r According to the Finite Element Analysis (FEA), the breaking force calculated with classical theories does not cause the rupture tensile to occur in the entire section, but only in certain regions of the interface between the spherical inclusion and matrix, with theories of fracture mechanics being applicable. Increasing the value of the tensile above the ultimate strength will cause a micro-crack to appear at the interface between the spherical inclusion and the matrix in the perpendicular plane in the direction of the tensile force, in the case of tensile breaking tests. Compared with the concentration factor of a void (3 in the case of a sphere), the existence of the liquid reduces the concentration factor by half (1.5). Using Reuss’ rule for the mechanical properties of a material consisting of two constituents, we have the following relations: E = f E + f E f E , (4) 1 1 2 2 ≈ 1 1 where E is the elasticity modulus of the mixture, Ei is the elasticity modulus of the i component, fi is the volumetric fraction of the i component, and the modulus of elasticity of the liquid is much smaller than that of the matrix (it is considered equal to 0).

3. Results

3.1. Integration of PUF-DCPD in Epoxy Specimens Following the curing cycle of neat epoxy specimens, six integration methods were performed to establish an optimal integration process, as presented in Table4.

Table 4. Methods of microcapsule integration in the epoxy matrix.

Method No. Method for Optimization 1 Homogenization at 300 rpm 2 Heating at 40 ◦C and homogenization at 300 rpm 3 Heating at 40 ◦C and homogenization at 200 rpm 4 Heating at 40 ◦C and homogenization at 100 rpm 5 Preheating at 60 ◦C, homogenization at 100 rpm, and heating at 80 ◦C 10 vol % acetone dilution, heating at 60 C, and homogenization at 6 ◦ 100 rpm, and then acetone evaporation at 80 ◦C

3.1.1. Method 1 and Method 2 Following the first integration method and taking into consideration the viscosity of RESOLTECH 1050 resin, which is 1043 mPa.s at 23 ◦C, proper integration of the microcapsules was difficult to obtain. Additionally, due to the large size of PUF-DCPD microcapsules and the resulting high volumetric fraction, the integration of 15 wt % microcapsules into the epoxy resin was difficult to achieve. Because of this high volumetric fraction, the mixture could no longer be injected into the silicon molds, causing very large pores or a lack of material uniformity, which are defects that can be seen in Figure 19. Polymers 2020, 12, x FOR PEER REVIEW 17 of 27

Following the first integration method and taking into consideration the viscosity of RESOLTECH 1050 resin, which is 1043 mPa.s at 23 °C, proper integration of the microcapsules was difficult to obtain. Additionally, due to the large size of PUF-DCPD microcapsules and the resulting high volumetric fraction, the integration of 15 wt % microcapsules into the epoxy resin was difficult to achieve. Because of this high volumetric fraction, the mixture could no longer be injected into the silicon molds, causing very large pores or a lack of material uniformity, which are defects that can be seen in Figure 19. For the second method, the matrix viscosity was reduced by heating the epoxy resin to 40 °C while maintaining the stirring rate at 300 rpm for homogenization. By doing so, the microcapsule integration time was halved compared to method 1 and the mixture could be injected into the silicon molds to manufacture specimens at a curing cycle of 80 °C for 2 h. It was found that most microcapsules have a tendency to rise at the surface of the specimens, as seen in Figure 19b. The specimens fabricated using method 1 and method 2 presented a high elasticity and therefore, a loss in the matrix mechanical properties, as a result of the microcapsules breaking during homogenization Polymersat 300 rpm.2020, 12When, 1052 the microcapsules break, the cross-linked agent, DCPD, is released into the 17epoxy of 27 resin and because the monomer does not cross-link without a hardener, it increases the host’s elasticity.

(a) (b)

Figure 19.19. SpecimensSpecimens fabricated fabricated using using (a) ( methoda) method 1, showing 1, showing defects defects due to due the highto the volumetric high volumetric fraction offraction epoxy of matrix epoxy and matrix 250 µ mand PUF-DCPD 250 µm PUF-DCPD microcapsules, microcapsules, and (b) method and 2,(b presenting) method microcapsule2, presenting agglomerationmicrocapsule agglomeration at the surface. at the surface.

3.1.2.For Method the second 3 and method, Method the4 matrix viscosity was reduced by heating the epoxy resin to 40 ◦C while maintaining the stirring rate at 300 rpm for homogenization. By doing so, the microcapsule integration For methods 3 and 4, the homogenization rate was decreased to 200 and 100 rpm, respectively, time was halved compared to method 1 and the mixture could be injected into the silicon molds to to reduce the number of broken microcapsules and thus the elasticity given to the specimens. A slight manufacture specimens at a curing cycle of 80 C for 2 h. It was found that most microcapsules have increase of their aspect and mechanical properties◦ was obtained, but was insufficient for three-point a tendency to rise at the surface of the specimens, as seen in Figure 19b. The specimens fabricated bending tests, as the specimens could not reach the minimum 5N preload stated in the standardized using method 1 and method 2 presented a high elasticity and therefore, a loss in the matrix mechanical method. Considering the epoxy matrix curing cycle at 60 °C for 16 h, a pre-polymerization step for properties, as a result of the microcapsules breaking during homogenization at 300 rpm. When the the epoxy matrix when integrating the microcapsules was proposed, in order to avoid the microcapsules break, the cross-linked agent, DCPD, is released into the epoxy resin and because the agglomeration of microcapsules on the surface. monomer does not cross-link without a hardener, it increases the host’s elasticity. 3.1.3. Method 5 and Method 6 3.1.2. Method 3 and Method 4 In method 5, the epoxy matrix was preheated at 60 °C, the hardener was added, and the stirring For methods 3 and 4, the homogenization rate was decreased to 200 and 100 rpm, respectively, rate was set to 100 rpm, after which 15 wt % microcapsules were introduced into the mixture. After to reduce the number of broken microcapsules and thus the elasticity given to the specimens. A slight homogenization, the epoxy resin-containing microcapsules were injected into silicon molds and increase of their aspect and mechanical properties was obtained, but was insuﬃcient for three-point cured at 80 °C for 2 h. bending tests, as the specimens could not reach the minimum 5N preload stated in the standardized Method 6 consisted of pre-heating the epoxy resin at 60 °C. To further reduce its viscosity, 10 wt method. Considering the epoxy matrix curing cycle at 60 ◦C for 16 h, a pre-polymerization step for the % acetone was added before introducing the hardener, followed by homogenization at 100 rpm and epoxy matrix when integrating the microcapsules was proposed, in order to avoid the agglomeration the addition of 15 wt % microcapsules. The temperature was raised to 80 °C and the stirring rate was of microcapsules on the surface. kept at 100 rpm until acetone evaporation. Then, microcapsules were introduced in silicon the mold 3.1.3. Method 5 and Method 6

In method 5, the epoxy matrix was preheated at 60 ◦C, the hardener was added, and the stirring rate was set to 100 rpm, after which 15 wt % microcapsules were introduced into the mixture. After homogenization, the epoxy resin-containing microcapsules were injected into silicon molds and cured at 80 ◦C for 2 h. Method 6 consisted of pre-heating the epoxy resin at 60 ◦C. To further reduce its viscosity, 10 wt % acetone was added before introducing the hardener, followed by homogenization at 100 rpm and the addition of 15 wt % microcapsules. The temperature was raised to 80 ◦C and the stirring rate was kept at 100 rpm until acetone evaporation. Then, microcapsules were introduced in silicon the mold and cured at 80 ◦C for 2 h. This method was found to be the most eﬃcient for the integration process. The same procedures were applied for MUF-ENB microcapsules, where method 5 was considered the most eﬃcient. The fabricated specimens are presented in Figure 20. Polymers 2020, 12, x FOR PEER REVIEW 18 of 27 and cured at 80 °C for 2 h. This method was found to be the most efficient for the integration process.

The samePolymers procedures2020, 12, 1052 were applied for MUF-ENB microcapsules, where method 5 was considered18 of 27 the most efficient. The fabricated specimens are presented in Figure 20.

(a) (b)

FigureFigure 20. 20.SpecimensSpecimens containing containing (a ()a )MUF-DCPD MUF-DCPD and and ((bb)) PUF-ENBPUF-ENB systems systems and and Grubbs Grubbs catalyst catalyst fabricatedfabricated by method by method 6. 6. 3.2. Thermal Stability Study for PUF-DCPD and MUF-ENB Self-Healing Systems 3.2. Thermal Stability Study for PUF-DCPD and MUF-ENB Self-Healing Systems In order to determine the thermal stability of the synthesized microcapsules, a series of tests were performedIn order to with determine respect to the the thermal curing cycles stability of the of ﬁnal the composite synthesized material, microcapsules, in which the a self-healing series of tests weresystems performed were with integrated respect according to the curing to method cycles 6 forof the PUF-DCPD final composite microcapsules material, and in method which 5 the for self- healingMUF-ENB systems microcapsules. were integrated Therefore, according thermal to method stability 6 tests for werePUF-DCPD performed microcapsules at 60, 80, and and 120 ◦ methodC for 5 for MUF-ENBan interval microcapsules. of 1, 2, 3, and 4 h. Therefore, thermal stability tests were performed at 60, 80, and 120 °C for an interval of 1, 2, 3, and 4 h. Testing Procedure

Testing ProcedureThe oven was preheated to the testing temperature (60, 80, and 120 ◦C). Four empty beakers were weighted, along with four 0.20 g samples, for each beaker. Each beaker containing the sample wasThe exposedoven was to dipreheatedﬀerent periods to the of time,testing namely, temperat beakerure 1—one-hour (60, 80, and exposure, 120 °C). beaker Four 2—two-hour empty beakers wereexposure, weighted, beaker along 3—three-hour with four 0.20 exposure, g samples, and beaker for each 4—four-hour beaker. exposure.Each beaker Each containing of the four beakersthe sample was exposedwere left into adifferent desiccator periods after they of time, were removednamely, frombeaker the 1—one-hour oven to cool down,exposure, after beaker which they2—two-hour were exposure,weightedPolymers beaker to 2020 determine ,3—three-hour 12, x FOR thePEER weight REVIEW exposure, loss obtained and by beaker evaporation 4—four-hour of the healing exposure. system. Each The19 di ofﬀof 27erence the four beakersbetween were the left weight in a desiccator before temperature after they exposure were removed and after exposurefrom the represents oven to cool thetotal down, weight after loss which difference between the weight before temperature exposure and after exposure represents the total percentage and is presented in Figures 21 and 22. they wereweight weighted loss percentage to determine and is thepresented weight in Figureloss ob 21tained and Figure by evaporation22. of the healing system. The difference between the weight before temperature exposure and after exposure represents the total weight loss percentage and is presented in Figure 21 and Figure 22.

FigureFigure 21. 21.Weight Weight loss loss variation variation overover timetime forfor PUF-DCPD after thermal thermal exposure exposure at at 60, 60, 80, 80, and and 120 120 °C.◦C.

Figure 22. Weight loss variation over time for MUF-ENB after thermal exposure at 60, 80, and 120 °C.

All samples were analysed under an optical microscope after being exposed to temperature, in order to determine the thermal stability, to ensure that the microcapsules retained their structure. It was observed that the majority of the PUF-DCPD and MUF-ENB microcapsules were not damaged, with the mass loss following the exposure to temperature taking place by evaporation of the repair agents dicyclopentadiene and ethylidene-norbonene, respectively, through the walls of the microcapsules. Although, in the case of exposure to the temperature of 120 °C, which is the temperature related to the polymerization cycle of the final composite material, the mass loss is substantial, this does not greatly influence the efficiency of the integrated healing systems, since the host matrix at 120 °C is already cured, having passed through a prepolymerization cycle at 60 and 80 °C, respectively.

Polymers 2020, 12, x FOR PEER REVIEW 19 of 27 difference between the weight before temperature exposure and after exposure represents the total weight loss percentage and is presented in Figure 21 and Figure 22.

Polymers 2020, 12, 1052 19 of 27 Figure 21. Weight loss variation over time for PUF-DCPD after thermal exposure at 60, 80, and 120 °C.

FigureFigure 22. 22. WeightWeight loss loss variation variation over over time time for for MUF-ENB MUF-ENB after after thermal thermal exposure exposure at at60, 60, 80, 80, and and 120 120 °C.◦ C.

AllAll samples samples were were analysed analysed under under an an optical optical micr microscopeoscope after after being being exposed exposed to to temperature, temperature, in orderin order to determine to determine the thermal the thermal stabil stability,ity, to ensure to ensure that that the the microcapsules microcapsules retained retained their their structure. structure. It wasIt wasobserved observed that that the themajority majority of ofthe the PUF-DCPD PUF-DCPD and and MUF-ENB MUF-ENB microcapsules werewere not not damaged, damaged, withwith the the mass mass loss loss followingfollowing the the exposure exposure to temperatureto temperature taking taking place place by evaporation by evaporation of the repair of the agents repair agentsdicyclopentadiene dicyclopentadiene and ethylidene-norbonene, and ethylidene-norbonene, respectively, respectively, through the through walls of thethe microcapsules. walls of the Although, in the case of exposure to the temperature of 120 C, which is the temperature related to the microcapsules. Although, in the case of exposure to the◦ temperature of 120 °C, which is the polymerization cycle of the final composite material, the mass loss is substantial, this does not greatly temperature related to the polymerization cycle of the final composite material, the mass loss is influence the efficiency of the integrated healing systems, since the host matrix at 120 C is already substantial, this does not greatly influence the efficiency of the integrated healing systems,◦ since the cured, having passed through a prepolymerization cycle at 60 and 80 C, respectively. host matrix at 120 °C is already cured, having passed through a prepolymerization◦ cycle at 60 and 80 °C,3.3. respectively. Three-Point Bending Test Campaign

3.3.1. Testing Procedure The testing method for both reference specimens containing neat epoxy resin and specimens containing healing systems followed ISO 14125:2003 (Class IV). All fabricated specimens correspond to Class IV materials, having dimensions of 100 15 2 mm. The loading speed was calculated using × × Equation (1), and a 2 mm/min speed was determined. The flexural stress is given by Equation (6) and the flexural modulus is given by Equation (9), calculated by measuring the deflection points (s0 and s”), in Equations (7) and (4). ε L2 v = 0 , (5) 6h

where L is the support span (mm), h is the specimen thickness (mm), and ε0 is the strain rate of 0.01/min or 1%/min. 3FL σ f = , (6) 2bh2

where σ f is the ﬂexural stress (MPa), F is the applied force (N), L is the support span (mm), h is the specimen thickness (mm), and b is the width of the specimen (mm)

2 ε0f L s0 = , (7) 6h

00 2 ε f L s00 = , (8) 6h Polymers 2020, 12, x FOR PEER REVIEW 20 of 27

= , (7)

= , (8)

∆ = , (9) ∆ where, s’ and s” represent the deflection in the middle of the specimen (mm), is the flexural stress ” for deflectionPolymers 2020 s’, 12 and, 1052 its corresponding value is 0.0005, and is the flexural stress for deflection20 of 27 s” and its corresponding value is 0.0025.

L3 ∆F 3.3.2. Testing of Neat Resin Specimens E f = , (9) 4bh3 ∆s

where,Three-points0 and bendings” represent tests the were deflection performed in the middle with Instron of the specimen 3360 Series (mm), Universalε0f is the flexuralTesting stressSystems. The load was applied to the specimens at mid-span until00 rupture. Table 5 and Table 6 present the for deflection s0 and its corresponding value is 0.0005, and ε f is the flexural stress for deflection s” and flexuralits corresponding stress at break value values is 0.0025. for specimens cured at 60 °C for 16 h and at 80 °C for 2 h, respectively, and the corresponding stress–strain curves are presented in Figure 23 and Figure 24. 3.3.2. Testing of Neat Resin Specimens Three-point bendingTable tests5. Flexural were performedtests for specimens with Instron cured 3360 at 60 Series °C for Universal 16 h. Testing Systems. The load was applied to the specimens at mid-span until rupture. Tables5 and6 present the flexural Load stressSpecimen at break values No. for specimens Flexural cured Strength at 60 ◦C (MPa) for 16 h and at 80 ◦ElongationC for 2 h, respectively, at Break (mm) and the corresponding stress–strain curves are presented in Figures 23(N) and 24. P1 65.21 32.03 17.60 P2 Table 5. Flexural tests61.79 for specimens cured32.32 at 60 ◦C for 16 h. 16.44

SpecimenP3 No. Flexural Strength60.72 (MPa) Load30.66 (N) Elongation15.99 at Break (mm) P4 67.19 33.10 17.52 P1 65.21 32.03 17.60 P5P2 61.7960.16 32.3231.50 16.4416.58 MeasurementP3 error (%) 60.72 ±3 30.66 ±1 15.99 ±0.5 P4 67.19 33.10 17.52 P5 60.16 31.50 16.58 Measurement errorTable (%) 6. Flexural tests3 for specimens cured1 at 80 °C for 2 h. 0.5 ± ± ± Load Specimen No. Flexural Strength (MPa) Elongation at Break (mm) Table 6. Flexural tests for specimens cured(N) at 80 ◦C for 2 h. SpecimenP1.1 No. Flexural Strength59.62 (MPa) Load30.53 (N) Elongation15.97 at Break (mm) P2.1P1.1 59.6262.55 30.5330.73 15.9715.40 P3.1P2.1 62.5559.02 30.7327.16 15.4015.67 P3.1 59.02 27.16 15.67 P4.1 61.48 29.18 16.63 P4.1 61.48 29.18 16.63 P5.1P5.1 62.2162.21 29.4029.40 15.90 Measurement error (%) 3 1 0.5 Measurement error (%) ± ±3 ± ±1 ± ±0.5

FigureFigure 23. 23. Stress–strainStress–strain curves curves for for specimens cured cured at at 60 60◦C °C for for 16 16 h. h.

Polymers 2020, 12, x FOR PEER REVIEW 21 of 27

Polymers 2020, 12, 1052 21 of 27 Polymers 2020, 12, x FOR PEER REVIEW 21 of 27

Figure 24. Stress–strain curves for specimens cured at 80 °C for 2 h.

3.3.3. Testing of PUF-DCPD Specimens Fabricated by Method 5 FigureFigure 24. 24. Stress–strainStress–strain curves curves for for specimens specimens cured cured at at 80 80 °C◦C for for 2 2 h. h. Specimens obtained by method 5 showed an easier integration of microcapsules and maintained their3.3.3.3.3.3. dispersion Testing Testing of ofafter PUF-DCPD PUF-DCPD the curing Specimens Specimens cycle. Fabricated Although by the Method amount 5 of broken microcapsules decreased during homogenization,SpecimensSpecimens obtained obtained the by by specimens method method 5 5 showed showed displayed an an easi easier laerrge integration integration differences of of microcapsules microcapsulesduring the three-point and and maintained maintained bending tests,theirtheir as showndispersion dispersion in Table after after 7the and curing Figure cycle. 25. Although Although the the amount amount of of broken broken microcapsules decreased decreased duringduring homogenization, homogenization, the the specimens specimens displayed displayed la largerge differences diﬀerences during during the the three-point bending tests,tests, as as shown shown in in Table Table 77 andandTable FigureFigure 7. Flexural 25 25.. tests for method 5 specimens.

Table 7. Flexural tests for method 5Load specimens. Specimen No. Table Flexural 7. Flexural Strength tests for (MPa) method 5 specimens.Elongation at Break (mm) (N) Specimen No. Flexural Strength (MPa) LoadLoad (N) Elongation at Break (mm) SpecimenP1 No. Flexural Strength2.33 (MPa) 0.13 Elongation at Break0.01 (mm) P1 2.33 0.13(N) 0.01 P2 22.99 15.71 14.14 P2P1 22.992.33 15.710.13 0.01 14.14 P3P3 2.262.26 0.05−0.05 0.040.04 P2 22.99 −15.71 14.14 Measurement error (%) 3 1 0.5 MeasurementP3 error (%) ± 2.26 ±3 −±0.05 ±1 0.04± ±0.5 Measurement error (%) ±3 ±1 ±0.5

FigureFigureFigure 25. 25. 25.Stress–strain Stress–strainStress–strain curves curves curves forfor for PUF-DCPD PUF-DCPD specimens specimensspecimens fabricated fabricated fabricated with with with method method method 5. 5. 5.

3.3.4. Testing of PUF-DCPD Specimens Fabricated by Method 6 3.3.4.3.3.4. Testing Testing of PUF-DCPDof PUF-DCPD Specimens Specimens FabricatedFabricated by by Method Method 6 6 The specimens obtained through method 6 showed a good dispersion of the microcapsules in TheThe specimens specimens obtained obtained through through methodmethod 66 showedshowed a agood good dispersion dispersion of theof themicrocapsules microcapsules in in theirtheir structure structure and and mechanical mechanical properties properties comparable comparable to to those those of of neat neat resin resin specimens specimens with with the the same same their structure and mechanical properties comparable to those of neat resin specimens with the same curingcuring cycle cycle (Table (Table 88 andand FigureFigure 26 26).). curing cycle (Table 8 and Figure 26). Table 8. Method 6 flexural test results for PUF-DCPD specimens. Table 8. Method 6 flexural test results for PUF-DCPD specimens. Load Specimen No. Flexural Strength (MPa) Elongation at Break (mm) (N)Load Specimen No. Flexural Strength (MPa) Elongation at Break (mm) P1 48.07 24.89(N) 15.41 P1 48.07 24.89 15.41

Polymers 2020, 12, 1052 22 of 27

Table 8. Method 6 ﬂexural test results for PUF-DCPD specimens.

Polymers Specimen2020, 12, x FOR No. PEER REVIEW Flexural Strength (MPa) Load (N) Elongation at Break (mm)22 of 27 P1 48.07 24.89 15.41 P2P2 48.3148.31 25.1225.12 15.38 15.38 P3P3 50.2450.24 24.1624.16 16.05 16.05 P4P4 52.4052.40 P5P5 43.3143.31 22.6022.60 16.33 16.33 Measurement error (%) 3 1 0.5 Measurement error (%) ± ±3 ± ±1 ±0.5 ±

FigureFigure 26. 26.Stress–strain Stress–strain curvescurves forfor PUF-DCPDPUF-DCPD specimens specimens fabricated fabricated with with method method 6. 6.

3.3.5.3.3.5. Testing Testing of of MUF-ENB MUF-ENB Specimens Specimens FabricatedFabricated by Method 5 5 TableTable9 presents 9 presents the the results results of of ﬂexural flexural tests tests performed performed forfor each MUF-ENB specimen. specimen. Figure Figure 27 27 showsshows the the stress–strain stress–strain curves curves for testedfor tested specimens. specim Theens. supportThe support span ofspan each of specimen each specimen was measured was at 80measuredPolymers mm. 2020 at, 12 80, x mm.FOR PEER REVIEW 23 of 27

TableTable 9. Flexural9. Flexural test test results results for for MUF-ENB MUF-ENB specimens,specimens, curedcured at 80 80 °C◦C for for 2 2h, h, according according to to method method 5. 5.

SpecimenSpecimen No. No. Flexural Flexural Strength Strength (MPa) (MPa) LoadLoad (N) (N) Elongation Elongation at at Break Break (mm) (mm) P1P1 16.3216.32 9.409.40 4.91 4.91 P2P2 17.8717.87 10.8110.81 5.34 5.34 P3P3 19.5919.59 10.3010.30 4.66 4.66 P4P4 22.1322.13 10.4810.48 5.08 5.08 P5P5 24.4524.45 16.7116.71 5.00 5.00 Measurement error (%) 3 1 0.5 Measurement error (%) ± ±3 ± ±1 ±0.5±

FigureFigure 27. 27.Stress–strain Stress–strain curves curves forfor MUF-ENBMUF-ENB specimens fabricated fabricated according according to to method method 6. 6.

3.3.6. Self-Healing Evaluation Materials used in the specimen manufacturing process were M9.6GF/37%/300H8/G prepreg (glass fiber reinforced), Resoltech 1050/1058 epoxy resin (compatible with prepreg matrix), PUF- DCPD, MUF-ENB, and first generation Grubbs catalyst. Composite specimens containing a 15 vol % PUF-DCPD system and 10 vol % MUF-ENB system, respectively, were fabricated with respect to method 6 and in accordance with ISO 14125:2003 (Class IV). For the specimens containing the PUF-DCPD system, three-point bending tests showed a 4% average decrease in flexural strength, while for the specimens containing the MUF-ENB system, an average of a 10% decrease was observed when compared to the neat specimens. These values are acceptable, as these microcapsules are considered an induced defect in the composite structure. Moreover, specimens containing the PUF-DCPD system showed an average displacement of 13%, whilst for MUF-ENB specimens, an increase of 90% was observed when compared to neat specimens. This may be due to the larger amount of MUF-ENB microcapsules used for the specimen fabrication, as their dimensions are considerably smaller than those of PUF-DCPD microcapsules. Specimens were tested until the load was constant, which is the starting point of material degradation. Tested specimens were introduced in an oven (ambient atmosphere) at 40 °C, left for 48 h, and retested in the same conditions. After 48 h, specimens containing PUF-DCPD showed an average flexural strength of 81% compared to the reference specimens, and those containing MUF-ENB exhibited a value of 77%. Retested specimens presented an average displacement of 23% for PUF-DCPD specimens and 109% for MUF-ENB, respectively, until full specimen rupture. Due to the large number of tested specimens and in order to obtain a better understanding of the results, the load–displacement curves are presented as two separate charts, as seen in Figure 28 and Figure 29.

Polymers 2020, 12, 1052 23 of 27

3.3.6. Self-Healing Evaluation Materials used in the specimen manufacturing process were M9.6GF/37%/300H8/G prepreg (glass fiber reinforced), Resoltech 1050/1058 epoxy resin (compatible with prepreg matrix), PUF-DCPD, MUF-ENB, and first generation Grubbs catalyst. Composite specimens containing a 15 vol % PUF-DCPD system and 10 vol % MUF-ENB system, respectively, were fabricated with respect to method 6 and in accordance with ISO 14125:2003 (Class IV). For the specimens containing the PUF-DCPD system, three-point bending tests showed a 4% average decrease in flexural strength, while for the specimens containing the MUF-ENB system, an average of a 10% decrease was observed when compared to the neat specimens. These values are acceptable, as these microcapsules are considered an induced defect in the composite structure. Moreover, specimens containing the PUF-DCPD system showed an average displacement of 13%, whilst for MUF-ENB specimens, an increase of 90% was observed when compared to neat specimens. This may be due to the larger amount of MUF-ENB microcapsules used for the specimen fabrication, as their dimensions are considerably smaller than those of PUF-DCPD microcapsules. Specimens were tested until the load was constant, which is the starting point of material degradation. Tested specimens were introduced in an oven (ambient atmosphere) at 40 ◦C, left for 48 h, and retested in the same conditions. After 48 h, specimens containing PUF-DCPD showed an average flexural strength of 81% compared to the reference specimens, and those containing MUF-ENB exhibited a value of 77%. Retested specimens presented an average displacement of 23% for PUF-DCPD specimens and 109% for MUF-ENB, respectively, until full specimen rupture. Due to the large number of tested specimens and in order to obtain a better understanding of the results, the load–displacement curves are presented as two separate charts, as seen in Figures 28 and 29. Polymers 2020, 12, x FOR PEER REVIEW 24 of 27

FigureFigure 28. 28.Load Load valuesvalues forfor thethe three-pointthree-point bending bending tested tested specimens. specimens.

Figure 29. Displacement values for the three-point bending tested specimens.

4. Discussion Because the stirring rate is the main factor of microcapsule formation, the stirring rate was raised to 500 rpm to overcome the traces of non-encapsulated DCPD, thus halving the unreacted monomer. Adding acetone to the process of washing the microcapsules helps remove small traces of unreacted monomer and dries the capsules faster. The same principles were applied for MUF-ENB healing systems. After an evaluation of the six proposed methods for the integration of healing systems in the epoxy matrix, it was concluded that method 6 showed the optimal results for PUF-DCPD microcapsules and method 5 was found to be optimal for MUF-ENB microcapsules.

Polymers 2020, 12, x FOR PEER REVIEW 24 of 27

Polymers 2020, 12, 1052 24 of 27 Figure 28. Load values for the three-point bending tested specimens.

FigureFigure 29. 29.Displacement Displacement values values forfor the three-point bending bending tested tested specimens. specimens.

4. Discussion4. Discussion BecauseBecause the the stirring stirring rate rate isis the the main main factor factor of mi ofcrocapsule microcapsule formation, formation, the stirring the rate stirring was raised rate was raisedto to500 500 rpm rpm to overcome to overcome the traces the of traces non-encaps of non-encapsulatedulated DCPD, thus DCPD, halving thusthe unreacted halving monomer. the unreacted Adding acetone to the process of washing the microcapsules helps remove small traces of unreacted monomer. Adding acetone to the process of washing the microcapsules helps remove small traces of monomer and dries the capsules faster. The same principles were applied for MUF-ENB healing unreactedsystems. monomer and dries the capsules faster. The same principles were applied for MUF-ENB healing systems.After an evaluation of the six proposed methods for the integration of healing systems in the Afterepoxy an matrix, evaluation it was of theconcluded six proposed that method methods 6 forshowed the integration the optimal of healingresults systemsfor PUF-DCPD in the epoxy matrix,microcapsules it was concluded and method that method5 was found 6 showed to be optimal the optimal for MUF-ENB results formicrocapsules. PUF-DCPD microcapsules and method 5 was found to be optimal for MUF-ENB microcapsules.

Three-point bending tests were performed to evaluate the optimal integration methods. When compared to neat epoxy specimens, PUF-DCPD specimens showed a 20% average decrease in flexural strength and MUF-ENB specimens displayed a 40% decrease. Due to the smaller dimensions of the MUF-ENB microcapsules obtained, the defects induced in the matrix through the integration of the system should be smaller, being directly proportional to the mechanical strength of the neat material. However, the fact that the results obtained during the three-point bending test of the specimens with the MUF-ENB self-healing system are smaller than those of the PUF-DCPD specimens shows that the integration method can be further optimized. The results of the thermal stability tests demonstrate that the MUF-ENB microcapsules are two times more stable than the PUF-DCPD microcapsules. Therefore, for the thermal stability at 60 ◦C, the average weight loss for PUF-DCPD microcapsules is 20.49%, compared to only 8.39% in the case of MUF-ENB microcapsules. When exposed to the temperature of 80 ◦C, the PUF-DCPD microcapsules had an average weight loss percentage of 13.93%, compared to only 9.63% in the case of MUF-ENB microcapsules, and in the case of thermal stability testing at 120 ◦C, there was a 57.49% drop in mass for UPF-DCPD microcapsules, compared to only 19.23% for MUF-ENB microcapsules. To evaluate the healing efficiency of the proposed integration methods, PUF-DCPD and MUF-ENB microcapsules were integrated along with the Grubbs’ catalyst in glass fiber reinforced polymer (GFRP) specimens and subjected to three-point bending tests. An initial test was conducted to evaluate the difference in flexural strength for the specimens containing healing systems when compared to the neat epoxy specimens. The PUF-DCPD and MUF-ENB specimens were retested after conditioning at 40 ◦C for 48 h, from the starting point of material degradation, at which the specimens were initially tested. Polymers 2020, 12, 1052 25 of 27

The results showed a healing eﬃciency of 81% for the PUF-DCPD specimens and 77% for MUF-ENB specimens, when compared to the reference samples.

5. Conclusions Within this work, the synthesis of self-repair system components was conducted and methods for their integration in the host polymeric (epoxy) matrix were developed. Two different microcapsule-based regenerative systems whose shells were made of poly-urea- formaldehyde with embedded dicyclopentadiene monomer and melamine-urea-formaldehyde with embedded ethylidene-norbonene, were studied and compared in this work. Different integration methods were performed to identify an optimal integration process of both PUF-DCPD and MUF-ENB systems. Following identification of the optimal method, flexural tests were conducted to validate the integration methods. FT-IR analysis was conducted to validate the presence of constitutive elements in the obtained PUF-DCPD and MUF-ENB microcapsules. Additionally, thermogravimetric analysis showed thermal degradation of 73.6% for the PUF-DCPD system and 74% for MUF-ENB. Microstructural analysis was performed to observe the synthesized microcapsules and to confirm the core-shell structure. The three-point bending test results for the evaluation of self-healing efficiency confirmed the healing capacity of the proposed approach, where at least a 77% healing efficiency was obtained. Moreover, the displacement values for the retested specimens strengthened the healing ability of the optimized integration method.

Author Contributions: I.S.V. conceived of the presented idea. I.S.V. wrote the manuscript with support from A.A., A.C.M., A.C. and B.S.V., I.S.V. and A.C.M. performed the synthesis of microcapsules, sample fabrication and testing. A.A. performed the numerical simulations. A.C. took the lead in thermal and spectroscopy analysis. B.S.V. and I.S.V. were involved in microstructural investigations. I.S.V. and H.I. contributed to the interpretation of the results, review and editing of the manuscript. All authors provided critical feedback and helped shape the research, analysis and manuscript. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Romanian Space Agency—ROSA grant number 187/2017. The APC was funded by Program 1—Development of the national research-development system, Subprogram 1.2—Institutional performance—Funding projects of excellence in RDI, Contract no. 3PFE, supported by the Romanian Minister of Research and Innovation. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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