Synthesis of Thermally Stable Reactive Polyurethane and Its Physical Effects in Epoxy Composites

applied sciences

Article Synthesis of Thermally Stable Reactive Polyurethane and Its Physical Effects in Epoxy Composites

Jin Hong Kim 1,2, Wonjoo Lee 1, Taehee Kim 1, Hyeon-Gook Kim 1, Bongkuk Seo 1, Choong-Sun Lim 1,* and In Woo Cheong 2,* 1 Center for Chemical Industry Development, Korea Research Institute of Chemical Technology, 45, Jongga-ro, Yugok-dong, Jung-gu, Ulsan 44412, Korea; [email protected] (J.H.K.); [email protected] (W.L.); [email protected] (T.K.); [email protected] (H.-G.K.); [email protected] (B.S.) 2 School of Applied Chemical Engineering, Kyungpook National University, Daegu 41566, Korea * Correspondence: [email protected] (C.-S.L.); [email protected] (I.W.C.); Tel.: +82-52-241-6021 (C.S.L.); Fax: +82-52-241-6029 (C.S.L.) Received: 19 July 2018; Accepted: 5 September 2018; Published: 7 September 2018 Abstract: A flame retardant polyol (EP-DOPO) with epoxy functional groups was synthesized by reacting a 1,6-hexanediol glycidyl ether with a flame retardant 10-(2,5-dihydroxyphenyl)- 10H-9-oxa-10-phospha-phenanthrene-10-oxide (DOPO). The polyurethane (EPPU) with enhanced heat resistance was prepared by the reaction of a polyol blend of EP-DOPO and polytetrahydrofuran (PolyTHF) at a ratio of 1:1 with isophorone diisocyanate. EPPU useful for the preparation of cables or coatings showed higher thermal decomposition temperature rather than that of reference polyurethane synthesized by the reaction between pure PolyTHF and isophorone diisocyanate by thermogravimetric analysis. Further study of the polyurethane as a toughening agent for epoxy polymers was carried out. Epoxy compositions consisting of bisphenol A epoxy resin and dicyandiamide as a hardener have a brittle property allowing crack propagation after cure. Polyurethane plays an important role as an impact modifier to prevent from cracks of epoxy polymers. Various contents of EPPU were added into epoxy compositions to measure the physical property changes of epoxy polymers. The tensile and flexural strengths of the cured specimen were compared with those of epoxy compositions including reference polyurethane. Furthermore, the crosslink density of the cured epoxy compositions was compared.

Keywords: polyurethane; ﬂame retardant; epoxy; crosslink density; impact resistance

1. Introduction Polyurethanes (PUs) are polymers composed of a soft segment and a hard segment prepared by the reaction between hard isocyanates and soft polyols. Polypeptides of the hard segments form strong hydrogen interactions with each other. In contrast, soft segments comprising linear long-chain polyols have flexible properties. The physical properties of synthesized polyurethanes depend on the reaction ratio between isocyanate and polyol and the types of reactants, such as polyols, chain extenders, or isocyanates. Therefore, a variety of synthetic polyurethanes prepared with various combinations of reactants have been widely adopted in automotive interior materials, adhesives, coatings, shoes, clothing, etc. However, the low thermal stabilities of polyurethanes limit their use in high-temperature processes or applications. Furthermore, polyurethanes begin to decompose at temperatures >180 ◦C, resulting in the production of toxic gases, such as HCN or CO, that are harmful to the human body or environment [1–4]. Recently, researchers have been interested in introducing flame retardants into polyurethanes to compensate for the low thermal stability. The flame retardants can be classified into two types:

Appl. Sci. 2018, 8, 1587; doi:10.3390/app8091587 www.mdpi.com/journal/applsci Appl. Sci. 2018, 8, 1587 2 of 14 an additive type, in which a flame retardant is physically added to the polymeric resin, and a reactive type, in which a flame retardant reacts with reactant monomers, forming polymers. In general, the most often used additive flame retardants need to be highly miscible with the raw materials and not deteriorate the mechanical properties of the produced polymers nor produce any toxic gases. Since environmental regulations prohibit the use of halogenated flame retardants, phosphoric flame retardants have attracted attention as one alternative. In particular, the flame retardancy of polyurethane is improved by introducing a monomer having flame retardancy into the main chain, using a method of encapsulating the flame retardant at the end of the polyurethane or by using organic–inorganic additives [5–8]. Epoxy resins are thermosetting polymers constructed of three-dimensional network structures after a curing reaction with hardeners in the presence of catalysts. Epoxy resin derivatives can exhibit various physical properties by reacting with different hardeners, and they are widely used in coatings, adhesives, and moldings because the polymerized matrices provide excellent thermal and mechanical properties and chemical resistance [9,10]. However, the high brittleness of cured epoxy polymers promotes easy crack propagation. Therefore, numerous scientists have reported methods to reduce this brittleness by adding thermoplastic resin, rubber resins, or elastomeric polyurethanes [11,12]. Specifically, polyurethanes in epoxy compositions, which are known to be phase separated after curing, have been added to absorb external impact [13,14]. In this report, a phosphoric monomer was reacted with an epoxy resin to prepare a reactive polyol. The polyol mixed with a polyether polyol in a 1:1 ratio was reacted with 2 mol of isophorone diisocyanate to enhance thermal resistance. In addition, the prepared polyurethane was added to epoxy compositions to investigate physical changes, where bisphenol A epoxy resin and dicyandiamide (dicy) were stoichiometrically blended. The effects of the prepared polyurethane addition to the epoxy compositions were further analyzed with tensile and flexural strength measurements, the Izod impact method, thermogravimetric analysis, and scanning electron microscopy.

2. Materials and Methods

2.1. Materials The phosphorous flame retardant 10-(2,5-dihydroxyphenyl)-10H-9-oxa-10-phospha-phenanthrene- 10-oxide (DOPO) was obtained from Pharmicell Co., Ltd. (Ulsan, Korea) and 1,6-Hexanediol glycidyl ether (HDGE) was purchased from HJ Chemco (Ulsan, Korea). Tetrabutylammonium iodide (TBAI), polytetrahydrofuran (PolyTHF) (Mn = 2000 g/mol), isophorone diisocyanate (IPDI), and 2-allyl phenol (2-AP, 98%) were purchased from Sigma-Aldrich (St. Louis, MO, USA). Dibutyltindilaurylmercaptide (DBTDL) was obtained from Gelest Co (Morrisville, PA, USA). Bisphenol A epoxy resin (DGEBA, 187 g/eq) was purchased from Momentive Co (USA). The hardener, dicyandiamide, and 1,1-dimethyl-3-phenyl urea (DPU) were obtained from Air Products (Arlington, PA, USA).

2.2. Preparation of Polyol (EP-DOPO) DOPO (64.8 g) and HDGE (92 g) were poured into a 500 mL beaker and stirred at 60 ◦C until the DOPO completely dissolved. TBAI, a catalyst, was added to the beaker, which was heated at 150 ◦C for 1 h (Figure1). The product was cooled to room temperature and characterized (Figure2). Appl. Sci. 2018, 8, 1587x FOR PEER REVIEW 33 ofof 1415

Figure 1. Preparation of polyol (EP-DOPO) reaction scheme.

2.3. Synthesis of Polyurethane Reference polyurethane (Ref PU) was prepared by reacting PolyTHF (80 g, 0.04 mol) with IPDI ◦ (34 mL, 0.08 mol) at 90 °CC for 90 min under nitrogen in the presence ofof DBTDLDBTDL catalyst.catalyst. Then, 2-AP ◦ (23 mL, 0.18 mol) waswas addedadded and thethe mixturemixture waswas heatedheated atat 120120 °CC for 11 hh [[15].15]. The product was cooled to room temperature. Modified Modiﬁed polyurethane (EPPU) (EPPU) was was prepared by reacting IPDI (34 (34 mL, ◦ 0.08 mol) with the mixture of PolyTHF (40 g, 0.02 mol) and EP-DOPO (15.6 g, 0.02 mol) at 90 °CC for ◦ 2.5 h underunder nitrogen.nitrogen. AfterAfter the the addition addition of of 2-AP 2-AP (26 (26 mL, mL, 0.2 0.2 mol), mol), the the solution solution was was stirred stirred at 120at 120C for°C 1for h. 1 Two h. Two types types of ﬂame of flame retardant retardant polyurethanes polyurethanes (EPPU, (EPPU, and and EPPU-2) EPPU-2) were were synthesized synthesized based based on theon molarthe molar ratio ratio of PTMG, of PTMG, EP-DOPO, EP-DOPO, and and IPDI. IPDI. EPPU EPPU had had a molar a molar ratio ratio of 0.2:0.2:0.8, of 0.2:0.2:0.8, while while EPPU-2 EPPU-2 had ahad ratio a ratio of 0:0.4:0.8, of 0:0.4:0.8, as shown as shown in Figure in Figure3. 3. 2.4. Preparation of Epoxy Compositions and Curing Process 2.4. Preparation of Epoxy Compositions and Curing Process DGEBA and dicy (1:1 mole ratio) with DPU (0.2%) were stirred with a mechanical stirrer at 80 DGEBA and dicy (1:1 mole ratio) with DPU (0.2%) were stirred with a mechanical stirrer at 80 ◦C under vacuum for 30 min. Elastic polyurethanes were added to the epoxy binder in the range of °C under vacuum for 30 min. Elastic polyurethanes were added to the epoxy binder in the range of 10 to 30 parts per hundred resin (phr) to provide a toughening effect to the cured epoxy compositions. 10 to 30 parts per hundred resin (phr) to provide a toughening effect to the cured epoxy The resin composition was poured into a metal mold, followed by heating at 150 ◦C for 1 h, 170 ◦C for compositions. The resin composition was poured into a metal mold, followed by heating at 150 °C 1 h, and 190 ◦C for 1 h to complete the curing. for 1 h, 170 °C for 1 h, and 190 °C for 1 h to complete the curing. 2.5. Characterization and Analysis 2.5. Characterization and Analysis Elemental analysis was performed with an elemental analyzer (EA, Flash 2000, Thermo Fisher, Basingstok,Elemental UK). analysis The molecularwas performed weight with of an the elemental synthesized analyzer polyol (EA, was Flash measured 2000, Thermo using Fisher, liquid chromatography–massBasingstok, UK). The molecular spectrometry weight (LC-MS, of the G6130, synthesized Agilent Technologies,polyol was measured New York, using NY, liquid USA), whilechromatography–mass the structure was spectrometry analyzed using (LC-MS, Fourier-transform G6130, Agilent nuclear Technologies, magnetic New resonance York, NY, (FT-NMR, USA), Avancewhile the 3300, structure Bruker, was Karlsruhe, analyzed Germany). using Fourier-transform The LC-MS system nuclear ionized magnetic the sample resonance using electron (FT-NMR, spray ionizationAvance 3300, (ESI), Bruker, and theKarlsruhe, eluent had Germany). a 1:1 ratio The of LC acetonitrile:dried-MS system ionized water the (formic sample acid using 0.2%) electron under aspray 0.7 mL/min ionization flow (ESI), rate. and The molecularthe eluent weighthad a of1:1 the ratio synthesized of acetonitrile:dried polyurethane water was measured(formic acid using 0.2%) gel permeationunder a 0.7 mL/min chromatography flow rate. (GPC, The molecular 1260 Series, weight Agilent of Technologies,the synthesized New poly York,urethane NY, USA). was measured The glass using gel permeation chromatography (GPC, 1260 Series, Agilent Technologies, New York, NY, Appl. Sci. 2018, 8, x FOR PEER REVIEW 4 of 15

Appl.USA). Sci. The2018, 8glass, 1587 transition temperature (Tg) of polyurethane was measured using differential4 of 14 scanning calorimetry (DSC, Q2000, TA instrument, New Castle, DE, USA) from −80 to −20 °C at a heating rate of 10 °C/min. The thermal stability of the polyurethane was examined using T transitionthermogravimetric temperature analysis ( g) of (TGA, polyurethane Q500, TA was instrume measurednt, usingNew differentialCastle, DE, scanningUSA) in calorimetrythe range of (DSC, 25 to − − ◦ ◦ Q2000,800 °C TAat a Instrument,heating rate New of 10 Castle, °C/min. DE, Then, USA) the from tensile80 strengths to 20 Cof atthe a heatingcured epoxy rate ofcompositions 10 C/min. The thermal stability of the polyurethane was examined using thermogravimetric analysis (TGA, Q500, were measured with a universal testing machine (UTM, INSTRON◦ 5982, Instron, ◦USA) for test TAspecimens Instrument, processed New Castle, to sizes DE, of USA) 150 mm in the × range13 mm of × 25 3 tomm 800 basedC at on a heating the ASTM rate ofD 10638C/min. method. Then, The theflexural tensile strengths strengths of of the the cured cured epoxycompositions compositions processed were to measured sizes of with60 mm a universal × 25 mm testing × 3 mm machine were × × (UTM,tested INSTRONby using the 5982, ASTM Instron, D 790M USA) method. for test specimensThe tensile processed and flexural to sizes tests of were 150 mmrepeated13 mmfive times3 mm to basedobtain on average the ASTM values. D 638 The method. Izod impact The flexural strengths strengths were measured of the cured using compositions the ASTM processedD 256 method to sizes for × × ofcured 60 mm compositions25 mm with3 mm sizes were of tested 63.5 bymm using × 12 the.7 ASTMmm × D3 790Mmm with method. a pendulum The tensile impact and flexural tester tests(HIT-2492, were repeated JJ-TEST, five Chengde, times to obtainChina). average The dynamic values. Themechanical Izod impact analysis strengths (DMA, were Q800, measured TA × × usingInstrument, the ASTM New D Castle, 256 method DE, USA) for cured of the compositions epoxy comp withositions sizes was of 63.5performed mm 12.7using mm a Q8003 mmwith with test aspecimens pendulum measuring impact tester 60 mm (HIT-2492, × 12 mm JJ-TEST, × 3 mm. Chengde, The specimens China). were The installed dynamic on mechanical the dual cantilever analysis (DMA,probe while Q800, TAthe Instrument,operating temperature New Castle, DE,increased USA) ofat the a epoxyheating compositions rate of 5 °C/min was performed under a using1 Hz a Q800 with test specimens measuring 60 mm × 12 mm × 3 mm. The specimens were installed on frequency to measure both the storage moduli and tan δ values. Furthermore, the thermal stabilities◦ theof the dual cured cantilever epoxy probe compositions while the were operating measured temperature using TGA increased (TGA, at Q500, a heating TA instrument, rate of 5 C/min New δ underCastle aDE. 1 Hz USA) frequency in the torange measure of 25 bothto 800 the °C storage at a heating moduli rate and of tan 10 °C/min.values. Furthermore, the thermal stabilities of the cured epoxy compositions were measured using TGA (TGA, Q500, TA Instrument, ◦ ◦ New3. Results Castle and DE. Discussion USA) in the range of 25 to 800 C at a heating rate of 10 C/min.

3. Results and Discussion 3.1. Characterization

3.1. CharacterizationThe structures of the prepared polyurethanes were analyzed with 1H-NMR, ESI-MS, and elementalThe structures analysis. of The the preparedproton peaks polyurethanes of secondary were carbon analyzed (i) withand 1tertiaryH-NMR, carbon ESI-MS, (h) and formed elemental by a analysis.ring opening The protonreaction peaks of the of epoxy secondary group carbon were (i)obse andrved tertiary in the carbon 4.05–4.32 (h) formed(ppm) range, by a ring as shown opening in reactionFigure 2. of the epoxy group were observed in the 4.05–4.32 (ppm) range, as shown in Figure2.

Figure 2. 1H-NMR spectrum of EP-DOPO. Figure 2. 1H-NMR spectrum of EP-DOPO. 1H NMR (300 MHz, Acetone): δ (ppm) = 1.36 (8H, m), 1.53 (8H, m), 2.52, 2.69 (4H, d), 3.05 (2H, m), 3.26~3.90 (12H, m), 4.32 (2H, m), 4.05 (4H, d). EA calc’d for C42H57O12P: C, 64.27; H, 7.32; O, 24.46; P, 3.95. found: C, 60.95; H, 5.99; O, 22.86. LC-ESI-MS: found 785.3 (M + H)+, calc’d 784.3. Appl. Sci. 2018, 8, x FOR PEER REVIEW 5 of 15

Appl. Sci. 20181H, 8NMR, 1587 (300 MHz, Acetone): δ (ppm) = 1.36 (8H, m), 1.53 (8H, m), 2.52, 2.69 (4H, d), 3.05 (2H,5 of 14 m), 3.26~3.90 (12H, m), 4.32 (2H, m), 4.05 (4H, d). EA calc’d for C42H57O12P: C, 64.27; H, 7.32; O, 24.46; P, 3.95. found: C, 60.95; H, 5.99; O, 22.86. LC-ESI-MS: found 785.3 (M + H)+, calc’d 784.3. PolyurethanePolyurethane was preparedwas prepared by reacting by reacting polyol withpolyol IPDI with followed IPDI followed by capping by thecapping terminal-NCO the (isocyanate)terminal-NCO group (isocyanate) with a phenol group OH with group, a phenol as shown OH group, in Figure as shown3. in Figure 3.

FigureFigure 3. 3.Reaction Reaction schemescheme for for polyurethanes. polyurethanes. Appl. Sci. 2018, 8, x FOR PEER REVIEW 6 of 15 The molecularThe molecular weights weights of theof the prepared prepared polyurethanes polyurethanes were were analyzed analyzed with GPC (Figure (Figure 4)4) and and the molecularthe molecular weights weights and dispersity and dispersity data aredata shown are shown in Table in Table1. 1.

Figure 4. GPC data for polyurethanes, intensity (arbitrary unit) vs retention time (min)). Figure 4. GPC data for polyurethanes, intensity (arbitrary unit) vs retention time (min)). Table 1. Molecular weights of polyurethanes. Table 1. Molecular weights of polyurethanes. Mn (g/mol) Mw (g/mol) D Ref PUM 7556n (g/mol) M 13,158w (g/mol) D 1.74 EPPURef PU 93387556 21,11213,158 1.74 2.26 EPPU-2EPPU 31339338 414321,112 2.26 1.32 EPPU-2 3133 4143 1.32

The prepared polyurethanes were analyzed with GPC to check whether two polyurethanes have a similar range of molecular weight. It turned out that both polyurethanes have a similar range of molecular weight: 9338 g/mol for EPPU and 7556 g/mol for Ref PU. However, EPPU-2 has a small molecular weight because it is synthesized by reacting small molecular weight of EP-DOPO with diisocyanate (Figure S1).

3.2. Thermal Properties of Polyurethane

3.2.1. Differential Scanning Calorimetry Polyurethane composed of a hard segment from diisocyanate and soft segment from polyols had two glass transition temperatures due to the individual segments. The glass transition temperature of the soft segment was usually observed at low temperatures, below 0 °C. The DSC data (Figure 5) shows the Tg values of Ref PU and EPPU at −69, and −60 °C, respectively. This suggests that EP-DOPO, which is more rigid than PTMG, causes the higher Tg of the EPPU soft segment. For the same reason, Tg of EPPU-2 synthesized by reacting EP-DOPO polyol with IPDI was −8.8 ℃ (Figure S2) Appl. Sci. 2018, 8, 1587 6 of 14

3.2. Thermal Properties of Polyurethane

3.2.1. Differential Scanning Calorimetry Polyurethane composed of a hard segment from diisocyanate and soft segment from polyols had two glass transition temperatures due to the individual segments. The glass transition temperature of the soft segment was usually observed at low temperatures, below 0 ◦C. The DSC data (Figure5) shows ◦ the Tg values of Ref PU and EPPU at −69, and −60 C, respectively. This suggests that EP-DOPO, which is more rigid than PTMG, causes the higher Tg of the EPPU soft segment. For the same reason, Tg of EPPU-2Appl. Sci. 2018 synthesized, 8, x FOR PEER by REVIEW reacting EP-DOPO polyol with IPDI was −8.8 °C (Figure S2)7 of 15

FigureFigure 5. Differential 5. Differential scanning scanning calorimetry calorimetry (DSC) (DSC) data data forfor Ref RefPU and PU EPPU. and EPPU.

3.2.2. Thermogravimetric3.2.2. Thermogravimetric Analysis Analysis Van KrevelenVan Krevelen suggested suggested that one that way one to reduceway to thereduce burning the ofburning polymers of polymers is to add charcoal-formingis to add additivescharcoal-forming like phosphor additives [16]. Lyon like developed phosphor a[16]. method Lyon fordeveloped measuring a method flame retardancyfor measuring from flame polymers that produceretardancy charcoal from withpolymers TGA that [17 ].produce The phosphor charcoal with used TGA in this [17]. studyThe phosphor for flame used retardancy in this study is known to for flame retardancy is known to react with a combustible material during combustion and form a react withcarbonized a combustible layer on material the surface during of the combustion material. In and addition, form a the carbonized phosphorous layer flame on the retardant surface of the material.produces In addition, phosphoric the phosphorous acid and polyphosphoric flame retardant acid by produces pyrolysis, phosphoric resulting in acid the andformation polyphosphoric of a acid byHPO pyrolysis,2 or PO radical resulting that instabilizes the formation the active of OH a HPOor ∙H2 radicalsor PO [18,19]. radical TGA that analysis stabilizes was the carried active OH or ·H radicalsout to evaluate [18,19 the]. TGAthermal analysis stabilities was of Ref carried PU and out EPPU to by evaluate comparing the the thermal pyrolysis stabilities temperatures of Ref PU and EPPUat 5% by and comparing 20% weight the loss pyrolysis ratios. Furthermore, temperatures flame at retardancy 5% and 20% was weightevaluated loss by ratios.measuring Furthermore, the flame retardancytotal residual was amount evaluated of material by measuring remaining afte ther totalthermal residual decomposition amount related of material to the remainingdegree of after formation of char. Plots of the weight reduction rate versus the temperature measured under a N2, thermalor decomposition air atmosphere are related displayed to the in Figures degree 6 ofand formation 7, and the ofresults char. are Plots summarized of the weight in Tables reduction 2 and rate versus3. the In temperatureFigure 6, EP-DOPO measured with a large under amount a N2 ,of or phosphorus air atmosphere showed area residual displayed amount in of Figures 11.9% at6 and7, and theover results 400 °C are under summarized nitrogen gas. in Ref Tables PU 2without and3. phosphorus In Figure6 showed, EP-DOPO a small with residual a large amount amount of of phosphorus0.2%. However, showed aEPPU residual and EPPU-2 amount have of phosphorus 11.9% at over in their 400 structures,◦C under but nitrogen their residual gas. Ref amounts PU without phosphoruswere 1.2% showed and 3.0%, a small respectively. residual amountThe results of of 0.2%. TGA However, analysis under EPPU an andoxygen EPPU-2 atmosphere have phosphorusshow that the residual amount of EP-DOPO at temperatures over 400 °C was 28.2% (Figure 7), which is higher than that measured in the N2 atmosphere (Figure 6). In addition, the residual amount of EPPU and EPPU-2, which showed a small residual amount in the N2 atmosphere, increased to 13.9% and 16.8% in the oxygen atmosphere, respectively. It was confirmed that the flame-retardant effect of phosphorus in the air atmosphere was better than that in the nitrogen atmosphere. This result agrees with a previous report which showed that the residual amount increases as the phosphorus content increases [20]. The temperatures at which EPPU lost 5% and 20% of its initial weight were as high as 16 and 58 °C compared to those of Ref PU, respectively. Since EPPU is rich in aromatic groups that improve heat resistance, the pyrolysis temperature of EPPU is higher than that of Ref PU. Furthermore, EPPU-2 having higher content of phosphorous than that of EPPU showed better thermal stability than EPPU. The high thermal stability of EPPU is useful in industrial applications, such as cables or wires. Appl. Sci. 2018, 8, 1587 7 of 14 in their structures, but their residual amounts were 1.2% and 3.0%, respectively. The results of TGA analysis under an oxygen atmosphere show that the residual amount of EP-DOPO at temperatures ◦ over 400 C was 28.2% (Figure7), which is higher than that measured in the N 2 atmosphere (Figure6). In addition, the residual amount of EPPU and EPPU-2, which showed a small residual amount in the N2 atmosphere, increased to 13.9% and 16.8% in the oxygen atmosphere, respectively. It was confirmed that the flame-retardant effect of phosphorus in the air atmosphere was better than that in the nitrogen atmosphere. This result agrees with a previous report which showed that the residual amount increases as the phosphorus content increases [20]. The temperatures at which EPPU lost 5% and 20% of its initial weight were as high as 16 and 58 ◦C compared to those of Ref PU, respectively. Since EPPU is rich in aromatic groups that improve heat resistance, the pyrolysis temperature of EPPU is higher than that of Ref PU. Furthermore, EPPU-2 having higher content of phosphorous than that of EPPU showed better thermal stability than EPPU. The high thermal stability of EPPU is useful in industrial applications, such as cables or wires.

Table 2. Thermal degradation temperatures of polyurethanes obtained with thermogravimetric analysis

(TGA) (N2).

Temp. at 5% Weight Temp. at 20% Weight N Atmosphere Char at 450 ◦C (%) 2 Loss (◦C) Loss (◦C) EP-DOPO 226 341 11.9 REF PU 152 230 0.2 EPPU 168 288 1.2 EPPU-2 207 341 3.0

Table 3. Thermal degradation temperature of polyurethanes obtained with TGA (air).

Temp at 5% Weight Loss Temp at 20% Weight Loss Air Atmosphere Char at 450 ◦C (%) (◦C) (◦C) EP-DOPO 242 337 28.2 REF PU 157 248 6.5 EPPU 164 300 13.9 EPPU-2 214 328 16.8 Appl. Sci. 2018, 8, x FOR PEER REVIEW 8 of 15

Figure 6. Thermogravimetric analysis (TGA) data for Ref PU and EPPU in N2 DOPO: Figure 6. Thermogravimetric analysis (TGA) data for Ref PU and EPPU in N2 DOPO: 10-(2,5-dihydroxyphenyl)-10H-9-oxa-10-phospha-phenanthrene-10-oxide.10-(2,5-dihydroxyphenyl)-10H-9-oxa-10-phospha-phenanthrene-10-oxide.

Figure 7. TGA data for Ref PU and EPPU in air.

Table 2. Thermal degradation temperatures of polyurethanes obtained with thermogravimetric

analysis (TGA) (N2).

Temp. at 5% Weight Temp. at 20% Weight N2 Atmosphere Char at 450 °C (%) Loss (°C) Loss (°C) EP-DOPO 226 341 11.9 Appl. Sci. 2018, 8, x FOR PEER REVIEW 8 of 15

Appl. Sci. 2018Figure, 8, 1587 6. Thermogravimetric analysis (TGA) data for Ref PU and EPPU in N2 DOPO: 8 of 14 10-(2,5-dihydroxyphenyl)-10H-9-oxa-10-phospha-phenanthrene-10-oxide.

Appl. Sci. 2018, 8, x FOR PEER REVIEW 9 of 15

REF PU 152 230 0.2 EPPU 168 288 1.2 EPPU-2 207 341 3.0

Table 3. Thermal degradation temperature of polyurethanes obtained with TGA (air).

Temp at 5% Weight Temp at 20% Weight Air Atmosphere Loss Loss Char at 450 °C (%) (°C) (°C) EP-DOPO 242 337 28.2 REF PU 157 248 6.5

EPPU 164 300 13.9 EPPU-2 FigureFigure214 7. 7.TGA TGA data data forfor Ref PU PU and and EPPU 328 EPPU in air. in air. 16.8

3.3. CharacterizationTable 2. Thermal of Epoxy degradation Compositions temperatures with Polyurethanes of polyurethanes obtained with thermogravimetric 3.3. Characterizationanalysis (TGA) of Epoxy (N2). Compositions with Polyurethanes In addition, the properties of the prepared polyurethanes as a toughener were studied by adding In addition, the propertiesTemp. ofat 5%the Weight prepared Temp.polyurethanes at 20% Weight as a toughener were studied by them to epoxyN2 Atmosphere compositions composed of an epoxy resin and a curingChar agent: at 450 dicy.°C (%) The epoxy adding them to epoxy compositionsLoss (°C)composed of an epoxyLoss (°C) resin and a curing agent: dicy. The compositions include various amounts of PU, except for EPPU-2 because it has a high viscosity; it was epoxy compositionsEP-DOPO include various226 amounts of PU, except 341 for EPPU-2 because 11.9 it has a high cured as described previously to prepare the test specimens for the measurements of the mechanical viscosity; it was cured as described previously to prepare the test specimens for the measurements andof the thermal mechanical properties. and thermal Figure 8properties.a shows that Figure epoxy 8a shows compositions that epoxy with compositions synthetic EPPU with synthetic had higher tensileEPPU strengths had higher than tensile those strengths of the epoxy than compositionsthose of the epoxy with composit Ref PU. Furthermore,ions with Ref PU. epoxy Furthermore, compositions containingepoxy compositions EPPU have highercontaining ﬂexural EPPU strength have thanhigher epoxy flexural compositions strength containingthan epoxy Ref compositions PU (Figure8 b). Thiscontaining indicates Ref that PU the (Figure rigid EPPU8b). This in the indicates epoxy compositionsthat the rigid compensatedEPPU in the forepoxy the ﬂexibilitycompositions of the overallcompensated PU and for enhanced the flexibility the mechanical of the overall strength. PU and enhanced In addition, the themechanical impact strength. resistance In of addition, the epoxy binderthe impact with 10resistance phr of EPPUof the increasedepoxy binder by ~17%with 10 compared phr of EPPU to that increased of neat by epoxy ∼17% (Figure compared8c). However,to that whenof neat more epoxy than (Figure 10 phr 8c). of However, EPPU was when included more inthan the 10 epoxy phr of compositions, EPPU was included the impact in the resistanceepoxy becamecompositions, lower than the thatimpact of theresistance epoxy withbecame Ref lower PU due than to that the increasedof the epoxy rigidity withfrom Ref PU aromatic due to groupsthe inincreased EPPU [21 rigidity]. from aromatic groups in EPPU [21].

Figure 8. Cont. Appl. Sci. 2018, 8, 1587 9 of 14 Appl. Sci. 2018, 8, x FOR PEER REVIEW 10 of 15

Figure 8. Mechanical strength data for epoxy composites including various amounts of Ref PU or EPPU: (a) tensile strength~(b) flexural strength~and (c) impact resistance. phr: parts per hundred resin. Figure 8. Mechanical strength data for epoxy composites including various amounts of Ref PU or The viscoelastic properties of each epoxy composition were measured using DMA. The storage EPPU: (a) tensile strength~ (b) flexural strength~ and (c) impact resistance. phr: parts per hundred modulus and tan δ curves are displayed in Figure9a,b, and the calculated crosslink density values areresin shown in Table4. The storage modulus ( E0) of neat epoxy exhibited the highest value, while the E0 of the epoxy compositions decreased as the amount of PU in the epoxy matrix increased. Meanwhile,The viscoelastic the storage properties moduli of each of the epoxy EPPU comp epoxyosition compositions were measured were higher using than DMA. those The of thestorage Ref modulusPU epoxy and compositions.tan δ curves are The displayed high crosslink in Figure density 9a,b, of the an EPPUd the calculated epoxy matrix crosslink can cause density this, because values are shownthe diglycidyl in Table ethers 4. The of storage EPPU canmodulus form a ( polymerE′) of neat network epoxy exhibited structure withthe highest an amine value, hardener while [ 22the]. E′ ofTo the calculate epoxy the compositions crosslink density decreased of each epoxyas the composition amount of in thePU rubberyin the plateauepoxy regionmatrix in increased. the range Meanwhile,of 150–250 the◦ C,storage the following moduli equationof the EPPU was used:epoxy compositions were higher than those of the Ref PU epoxy compositions. The high crosslink density of the EPPU epoxy matrix can cause this, E0 because the diglycidyl ethers of EPPU can Vformc = a polymer network structure with an amine(1) hardener [22]. To calculate the crosslink density of each3RT epoxy composition in the rubbery plateau 0 regionwhere in theVc rangeis the of crosslink 150–250 density, °C, theE followingis the tensile equation storage was modulus used: in the rubbery plateau, T is the temperature in K corresponding to the storage modulusE ′ value, and R is the gas constant. The calculated values for the EPPU epoxy compositions wereV=c higher than those of the Ref PU epoxy binder with(1) the same amount of polyurethanes. These results3 correspondRT to the storage modulus values listed in whereTable Vc 4is. However,the crosslink the Vdensity,c values ofE′ theis the epoxy tensile compositions storage modu with EPPUlus in alsothe decreasedrubbery plateau, as the content T is the of temperaturepolyurethane in K increased. corresponding This was to causedthe storage by the modulus decreased value, amount and of epoxyR is the resin gas forming constant. network The calculated values for the EPPU epoxy compositions were higher than those of the Ref PU epoxy binder with the same amount of polyurethanes. These results correspond to the storage modulus values listed in Table 4. However, the Vc values of the epoxy compositions with EPPU also decreased as the content of polyurethane increased. This was caused by the decreased amount of Appl. Sci. 2018, 8, 1587 10 of 14 Appl. Sci. 2018, 8, x FOR PEER REVIEW 11 of 15 polymersepoxy [23 resin,24]. forming In Figure network9b, one polymers can see [23,24]. that the In curvesFigure in9b, theone tan can δseeplot that of the the curves epoxy in compositions the tan δ tend toplot decrease of the epoxy owing compositions to the increase tend ofto thedecrease flexible ow segmenting to the ofincrease the polyurethane, of the flexible giving segment rise of tothe chain motion.polyurethane, Specifically, giving it is rise obvious to chain that motion. epoxy Specifically, binders containing it is obvious EPPU that haveepoxy higher binders glass containing transition temperaturesEPPU have than higher those glass of thetransition epoxy temperatures compositions th withan those Ref of PUs the becauseepoxy compositions both phenanthrene with Ref PUs and the benzenebecause group both of EPPUphenanthrene function and as the hard benzene segments group that of EPPU restrain function the chain as hard motion segments of PU that in therestrain polymer the chain motion of PU in the polymer matrix. In addition, parameters such as the aromatic density matrix. In addition, parameters such as the aromatic density can change the thermal properties of the can change the thermal properties of the polymer, increasing Tg or improving the degradation polymer, increasing T or improving the degradation characteristics [25]. characteristics [25].g

FigureFigure 9. ( a9.) ( Storagea) Storage moduli moduli of of epoxy epoxy compositionscompositions and and (b ()b tan) tan δ values.δ values.

The thermalTable 4. Storage stabilities moduli of theand V epoxyc of epoxy compositions compositionscontaining containing PUs. PUs phr: were parts measured per hundred using resin; TGA and observingDMA: the weight dynamic loss mechanical as a function analysis. of temperature (Figure 10). The thermal data are collected in Table5. For a weight loss of 5%, theNeat thermal EPPU degradation EPPU temperatureEPPU Ref PU ( T 5%Ref) decreased PU Ref proportionallyPU to the content of polyurethane in the epoxy binder, indicating reduced thermal stability. Based on the values at T5%, and T10%, the epoxy with 10 phr of Ref PU had the highest temperature. This is thought to be related to the morphology of PU in the cured epoxy matrix, as shown in Figure 11, which shows Appl. Sci. 2018, 8, 1587 11 of 14 the scanning electron microscopy (SEM) images of each epoxy sample. Figure 11b1 shows a large amount of phase-separated PUs in the epoxy binder with 10 phr of Ref PU. The thermal degradation of the volatile segments of the phase-separated PUs was delayed by the surrounding epoxy matrix and resultedAppl. inSci. the 2018 highest, 8, x FOR PEER thermal REVIEW degradation temperature. Neat epoxy and epoxy compositions12 of 15 with PUs exhibited similar degradation temperatures within a ±2% range. This indicates that the content of the flame-retardant phosphor wasEpoxy too low10 tophr provide 20 phr flame 30 phr retardancy. 10 phr 20 phr 30 phr SurfaceR images (L∙MPa of∙K− neat1∙mol resin−1) (Figure 11a1,a2) showed0.008314 mainly smooth surfaces with partial wavy Storage modulus, E′ patterns. However, an epoxy composition1.1 1.1 with 100.965 phr of0.873 Ref PU (Figure0.946 110.908b1) or EPPU0.811 (Figure 11cl) (MPa at 180 °C) showed numerous holes resulting from impact-detached PUs. Other images, including those with >10 Tg (°C, with DMA) 160.8 153.6 142.8 138.2 146.8 136.7 129.4 phr of polyurethanes, did not show phase-separated polyurethanes but had rough surfaces as the PU Vc (mol/L) 0.097 0.097 0.085 0.077 0.084 0.080 0.072 content increased.The thermal This stabilities suggests of the that epoxy the high compositions polyurethane containing content PUs inwere the measured epoxy matrix using promoted TGA flexibility,and observing compensating the weight for the loss embrittlement as a function of theof temperature cured epoxy (Figure product, 10). but The also thermal lowered data the are tensile and flexuralcollected strengths in Table 5. of For the a epoxy. weight loss of 5%, the thermal degradation temperature (T5%) decreased proportionally to the content of polyurethane in the epoxy binder, indicating reduced thermal Tablestability. 4. Storage Based modulion the values and Vc atof epoxyT5%, and compositions T10%, the epoxy containing with 10 PUs. phr phr: of Ref parts PU per had hundred the highest resin; DMA:temperature. dynamic This mechanical is thought analysis. to be related to the morphology of PU in the cured epoxy matrix, as shown in Figure 11, which shows the scanning electron microscopy (SEM) images of each epoxy sample. Figure 11b1 shows a large amountNeat ofEPPU phase-separatedEPPU EPPUPUs in theRef epoxy PU binderRef PU with 10Ref phr PU Epoxy 10 phr 20 phr 30 phr 10 phr 20 phr 30 phr of Ref PU. The thermal degradation of the volatile segments of the phase-separated PUs was R (L·MPa·K−1·mol−1) 0.008314 delayed by the surrounding epoxy matrix and resulted in the highest thermal degradation Storage modulus, E0 (MPa at 180 ◦C) 1.1 1.1 0.965 0.873 0.946 0.908 0.811 temperature.◦ Neat epoxy and epoxy compositions with PUs exhibited similar degradation Tg ( C, with DMA) 160.8 153.6 142.8 138.2 146.8 136.7 129.4 temperaturesVc (mol/L) within a ±2% range. This 0.097 indicates 0.097 that 0.085the content 0.077 of the 0.084flame-retardant 0.080 phosphor 0.072 was too low to provide flame retardancy.

Table 5. Thermogravimetric analysis data at T5% and T10% and remaining contents. Table 5. Thermogravimetric analysis data at T5% and T10% and remaining contents.

Neat NeatRef PURef PURef Ref PU PU RefRef PU PU EPPUEPPU EPPUEPPU EPPU EPPU Epoxy 10 phr 20 phr 30 phr 10 phr 20 phr 30 phr Epoxy 10 phr 20 phr 30 phr 10 phr 20 phr 30 phr T5% T5% 327.5327.5 334.2 334.2 323.5 323.5 323.4 323.4 325.2 325.2 323.2 323.2321.7 321.7 T10% 346.6 353.5 346.1 347.2 345.8 345 345.4 ◦ T10% 346.6 353.5 346.1 347.2 345.8 345 345.4 Char at 600CharC at (%) 600 °C 11.4(%) 11.4 12.2 12.2 8.9 8.9 8.1 8.1 10.2 10.2 9.4 9.4 9.0 9.0

FigureFigure 10. 10.TGA TGA curves curves for forepoxy epoxy compositions including including PUs. PUs.

Surface images of neat resin (Figure 11a1,a2) showed mainly smooth surfaces with partial wavy patterns. However, an epoxy composition with 10 phr of Ref PU (Figure 11b1) or EPPU (Figure 11cl) showed numerous holes resulting from impact-detached PUs. Other images, including those with >10 phr of polyurethanes, did not show phase-separated polyurethanes but had rough Appl. Sci. 2018, 8, x FOR PEER REVIEW 13 of 15 surfaces as the PU content increased. This suggests that the high polyurethane content in the epoxy matrixAppl. Sci. promoted2018, 8, 1587 flexibility, compensating for the embrittlement of the cured epoxy product,12 ofbut 14 also lowered the tensile and flexural strengths of the epoxy.

Figure 11. Scanning electron microscopy (SEM) images of epoxy compositions containing Ref PU PU or × EPPU: ((a1a1)) neatneat epoxy epoxy at at 5000 5000×× magniﬁcation, magnification, (a2 )(a2 neat) neat epoxy epoxy at 10,000 at 10,000× magniﬁcation; magnification; (b1–b3 (b1) epoxy–b3) epoxycompositions compositions with 10, with 20, and10, 20, 30 phrand Ref 30 PU;phr (Refc1– c3PU;) epoxy (c1–c3 compositions) epoxy compositions with 10, 20, with and 10, 30 20, phr and EPPU. 30 phr EPPU. Appl. Sci. 2018, 8, 1587 13 of 14

4. Conclusions In this study, a flame-retardant polyol (EP-DOPO) was prepared and characterized with 1H-NMR, electron spray ionization mass spectrometry (ESI-MS), and elemental analysis. In addition, flame retardant polyurethanes (EPPU, and EPPU-2) were prepared by reacting the blend of EP-DOPO and PolyTHF polyol with IPDI. The physical properties of the EPPUs were compared with polyurethanes composed of PolyTHF and IPDI (Ref PU). DSC data showed that EPPU, having a rigid aromatic group, has the disadvantage of having a higher Tg, due to its soft segment, compared to that of Ref PU. However, EPPU exhibited a higher thermal degradation temperature than Ref PU, as revealed by TGA. Therefore, these results suggest that EPPU itself can be useful for industrial applications requiring high thermal stability, such as wires and cables. Furthermore, the effect of polyurethane in the epoxy binder as a toughener was investigated in terms of mechanical and thermal properties. The tensile strengths of the epoxy compositions with EPPU and Ref-PU were similar, but the EPPU-epoxy compositions had higher flexural strengths than the Ref PU–epoxy compositions, which is beneficial for preparing high-performance epoxy composites. Moreover, EPPU–epoxy compositions exhibited higher impact resistance than neat epoxy, though epoxy compositions containing 20 and 30 phr had lower values than those of epoxy compositions with Ref PU. Furthermore, the viscoelastic properties of the epoxy compositions with various amounts of EPPU were analyzed using DMA. According to the results, the epoxy polymer with 10 phr of EPPU has higher tan δ value than that of the cured epoxy with 10 phr of Ref PU. Therefore, EPPU is useful for increasing the glass transition temperature (Tg) of the polymer. Moreover, the calculated crosslink density values of the compositions based on the DMA results were also found to increase proportionally to the values of the storage modulus.

Supplementary Materials: The following are available online at http://www.mdpi.com/2076-3417/8/9/1587/s1, Figure S1: GPC data of EPPU-2, intensity vs retention time (min), Figure S2: DSC data for EPPU-2. Author Contributions: J.H.K., and T.K. carried out experiments. W.L., and H.-G.K. analyzed the data. B.S. designed the experiment. C.-S.L., and I.W.C. wrote the paper. Funding: This research was supported by the Korea Research Institute of Chemical Technology (No. SI1809), and by the Technology Innovation Program (10063520) funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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