Synthesis and Characterization of a Polyurethane Phase Separated to Nano Size in an Epoxy Polymer

Article Synthesis and Characterization of a Polyurethane Phase Separated to Nano Size in an Epoxy Polymer

Tae Hee Kim, Miri Kim, Wonjoo Lee, Hyeon-Gook Kim, Choong-Sun Lim * and Bongkuk Seo *

The Center for Chemical Industry Development, Korea Research Institute of Chemical Technology, 45, Jongga-ro, Yugok-dong, Jung-gu, Ulsan 44412, Korea; [email protected] (T.H.K.); [email protected] (M.K.); [email protected] (W.L.); [email protected] (H.-G.K.) * Correspondence: [email protected] (C.-S.L.); [email protected] (B.S.)

Received: 2 April 2019; Accepted: 7 May 2019; Published: 13 May 2019

Abstract: Epoxy resins are widely applicable in the aircraft, automobile, coating, and adhesive industries because of their good chemical resistance and excellent mechanical and thermal properties. However, upon external impact, the crack propagation of epoxy polymers weakens the overall impact resistance of these materials. Therefore, many impact modifiers have been developed to reduce the brittleness of epoxy polymers. Polyurethanes, as impact modifiers, can improve the toughness of polymers. Although it is well known that polyurethanes (PUs) are phase-separated in the polymer matrix after curing, connecting PUs to the polymer matrix for enhancing the mechanical properties of polymers has proven to be challenging. In this study, we introduced epoxy functional groups into polyol backbones, which is different from other studies that focused on modifying capping agents to achieve a network structure between the polymer matrix and PU. We confirmed the molecular weight of the prepared PU via gel permeation chromatography. Moreover, the prepared material was added to the epoxies and the resulting mechanical and thermal properties of the materials were evaluated. Furthermore, we conducted tensile, flexural strength, and impact resistance measurements. The addition of PU to the epoxy compositions enhanced their impact strength and maintained their mechanical strength up to 10 phr of PU. Furthermore, the morphologies observed with field emission scanning electron microscopy and transmission electron microscopy proved that the PU was phase separated in the epoxy matrix.

Keywords: epoxy resin; polyurethane; physical properties

1. Introduction Epoxy resins are thermosetting polymers that form 3D network structures after exothermic reactions with amine or acidic anhydride hardeners [1,2]. Epoxy resins are interesting materials due to their excellent mechanical and thermal properties as well as chemical resistance [3–6]. A main disadvantage of the resins is their brittleness, which allows crack propagation upon outside impact. Therefore, many studies have been conducted to overcome this weakness by adding toughening agents that enhance impact resistance. In general, rubbery resins, such as carboxyl-terminated butadiene (CTBN) [7] and amine-terminated butadiene (ATBN) [8] have been used as toughening agents, whereas thermoplastic polyethersulfone [9] and polyetherimide [10] have also been applied. Elastomeric polyurethanes (PUs) are representative tougheners for thermosetting polymers. PUs are composed of a soft segment of polyol and a hard segment of diisocyanate [11], which are not compatible with each other and lead to phase separation. Upon addition of PUs to other polymers, the mechanical properties are enhanced because of the networking of the two polymers. The interaction between PU and the epoxy matrix is usually achieved

Coatings 2019, 9, 319; doi:10.3390/coatings9050319 www.mdpi.com/journal/coatings Coatings 2019, 9, 319 2 of 10 by mixing PU, which contains –NCO functional groups, with epoxy, which contains a hydroxyl group [12,13], or reacting polyol with diisocyanate in the presence of fillers [14,15]. On the other hand, the interaction between epoxy matrix and PU can be achieved by introducing glycidyl ethers in the polyol backbones of PUs instead of reacting epoxy groups with terminal NCO groups. Once the prepared polyols react with diisocyanate (DIC), the resulting PUs are furnished with epoxy functional groups. Therefore, the interaction between PU with the polyols and epoxy matrix can be enhanced by chemical reaction. In this study, dimeric fatty acids were reacted with two equivalents of epoxy groups to provide both polyols and epoxy functional groups. When the polyols were reacted with DIC, the obtained PU contained epoxy groups designed to react with an amine curer or epoxy groups in the epoxy matrix. Therefore, a fatty acid-modified epoxy polyol was reacted with diisocyanate to synthesize PU to increase the impact resistance and maintain the mechanical properties. Furthermore, the toughening properties of PU on the epoxy compositions were tested. Different amounts of PU were poured into the epoxy materials and cured to form thermosetting polymers, and their mechanical and viscoelastic properties were evaluated using a universal testing machine (UTM) and dynamic mechanical analysis (DMA), respectively. Furthermore, the fracture surfaces of the test specimens were observed by using field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM).

2. Experiments

2.1. Materials Diglycidyl ether of bisphenol A (DGEBA, YD-128, epoxy equivalent weight (EEW, g/eq unit), 187 g/eq) and a hydrogenated epoxy resin (ST-3000, EEW, 228.6 g/eq) were obtained from Kukdo Chemical (Seoul, Korea). D-230 (polyether diamine, Mw 230 g/mol) was also purchased from Kukdo Chemical. A dimeric fatty acid (pripol 1013-LQ, Mw 560 g/mol) was obtained from Croda Europe (Gouda, The Netherlands), LTD (Korea branch, Incheon, Korea). Polypropylene glycol (PPG, Mw 2000 g/mol), dibutyltin dilaurate (DBTDL), 2-hydroxyethyl methacrylate (HEMA), and isophorone diisocyanate (IPDI) were purchased from Sigma-Aldrich (St. Louis, MO, USA).

2.2. Synthesis of Polyol: Modification of Hydrogenated Epoxy Resin with Fatty Acids A dimeric fatty acid (45 g, 0.08 mol) was reacted with ST-3000 (73 g, 0.16 mol) in a 200-mL reactor at 100 ◦C for 40 min while triphenyl phosphine (0.3 g) was added as a catalyst (Figure1). The product was cooled to room temperature followed by analysis via 1H-NMR and FT-IR to confirm that the reaction was completed to produce the fatty-acid-modified hydrogenated epoxy polyol (FMEP). Coatings 2019, 9, x FOR PEER REVIEW 3 of 10

FigureFigure 1. Reaction 1. Reaction scheme scheme of of the the fatty fatty acid-modiﬁedacid-modified epoxy epoxy resin resin to synthesize to synthesize the polyol. the polyol.

Figure 2. Reaction scheme for PU by reacting the fatty acid-modified hydrogenated epoxy polyol with isophorone diisocyanate (FMEP-PU).

2.4. Preparation of the Epoxy Compositions and Cured Thermosetting Plastics The epoxy resin and D-230 were stoichiometrically mixed based on the equivalent molar ratio (Equation (1)). The amount of FMEP-PU in the epoxy resin was varied from 5 to 15 parts by weight per 100 parts of epoxy resin (phr). The formulation information for the epoxy compositions is summarized in Table 1. {100 g (of epoxy resin)}/{187 (g/epoxy resin)} × 60 (amine hydrogen equivalent wt.) (1) The epoxy resin and FMEP-PU were mixed and stirred for 20 min at 40 °C followed by the addition of D-230 for 20 min. The prepared paste was poured into a metal mold. The metal mold was placed in an oven and heated for 1 h at 80 °C to polymerize the epoxy resin and amine hardener.

Table 1. Formulation information for the epoxy compositions.

Compositions (g) YD-128 D-230 FMEP-PU Binder 100 32.1 – PU-5 100 32.1 5 PU-10 100 32.1 10 PU-15 100 32.1 15

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2.3. Synthesis of Polyurethane using FMEP (FMEP-PU) FMEP (60 g, 0.04 mol), polypropylene glycol (20 g, 0.04 mol), and IPDI (9.8 g, 0.16 mol) were added to a reactor and stirred for 40 min at 80 ◦C under nitrogen and a DBTDL catalyst was added. Subsequently, 2-hydroxyethyl methacrylate (HEMA, 21 g, 0.16 mol) was added to cap the terminal NCO groupFigure and 1. the Reaction solution scheme was stirredof the fatty for 3acid-modified h at 80 ◦C. The epoxy reaction resin to scheme synthesize is displayed the polyol. in Figure2.

Figure 2. ReactionReaction scheme scheme for for PU PU by by reacting reacting the the fatty fatty acid-modified acid-modiﬁed hydrogenated hydrogenated epoxy polyol with isophorone diisocyanatediisocyanate (FMEP-PU).

2.4. Preparation Preparation of the Epoxy Composit Compositionsions and Cured Thermosetting Plastics The epoxy resin and D-230 were stoichiometrically mixed based on the equivalent molar ratio (Equation (1)).(1)). TheThe amountamount ofof FMEP-PU FMEP-PU in in the the epoxy epoxy resin resin was was varied varied from from 5 to 5 15to parts15 parts by weightby weight per 100per parts100 parts of epoxy of epoxy resin (phr). resin The(phr). formulation The formulat informationion information for the epoxy for compositionsthe epoxy compositions is summarized is summarizedin Table1. in Table 1.

{100{100 g (of g epoxy (of epoxy resin)}/{187 resin)}/{187 (g/epoxy (g/epoxy resin)} resin)} × 60 (amine60 (amine hydrogen hydrogen equivalent equivalent wt.) wt.) (1) (1) × The epoxy resin and FMEP-PU were mixed and stirred for 20 min at 40 °C followed by the addition of D-230 for 20Table min. 1. TheFormulation prepared information paste was forpoured the epoxy into compositions.a metal mold. The metal mold was placed in an oven and heated for 1 h at 80 °C to polymerize the epoxy resin and amine hardener. Compositions (g) YD-128 D-230 FMEP-PU TableBinder 1. Formulation information 100 for the 32.1 epoxy compositions. – PU-5 100 32.1 5 CompositionsPU-10 (g) 100YD-128 D-230 32.1 FMEP-PU 10 PU-15Binder 100100 32.1 32.1 – 15 PU-5 100 32.1 5 PU-10 100 32.1 10 The epoxy resin and FMEP-PU were mixed and stirred for 20 min at 40 ◦C followed by the addition of D-230 for 20 min. The preparedPU-15 paste was poured100 into a32.1 metal mold.15 The metal mold was placed in an oven and heated for 1 h at 80 ◦ C to polymerize the epoxy resin and amine hardener.

2.5. Measurements and Analyses The synthesized FMEP and FMEP-PU were analyzed by Fourier transform-infrared spectrometry (FT-IR, Nicolet 6700/Nicolet Continuum; Thermo Fisher Scientific Inc., Walthum, MA, USA) and gel permeation chromatography (GPC, 1260 series, Agilent Technologies, New York, NY, USA, calibrated using polystyrene standards). The cured epoxy polymers in the metal mold were processed to achieve a 60 mm × 25 mm × 3 mm Coatings 9 test specimen2019, , 319 and used to measure the flexural strength based on the ASTM D790M standard4 [16] of 10 with a grinding machine (BESTPOL P2b2; SSAUL Bestech Co., Seoul, Korea) and diamond cutter (Manix, MBS 500500/E;/E; WoosungWoosung E&I E&I Co., Co., Sungnam, Sungnam, Korea). Korea). Additionally, Additionally, the the test test specimenspecimen forfor the tensile strength measurement measurement was was processe processedd to toa size a size of 150 of 150mm mm × 13 mm13 mm× 3 mm.3 mm.The tensile The tensile strengths strengths were × × testedwere tested with with10 specimens 10 specimens per percondition condition with with the theUTM UTM (UTM (UTM 5982, 5982, Instron, Instron, Norwood, Norwood, MA, MA, USA) USA) following thethe ASTM ASTM D638-14 D638-14 method method [17 ],[17], and theand mean the valuemean (excludingvalue (excluding the maximum the maximum and minimum and minimumvalues) was values) used. Thewas impactused. The strength impact was strength measured was usingmeasured an Izod using impact an Izod tester impact (JJHBT-6501, tester (JJHBT- JJ-test, 6501,Chengde, JJ-test, China) Chengde, based China) on the bas ASTMed on D256-10 the ASTM method D256-10 [18]. method [18]. The viscoelastic properties properties of of the the cured cured polymers polymers were were evaluated evaluated by by subjecting subjecting a a60 60 mm mm × 1212 mm mm × × 3 mm3 mm sample sample to todynamic dynamic mechanical mechanical analysis analysis (DMA, (DMA, Q800, Q800, TA TA Instruments, Instruments, Inc., Inc., New New Castle, Castle, DE, DE, × USA). The test sample was mounted on a dual cantilevercantilever probe and tested at a heating rate of 5 ◦°C/minC/min from 25 to 200 °C at a frequency of 1 Hz to measure both the storage moduli and tanδ values. The from 25 to 200 ◦C at a frequency of 1 Hz to measure both the storage moduli and tanδ values. The fractured surface obtained during the impact test test was was observed observed by by FE-SEM FE-SEM (MIRA (MIRA 3, 3, TESCAN, TESCAN, Brno, Brno, Czech Republic) and FE-TEM (JEM-2010, JEOL, Tokyo, Japan).Japan).

3. Results Results and and Discussion Discussion

3.1. Modification Modiﬁcation of th thee Epoxy Resin (FMEP) The prepared FMEP was analyzed via FT-IR, as shown in Figure3 3.. TheThe FT-IRFT-IR resultsresults showedshowed 1 1 that the fatty acidacid carbonylcarbonyl (C(C=O)=O) peak atat 17081708 cmcm−−1 shifted to 1740 cm−1 uponupon reaction reaction between the hydroxyl group of the fatty acid and the epoxy groupgroup of ST-3000 (Figure1 1).). AA chemicalchemical shiftshift inin thethe 1 hydroxyl group of ST-3000 was observedobserved from 34133413 toto 34723472 cmcm−−1 uponupon the reaction, along with an intensity increase. The The EEW EEW of of FMEP FMEP that that was measured using the ASTM D1652-11e1 method [19] [19] was 722 EEW, whichwhich isis similarsimilar toto thethe theoreticaltheoretical valuevalue (736(736 EEW)EEW) forfor thethe completecomplete reaction.reaction.

Figure 3.3. FT-IRFT-IR spectrum spectrum of (aof) the(a) fatty the acid,fatty ( bacid,) ST-3000, (b) andST-3000, (c) the and fatty ( acid-modiﬁedc) the fatty acid-modified hydrogenated hydrogenatedepoxy polyol (FMEP). epoxy polyol (FMEP).

3.2. Synthesis Synthesis of of PU PU The prepared polyol, FMEP, was mixed with PPG in a 1:1 molar ratio before the mixed polyol was reacted with IPDI to form the PU. The ratio of the mixed polyol to IPDI was 1:2, and the terminal 1 isocyanate group waswas observedobserved atat 22702270 cmcm−−1 (–N=C=O)(–N=C=O) in in the the FT-IR FT-IR spectrum, spectrum, as as shown shown in in Figure Figure 44a.a. In contrast, the isocyanate peak was minimal (in term termss of –NCO content, Figure 44b)b) afterafter thethe reactionreaction with HEMA. The molecular weights ofof the PU obtained via GPC analyses were asas follows: the number average molecular weight (Mn) = 6,665 g/mol and the weight molecular weight (Mw) = 51,260 g/mol with a 7.7 polydispersity index (PDI; Figure5). Coatings 2019, 9, x FOR PEER REVIEW 5 of 10 Coatings 2019, 9, x FOR PEER REVIEW 5 of 10 average molecular weight (Mn) = 6,665 g/mol and the weight molecular weight (Mw) = 51,260 g/mol with Coatingsaverage2019 molecular, 9, 319 weight (Mn) = 6,665 g/mol and the weight molecular weight (Mw) = 51,260 g/mol5 with of 10 a a7.7 7.7 polydispersity polydispersity index index (PDI; (PDI; Figure Figure 5). 5).

Figure 4. IR spectrum of polyurethane (FMEP-PU) (a) before mixing with the capping agent and (b) Figure 4. IR spectrum of polyurethane (FMEP-PU) ( (a)) before mixing with with the the capping agent agent and and ( (b)) afterafter 3 h 3 hof of mixing mixing with with the the capping capping agent. agent.

1.5 1.5 Mn = 6,665 g/mol Mn = 6,665 g/mol Mw = 51,260 g/mol Mw = 51,260 g/mol PDI = 7.7 1.0 PDI = 7.7 1.0

0.5

Intensity 0.5 Intensity

0.0 0.0

9 101112131415161718 9 101112131415161718 Retention time (min) Retention time (min)

Figure 5. Gel permeation chromatogram of FMEP-PU. FigureFigure 5. 5.Gel Gel permeation permeation chroma chromatogramtogram of of FMEP-PU. FMEP-PU. 3.3. Mechanical Properties of the Cured Epoxy Polymer 3.3. Mechanical Properties of the Cured Epoxy Polymer 3.3. MechanicalThe mechanical Properties properties, of the Cured including Epoxy tensile,Polymer ﬂexural, and impact strengths, of the cured epoxy polymersTheThe mechanical mechanical were measured properties, properties, using including including an UTM, tensile, tensile, as shown flex flexural,ural, in Figureand and impact impact6. Epoxy strengths, strengths, polymers of of the the have cured cured excellent epoxy epoxy polymersmechanicalpolymers were were properties measured measured but using using are an brittle an UTM, UTM, and as as su shown ﬀshowner from in in Figure severeFigure 6. crack6. Epoxy Epoxy propagation. polymers polymers have Therefore,have excellent excellent the mechanicalsynthesizedmechanical properties properties PU was addedbut but are toare thebrittle brittle epoxy and and compositions suffer suffer from from assevere severe a toughening crack crack propagation. propagation. agent to minimize Therefore, Therefore, cracking. the the synthesizedThesynthesized viscous PU liquidPU was was PUadded added was to thoroughlyto the the epoxy epoxy mixedcompositions compositions with the as compositionsas a atoughening toughening and agent agent phase-separated to to minimize minimize cracking. whencracking. the ThepastesThe viscous viscous formed liquid liquid a solidPU PU was polymerwas thoroughly thoroughly after the mixed mixed high-temperature with with the the compositions compositions curing reaction. and and phase-separated phase-separated However, when when when the the PUthe pastesphase-separated,pastes formed formed a asolid solid it loweredpolymer polymer the after after cross-linking the the high-tempe high-tempe densityraturerature of thecuring curing polymer reaction. reaction. and However, reducedHowever, thewhen when mechanical the the PU PU phase-separated,properties.phase-separated, The cross-linkingit itlowered lowered the densitythe cross-linking cross-linking of the compositions den densitysity of of the was the polymer calculatedpolymer and and using reduced reduced data obtainedthe the mechanical mechanical from the properties.DMAproperties. experiments The The cross-linking cross-linking in Table2 bydensity density Equation of of the the (2). compos compos The dataitionsitions shows was was that calculated calculated the cross-linking using using data data density obtained obtained decreased from from theuponthe DMA DMA addition experiments experiments of PU to in thein Table Table epoxy 2 2by compositions. by Equation Equation (2). (2). The The data data shows shows that that the the cross-linking cross-linking density density decreaseddecreased upon upon addition addition of of PU PU to to the the epoxy epoxy compositions. compositions. υe = E’/3RT (2) υe = E’/3RT (2) υe = E’/3RT (2) wherewhere υeυ ise isthe the cross-link cross-link density, density, E’E is’ is the the tensile tensile storage storage modulus modulus (MPa), (MPa), RR isis the the gas gas constant constant where υe is the cross-link density, E’ is the tensile storage modulus (MPa), R is the gas constant 3 1 1 (8.314(8.314 m m3∙PaPa∙KK−1−∙molmol−1),− T), isT temperatureis temperature in inK Kcorresponding corresponding to tothe the storage storage modulus modulus value value [20]. [20 ]. (8.314 m3·∙Pa·∙K−1∙·mol−1), T is temperature in K corresponding to the storage modulus value [20]. Coatings 2019, 9, 319 6 of 10 Coatings 2019, 9, x FOR PEER REVIEW 6 of 10

FigureFigure 6.6. CharacterizationCharacterization ofof the the cured cured epoxy epoxy polymers: polymers: (a ()a tensile) tensile strength, strength, (b )(b stress-strain) stress-strain curves curves of tensileof tensile test, test, (c) ( flexuralc) flexural strength strength (d) ( stress-straind) stress-strain curves curves of flexural of flexural test, test, and and (e) impact(e) impact strength. strength. Table 2. Calculation of cross-linking density of the cured epoxy compositions with DMA. Table 2. Calculation of cross-linking density of the cured epoxy compositions with DMA. Storage Modulus at Tg Cross-Linking Density Compositions Storage Modulus at Tg + TgTg(◦ (°C,C, with with DMA) Cross-Linking Density3 Compositions + 40 ◦C (MPa) (moles/cm− ) −3 40 °C (MPa) DMA) (moles/cm 3) Binder 10.78 108 1.03 10− × −3 BinderPU-5 10.78 3.17 108 72 1.030.33 × 1010 3 × − PU-5PU-10 3.17 4.75 72 78 0.330.49 × 1010−3 3 × − PU-10PU-15 4.75 5.45 78 82 0.490.55 × 1010−3 3 × − PU-15 5.45 82 0.55 × 10−3 Typically, decreases in tensile or flexural strength are proportional to the amount of PU added to the epoxy.Typically, To increase decreases strength, in tensile the synthesizedor flexural strength PU was designedare proportional to contain to glycidylthe amount groups of PU to bridgeadded theto the epoxy epoxy. polymer To increase via a reaction strength, between the synthesize the PU glycidyld PU was groups designed and to hardener contain amines.glycidyl Asgroups shown to inbridge Table the2, though epoxy polymer the cross-linking via a reaction density between of the epoxythe PU compositions glycidyl groups decreased and hardener upon addition amines. ofAs PUshown (PU-5), in Table it increased 2, though with the increasing cross-linking PU content density (PU-10 of the or epoxy PU-15), compositions indicating thedecreased formation upon of cross-linkingaddition of PU bonds (PU-5), between it increased the PU and with epoxy increasing polymers. PU Ascontent shown (PU-10 in Figure or 6PU-15),a, the tensile indicating strength the increasedformation whenof cross-linking 5 phr of PU bonds was added,between but the decreased PU and epoxy when polymers. 10 phr of PUAs wasshown added. in Figure In terms 6a, the of flexuraltensile strength strength increased (Figure6b), when even 5 a phr 5 phr of additionPU was added, of PU increased but decreased the strength, when 10 which phr of was PUmaintained was added. In terms of flexural strength (Figure 6b), even a 5 phr addition of PU increased the strength, which was maintained at 10 phr PU, but decreased at 15 phr PU. The impact strength data (Figure 6c) show that Coatings 2019, 9, 319 7 of 10

Coatings 2019, 9, x FOR PEER REVIEW 7 of 10 at 10 phr PU, but decreased at 15 phr PU. The impact strength data (Figure6c) show that the impact thestrength impact of strength the epoxy of polymersthe epoxy increased polymers from increased 46.3 to from 76.4 46.3 J/m to when 76.4 10J/m phr when PU 10 was phr added. PU was At added. 15 phr Atof PU,15 phr the of impact PU, the strength impact increased strength toincreased 96.4 J/m, to more 96.4 thanJ/m, twicemore than the increase twice the of increase the binder. of the The binder. results Theshow results that the show synthesized that the synthesized FMEP-PU increased FMEP-PU the incr impacteased strength,the impact while strength, the well-known while the well-known toughening tougheningagent CTBN agent enhanced CTBN tensile enhanced strength tensile [21 ].strength [21]. The viscoelastic properties of the cured epoxy polymers were subjected to DMA. The obtained storage modulus, loss loss modulus, modulus, and and tan tanδδ datadata are are shown shown in in Figure Figure 7.7 .The The initial initial storage storage modulus modulus at 25at 25°C◦ Cfor for the the epoxy epoxy polymers polymers increased increased from from 2,035 2,035 to to 3,000 3,000 MPa MPa upon upon addition addition of of PU, PU, while the storage modulusmodulus ofof thethe polymerspolymers decreased decreased with with increasing increasing temperature. temperature. Tan Tanδ providesδ provides a measurea measure of ofthe the viscous viscous to elasticto elastic portion portion ratio. ratio. As As shown shown in in Figure Figure7c, 7c, the the glass glass transition transition temperature temperature (tan (tanδ) ofδ) ofPU-5 PU-5 abruptly abruptly decreased, decreased, as as the the tan tanδ ofδ PU-10of PU-10 and and PU-15 PU-15 showed showed a higher a higher temperature temperature than than that that of ofPU-5. PU-5. This This indicates indicates that that the the cyclohexyl cyclohexyl groups groups of theof the modiﬁed modified polyol polyol imparted imparted rigidity rigidity to PU.to PU.

3000 500 Binder Binder PU-5 PU-5 2500 PU-10 400 PU-10 PU-15 PU-15 2000 300

1500 200 1000 age modulus (MPa) age modulus T

Loss modulus (MPa) 100 Sto 500

0 0

25 50 75 100 125 150 175 25 50 75 100 125 150 175 TemTeTatuTe (℃ ) TemTeTatuTe (℃ ) (a) (b) 2.0

Binder PU-5 1.5 PU-10 PU-15

1.0 Tan Delta 0.5

0.0

25 50 75 100 125 150 175 TemTeTatuTe (℃ ) (c)

Figure 7. DMADMA data data of of the the cured cured epoxy epoxy polymers: ( (aa)) storage storage modulus, modulus, ( b) loss loss modulus, modulus, and ( c) tan δ.

3.4. FE-SEM and TEM Images of the Fractured Epoxy Polymers Obtained after Impact Tests Both FE-SEM (Figure 88)) andand TEMTEM (Figure(Figure9 )9) images images of of the the fractured fractured cured cured epoxy epoxy matrix matrix were were obtained and are shown in Figure8 8.. TheThe surfacesurface ofof thethe neatneat resinresin waswas smooth,smooth, whereaswhereas thethe fracturedfractured surfaces of the epoxy polymers, including PU, contained small small holes holes approximately approximately 100 100 nm nm in in size. size. The holes were left from the phase-separated PU part particlesicles bouncing ooffﬀ due to the impact. In In addition, addition, similar sized protruding parts were observed, indica indicatingting that a strong interaction between the epoxy matrix and PU particles occurred upon curing of the system. Coatings 2019, 9, 319 8 of 10 Coatings 2019, 9, x FOR PEER REVIEW 8 of 10 Coatings 2019, 9, x FOR PEER REVIEW 8 of 10

Figure 8. FE-SEM images of the fractured surfaces: (a) binder, (b) PU-5, (c) PU-10, and (d) PU-15. Figure 8. FE-SEM images of the fractured surfaces: ( a)) binder, (b) PU-5, (c) PU-10, and (d)) PU-15.

Figure 9. TEM image of PU-15. Figure 9. TEM image of PU-15. 4. Conclusions 4. Conclusions In this study, aa bisphenolbisphenol AA epoxyepoxy resinresin waswas modiﬁedmodified withwith aa dimericdimeric fatty acid, both of which In this study, a bisphenol A epoxy resin was modified with a dimeric fatty acid, both of which contained polyol and diglycidyl groups. The prepared polyol was mixed with PPG and reacted with contained polyol and diglycidyl groups. The prepared polyol was mixed with PPG and reacted with IPDI followed by the addition of HEMA to form a stable stable elastomeric elastomeric PU (FMEP-PU). (FMEP-PU). The synthesized IPDI followed by the addition of HEMA to form a stable elastomeric PU (FMEP-PU). The synthesized FMEP-PU waswas usedused as as a a toughener toughener for for the the epoxy epoxy system system to compensate to compensate for the for brittleness the brittleness of the of epoxy the FMEP-PU was used as a toughener for the epoxy system to compensate for the brittleness of the polymerepoxy polymer formed byformed curing. by Various curing. amounts Various of FMEP-PUamounts of were FMEP-PU added to were the epoxy added compositions to the epoxy and epoxy polymer formed by curing. Various amounts of FMEP-PU were added to the epoxy werecompositions cured at highand temperaturewere cured at to high form temperature the epoxy polymer. to form The the tensile epoxy strength polymer. measurements The tensile strength showed compositions and were cured at high temperature to form the epoxy polymer. The tensile strength measurementsthat the epoxy compositionsshowed that withthe epoxy FMEP-PU compositions decreased with in proportion FMEP-PU to decreased the contents in ofproportion FMEP-PU to while the measurements showed that the epoxy compositions with FMEP-PU decreased in proportion to the contentsﬂexural strength of FMEP-PU increased while up flexural to 5 phr strength of FMEP-PU. increased However, up to 5 thephr impact of FMEP-PU. strength However, of the compositions the impact contents of FMEP-PU while flexural strength increased up to 5 phr of FMEP-PU. However, the impact proportionallystrength of the compositions increases from proportionally 46.3 J/m with increases the binder from to 96.4 46.3J /J/mmof with PU-15. the binder Thus, FMEP-PUto 96.4 J/m enhancedof PU-15. strength of the compositions proportionally increases from 46.3 J/m with the binder to 96.4 J/m of PU-15. Thus,the toughness FMEP-PU of enhanced the epoxy the compositions. toughness of Viscoelastic the epoxy compositions. properties obtained Viscoelastic from properties DMA experiments obtained Thus, FMEP-PU enhanced the toughness of the epoxy compositions. Viscoelastic properties obtained fromshowed DMA that experiments the Tg decreases showed upon that the the addition Tg decreases of 5 phr upon PU fromthe addition 108 to 72 of◦ 5C, phr but PU increases from 108 along to 72 with °C, from DMA experiments showed that the Tg decreases upon the addition of 5 phr PU from 108 to 72 °C, butthe increasingincreases along PU content with the to 82increasing◦C. The calculatedPU content cross-linking to 82 °C. The density calculated indicated cross-linking that the glycidyldensity but increases along with the increasing PU content to 82 °C. The calculated cross-linking density indicatedgroups of that PU reactedthe glycidyl with groups the amine of PU curer reacted to form with a the cross-linked amine curer structure. to form a Therefore, cross-linked PU-5 structure. with a indicated that the glycidyl groups of PU reacted with the amine curer to form a cross-linked structure. Coatings 2019, 9, 319 9 of 10 cross-linking density of 1.03 10 3 moles/cm 3 increased to 0.55 10 3 moles/cm 3 when 15 phr of × − − × − − PU was added to the epoxy composition.

Author Contributions: Conceptualization, B.S. and C.-S.L.; Methodology, W.L. and H.-G.K.; Formal Analysis, M.K.; Investigation, T.H.K.; Writing–Original Draft Preparation, T.H.K.; Writing–Review & Editing, B.S. and C.-S.L. Funding: This research (No. SI1941-20; Development of specialty chemicals for the automobile industry) was funded by the Korea Research Institute of Chemical Technology (KRICT), the Ministry of Trade, Industry and Energy (MOTIE), and Korea Institute for Advancement of Technology (KIAT) through the National Innovation Cluster R&D program (P0006669_ Development of Fiber-reinforced Composite Based Floor Cover for Micro Electric Vehicle Platform). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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