Preparation and Properties of Castor Oil-Based Dual Cross-Linked

Research Article pubs.acs.org/journal/ascecg

Preparation and Properties of Castor Oil-Based Dual Cross-Linked Polymer Networks with Polyurethane and Polyoxazolidinone Structures Mei Li,†,‡ Jianling Xia,†,‡ Wei Mao,† Xuejuan Yang,† Lina Xu,† Kun Huang,†,‡ and Shouhai Li*,†,‡ †

Institute of Chemical Industry of Forestry Products, CAF, Key Laboratory of Biomass Energy and Material, National Engineering Laboratory for Biomass Chemical Utilization, Key and Laboratory on Forest Chemical Engineering, SFA, Nanjing 210042, Jiangsu Province, China ‡ Institute of Forest New Technology, CAF, Beijing 10091, China ABSTRACT: Dual cross-linked polymer networks (DCPNs) are novel copolymers containing two different cross-linking polymers with superior comprehensive properties. The aim of this study was to explore the feasibility of preparation of novel castor oil-based DCPNs material. First, two novel biobased reactive monomers of epoxidized castor oil (ECO) and ricinoleic acid epoxy (RAE), containing both hydroxy and epoxy groups, were synthesized from castor oil (CO). Their chemical structures were characterized by Fourier transforminfrared spectroscopy (FTIR) and 1H-nuclear magnetic resonance (1H NMR) analysis. Both hydroxy and epoxy groups can react with the isocyanate groups of toluene-2,4diisocyanate (TDI), and then two renewable resource-based DCPNs of RAE/TDI and ECO/TDI, containing both polyurethane (PU) and polyoxazolidinone (POXDN) molecular structures, were designed and prepared. The tensile properties, gel content (GC), hardness, fractured surface morphologies, thermostabilities, and dynamic mechanical and thermal properties of these renewable resource-based copolymers were all investigated. Results show that the tensile properties, GC, hardness, and glass transition temperature (Tg) of the fabricated DCPNs were superior to those of the pure CO/TDI copolymerized system. The fabricated DCPNs, particularly RAE/TDI, were found to have good tensile strength of 12.47 MPa and the highest elongation at break of 154.99%, and its thermostability was also increased greatly compared with that of the pure CO/TDI copolymerized system. KEYWORDS: Castor oil, Dual cross-linked polymer networks, Combination of polyurethane and polyoxazolidinone structures, Improved comprehensive properties, Renewable resources

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INTRODUCTION Cross-linked polymers play an important role in industry due to their durability, chemical stability, and thermal resistances. They can be prepared via ultraviolet (UV), heat, or moisture curing process.1−3 However, lacking performance and low cross-linking density might limit their wider applicability as provided by single curing way. To extend to more highperformance industrial applications, many novel compositing cross-linked polymers networks have been explored, such as interpenetrating polymer networks (IPNs) and dual cured polymer networks. Interpenetrating polymer networks are blends of two or more cross-linked polymers.4,5 The tight interpenetration of these networks ensures high GC, high cross-linking density, and broad formulation. Cured IPN materials might have superior mechanical properties, heat resistance, electrical properties, and dimensional stability. Thus, IPN polymers offer improved properties over their components.6−11 In the recent 2 decades, considerable efforts have been made to design and fabricate © 2017 American Chemical Society

many novel IPN materials, such as natural rubber/polystyrene (NR/PS), polyurethane/epoxy resin (PU/EP), and vinyl ester resin/epoxy resin (VER/EP), poly(methyl methacrylate)/ poly(ethylene oxide) (PMMA/PEO), polydivinylbenzene/ polyacryldiethylenetriamine (PDVB/PADETA), hydroxytelechelic polybutadiene/poly(ethylene oxide) (HTPB/PEO), hydroxytelechelic polybutadiene/polystyrene (HTPB/PS) IPNs. Different IPN systems have particular properties due to their different components.12−15 PMMA/PEO IPNs could represent an interesting medium as solid polymer electrolytes (SPEs) in practical electrochemical devices due to their excellent dimensional stability.9 PU/EP IPNs could be selected as the matrix of magnetorheological elastomer (MRE) material due to their superior compatibility with carbonyl iron particles (CIPs).16 NR/PS IPNs exhibit excellent impact resistance, Received: April 11, 2017 Revised: June 16, 2017 Published: June 24, 2017 6883

DOI: 10.1021/acssuschemeng.7b01103 ACS Sustainable Chem. Eng. 2017, 5, 6883−6893

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. Molecular structure of castor oil and its common derivatives.

preparation of plasticizers, elastomers, biolubricants, coatings, and other functional resins.46−49 At present, scientific research on CO in the production of functional resins is still focused on exploring novel pure polyurethane polymers. Many studies have been reported on the use of CO-resources in polyurethane resin fields.50−54 However, the mechanical properties of CO-based polyurethane polymers are very poor due to the low density of cross-linking in polymer material.52,54 Dual cross-linking technology could offer an effective means to explore novel CO-based materials with excellent comprehensive properties. Few studies of renewable resource-based dual cross-linked materials have been reported in the previous literature. Futhermore, no CO-based dual cross-linked materials have been reported. The design of CO-based dual cross-linked polymer networks (DCPNs) materials is the objective of the present study. In this study, the combined structures of PU and POXDN cross-linking systems could endow prepared DCPNs with comprehensive properties superior to those of the parent materials. We report an “epoxidation, epoxy ring-opening, and controllable ring-closing” strategy to prepare CO-based reactive monomers. The fabrication of CO-based DCPNs was used to verify the polymerizability of these two copolymerized systems with one-step heating method. The molecular structure and comprehensive properties of the copolymerized systems were also investigated. Prepared DCPNs could exhibit superior mechanical properties and excellent thermal stabilities, allowing them to be widely used for making composites matrix, adhesives, sealants, membranes, and coating materials. And these materials are promising for applications in automotive industry, marine industry, military affairs, sports, pharmacy, construction, and other industries. As a renewable, eco-friendly, chemical feedstock, the use of CO in DCPNs preparation would reduce fossil fuel inputs and environmental impacts. Our study also can provide guidance for the design and fabrication of this kind of material in some degree.

which can be used successfully in automobile components, footwears, various molded articles, etc.17−19 Dual cured polymer networks are an overlapping combination of two or more cross-linked polymers. Commonly, dual curing systems combine UV irradiation and other curing ways, such as heat, microwave, or moisture.20−22 Dual curing resins possess different cross-linkable functional groups, and different cross-linking systems have been formed in different curing process. After exposure to UV light, such a cross-linked polyacrylate (PA) network has been first formed, and then an enhanced dual cross-linked polymer networks (DCPNs) were successfully fabricated after being heated or other curing ways. As the coating materials, dual cured materials have excellent heat resistance, moisture resistance, chemical stability, and dimensional stability.23−25 Dual curing technology has been explored to address the issue of insufficient UV curing on the shadow area of coated three-dimensional objects as well as to achieve a deep-through curing of thick pigmented coatings, and many dual curing systems have been designed and fabricated, such as PA/EP, PA/PU, PA/polysiloxane copolymerized systems.26−28 IPNs and dual cured polymer networks are the most efficient materials for the manufacture of reinforced rubbers, toughened plastics, chemical coatings, and functional materials due to their perfect combination of cross-linked polymers. IPN and dual curing technologies are proven advisible strategy for the design and fabrication of two or more polymer networks with different properties. However, IPNs are only the simple physical entanglement of different polymers, and no chemical actions exist among the mixed cross-linking polymers. Thus, there could appears a strong unnecessary phase-separation behavior especially between the hydrophobic polymer networks and the hydrophilic polymer networks. In addition, conventional dual curing technologies are only aimed at preparing coating materials. On the other hand, fossil fuel depletion and environmental pollution have brought increased attention to explore renewable resource-based materials.29−32 Renewable resources-based materials can offer many advantages, such as low cost, reduced environmental impact, and potential biodegradability.33−42 As an agricultural product, CO is a low cost, high output material. Obtained from the seeds of the castor plant, CO is a light yellow liquid used to produce many derivatives through familiar organic reactions (Figure 1). The functional groups of CO include hydroxyl, carboxyl, and unsaturated double bond groups.43−45 These functional groups can be epoxidized, esterified, and hydrogenated. The hydrolysis product of CO, ricinoleic acid, is a superior chemical feedstock for the

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EXPERIMENTAL SECTION

Chemicals. Castor oil (stabilized, 98.5%, hydroxyl value: 0.290 mol/100 g) was purchased from Shanghai Lingfeng Chemical Reagent Co., Ltd., China and used as received. H2O2 (stabilized, 30.0%) and ptoluenesulfonic acid (stabilized, 98.5%) were purchased from Nanjing Chemical Reagent Co., Ltd., China and used as received. Toluene-2,4diisocyanate (TDI) (stabilized, 98.5%) and ricinoleic acid (stabilized, 97.0%) were purchased from Aladdin Chemicals Co., Ltd., China. Ethyl acetate (stabilized, 99.5%). Benzyltrimethylammonium bromide (stabilized, 99.5%), sodium hydroxide (stabilized, 99.5%), and calcium oxide (stabilized, 99.5%, AR) were purchased from Sinopharm Chemical Reagent Co., Ltd., China and used as received. Formic 6884


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ACS Sustainable Chemistry & Engineering acid (stabilized, 88.0%) and anhydrous MgSO4 (stabilized, 99.5%) were purchased from Xilong Chemical Industry Co., Ltd., China and used as received. Preparation of CO-Based Reactive Monomers. Preparation of Epoxidized Castor Oil (ECO). To synthesize ECO, 93.20 g (0.10 mol) of CO, 21.90 g (0.32 mol) of formic acid, and 1.524 g of p-toluene sulfonic acid were added to a 250 mL round-bottomed flask and heated to 50 °C. Using a dropping funnel, 113.33 g (1.00 mol) of H2O2 was gradually added to the flask. The mixture was slowly heated to 65 °C and continued for 5.0 h. Next, 100.00 g of ethyl acetate and 50.00 g of H2O were added to the reacting system. The reaction was continued for 10 min before separating the oil layer using a separatory funnel. The separated oil layer was washed 3 times with water then dried using anhydrous MgSO4. Finally, ethyl acetate was removed by vacuum distillation to obtain ECO, a light yellow, highly viscous liquid (hydroxyl value, 0.276 mol/100g; epoxy value, 0.271 mol/100g; theoretical epoxy value, 0.297 mol/100g). The synthetic route is shown in Figure 2.

Table 1. Formula of Different Copolymerized Systems Sample

CO (g)

CO/TDI RAE/TDI ECO/TDI

10.00

RAE (g)

ECO (g)

TDI (g)

10.00

2.52 4.88 4.76

10.00

denoted as follows: CO/TDI, a copolymer with a theoretical reacting weight ratio of CO and TDI; ECO/TDI, an DCPN with a theoretical reacting weight ratio of ECO and TDI; and RAE/TDI, an DCPN with a theoretical reacting weight ratio of RAE and TDI. Characterizations. 1H Nuclear Magnetic Resonance (1H NMR) Analysis. 1H NMR (300 MHz) spectra were recorded on an ARX300 spectrometer (Bruker, Germany). The chemical shifts relative to that of deuterated chloroform (d = 7.26) was recorded. Fourier Transform Infrared (FTIR) Analysis. FTIR analysis was performed an IS10 spectrometer (Nicolet, USA) and attenuated total reflectance. Samples were analyzed as liquid, powder or films on a ZnSe window. Each sample was scanned from 4000 to 400 cm−1. Shore Hardness Analysis. The hardness of DCPNs and copolymer materials were all measured using TH 210 Shore D durometer (TIME, China) at 25 °C. Gel Content (GC) Analysis. The GC of copolymerized systems was tested by the Soxhlet extraction.55 First, 0.5 g of the copolymer was cut into ∼2.5 mm × 5.0 mm × 0.2 mm pieces. The pieces were wrapped in filter paper and introduced into a 250 mL elution device equipped with a reflux condenser. The sample was then eluted for 24 h. After elution, remaining monomer and linear chain micromolecule components were all washed away, and then the samples were ovendried to a constant weight. In this experiment, each sample was tested three times for the accuracy. The GC was calculated as eq 1:

Figure 2. Synthetic route scheme of ECO. Preparation of Ricinoleic Acid Epoxy (RAE). To synthesize RAE, 149.23 g (0.50 mol) of ricinoleic acid, 462.5 g (5.00 mol) of epichlorohydrin, and 2.76 g (0.008 mol) of benzyltrimethylammonium bromide were added to a 1000 mL flask equipped with a magnetic stirrer, a thermometer, and a reflux condenser. The mixture was heated to 100 °C, and the reaction was continued at this temperature for 3 h. The mixture was cooled to 60 °C before the addition of 20.00 g (0.50 mol) of sodium hydroxide and 28.00 g (0.50 mol) of calciumoxide under stirring at 60 °C for 6 h. The product was filtered using a funnel paved with silicone or diatomaceous earth and the filtrate was collected. The excess epichlorohydrin was distilled via rotary vacuum evaporation to obtain RAE, a light orange low viscosity liquid (hydroxyl value, 0.282 mol/100 g; epoxy value, 0.279 mol/100 g; theoretical epoxy value, 0.282 mol/100 g). The typical route of RAE synthesis is shown in Figure 3. Preparation of DCPN Copolymers. Three copolymers were prepared according to the weight ratios of CO, ECO, and RAE to TDI. Predetermined amounts of the mixture were mixed well with a magnetic stirrer until visual uniformity was obtained. Cured polymer products were prepared by casting the above mixture into a plane polytetrafluoroethylene mold, curing at 150 °C for 2 h, and postcuring at 180 °C for another 2 h. To prevent inhibition (the selfcopolymerization of TDI), the mixture must be rapidly heated to curing temperature(150 °C). Detailed data on the mixing weight ratios of the copolymerized systems are listed in Table 1. Figure 4 shows the reacting mechanism of prepared RAE- and ECO-based DCPNs. The copolymers were

gel content (GC) =

(M3 − M1 + M i) × 100% (M 2 − M1)

(1)

Where Mi is the weight loss of filter paper (g); M1 is original weight of filter paper (g); M2 is weight of wrapping paper (g); M3 is weight of wrapping paper after eluting (g). Tensile Property Tests. The liquid mixtures were transformed into the desired solid forms for testing in special molds according to the method described in Preparation of DCPN Copolymers. Tensile properties were measured in accordance with the GB/T 1040.3-2006 standard type II using a CMT4303 universal test machine (SANS, China). The tensile test speed was 10 mm/min. The test region was 0.8 mm thick, 6.0 mm wide, and 25.0 mm long. Tensile strength, Young’s modulus, and elongation at break were measured. For the sake of accuracy, five replicates were measured and the average values were obtained. All the samples were tested at 25 °C. Scanning Electron Microscopy (SEM) Analysis. The liquid nitrogen fracture surface of each sample was coated with a thin layer of gold using a high-vacuum gold sputter at low voltage. The micrographs were observed by magnifying 1000 and 2000 times with an S-3400N scanning electron microscope (Hitachi, Japan) under conventional secondary electron imaging conditions at an accelerating voltage of 20 kV. Dynamic Mechanical Thermal Analysis (DMA). The storage modulus (E′), and loss factor (tan δ) were measured by a Q800 dynamic mechanical thermal analyzer (TA, America) in tensile test

Figure 3. Synthetic route scheme of RAE. 6885


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Figure 4. Reacting mechanism of prepared RAE- and ECO-based DCPNs. mode. All samples had a dimension of 50 mm × 5 mm × 0.4 mm, and were tested from −70 to +100 °C at a heating rate of 3 °C/min and a frequency of 1 Hz. Thermogravimetric Analysis (TGA). Thermogravimetric analysis (TGA) was performed on a 409PC thermogravimetric analyzer (Netzsch, Germany). Each sample was tested from 40 to 600 °C at a heating rate of 15 °C/min under a nitrogen atmosphere.

stretching absorption.51 This peak indicates the existence of a hydrogen bond system among the molecules of the CO-based polymers. The peaks at 2923 and 2853 cm−1 are respectively assigned to the antisymmetric and symmetric stretching vibration absorptions of CH in CH3 and CH2. The peak at about 1400 cm −1 represents the bending vibration of CH, whereas the weak peak at about 3010 cm −1 represents the stretching vibration absorption of CH in the CHCH of fatty acid chains.43 The ECO spectrum clearly shows two absorption peaks emerging at about 980 and 870 cm−1 that correspond to epoxy group vibrations. Two additional absorption peaks at 1375 and 1162 cm−1 correspond to the asymmetric vibrations of C OC on the epoxy group. In addition, the CC stretching vibration absorption peak at 1648 cm−1 once appeared on the CO spectrum is disappeared. The above findings indicate a C C epoxidation reaction. In RAE, two clear absorption peaks were observed at 909 and 850 cm−1 corresponding to the vibrations of the terminal epoxy group. Peaks at 1194.3 and 1168.5 cm−1 correspond to the overlapped stretching vibrations of the COC of the ester and epoxy groups. In addition, that strong sharp peak at 1738 cm −1 corresponds to the CO of the ester group, and the peak at 1708 cm−1 corresponding to CO of the carboxyl group once appeared on the FTIR spectrum of RA is nearly disappeared. Additionally, the disappearance of an overlapped wide absorption peak at 3500−2500 cm−1 corresponding to the OH of the carboxyl group in the association state on the RA spectrum further confirms successful RAE synthesis. The FTIR analysis indicates that the two target products were successfully synthesized. 1 H NMR Analysis. The 1H NMR spectra of CO, RA, ECO, and RAE are shown in Figure 6, and their representive 1H chemical shift values are also listed in Table 3. Compared with other vegetable oils, the CO curve (Figure 6a) exhibits

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RESULTS AND DISCUSSION FTIR Analysis of Reactive Monomers. The FTIR spectra of CO, RA, ECO, and RAE are shown in Figure 5, and corresponding representative absorption bands are listed in Table 2. The weak absorption peaks ranged from 3750 to 3250 cm−1 on all spectra in the assigned frequency for hydroxyl

Figure 5. FTIR spectra of raw materials and prepared reactive monomers. 6886


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ACS Sustainable Chemistry & Engineering Table 2. Representative FTIR Absorption Band Values for CO, RA, ECO, and RAE CO, RA, ECO, RAE

ECO CO RAE

RA

Absorption bands (cm−1)

Functionality

3750−3250 3010 2923 2853 1400 980, 870 1375, 1162 1648 909, 850 1194, 1168 1738 1708 3500−2500

The hydroxyl stretching absorption stretching vibration absorption of CH in the CHCH of fatty acid chains The antisymmetric stretching vibration absorptions of CH in CH3 and CH2 The symmetric stretching vibration absorptions of CH in CH3 and CH2 The bending vibration of CH The vibrations of epoxy group The asymmetric vibrations of COC on the epoxy group The stretching vibration of CC group The vibrations of the terminal epoxy group The overlapped stretching vibrations of the COC of the ester and epoxy groups The CO of the ester group The CO of the carboxyl group The −OH of the carboxyl group in the association state

Table 3. Representive 1H Chemical Shift Values for CO, RA, ECO, and RAE Chemical shift (ppm) CO, RA, RAE

RA RAE

CO, ECO ECO

Protons

Mult

5.4−5.6

−CH=

m

3.6 2.2 2.1 1.9 1.6 1.4 1.2 0.8 7.0 4.4, 3.8 3.4 3.1 2.8, 2.5 2.3 5.3

Methine proton adjacent to −OH −CH2− adjacent to −CH= and −CH−OH −OH −CH2− adjacent to −CH= only −CH2− adjacent to −CH2−COO− −CH2− adjacent to −CH2−CH= Protons on other −CH2− −CH3 −COOH −COO−CH2− Methine proton adjacent to −OH Methine proton on epoxy group −CH2− on epoxy group −CH2−COO− Methine proton on glycerol moiety

s m d m m m m m s m s m m t m

4.1−4.3 5.2 3.6 2.8 2.2

−CH2− on glycerol moiety Methine proton on glycerol moiety Methine proton adjacent to −OH Methine proton on epoxy group −OH, −CH2− adjacent to epoxy group and −CHOH, −CH2−COO− (overlapped) −CH2− only adjacent to epoxy group −CH2− adjacent to −CH2−COO Protons on other −CH2− groups −CH3

m m s m m

1.8 1.6 1.2, 1.3 0.8

m m m m

about 2.2 ppm are associated with the methylene proton between the carbon connected to the hydroxyl group and the carbon of the −CHCH− group. The peaks at about 1.9 ppm are assigned to another methylene proton next to the −CH CH− group. The 1H NMR spectrum of ECO clearly shows the emergence of overlapped peaks at about 2.8 ppm corresponding to the chemical shift of the proton on the epoxy group. In addition, overlapped peaks appeared at 5.5, 2.2, 1.9, and 3.6 ppm on the 1H NMR spectrum of CO before nearly disappearing. These findings demonstrate that the −CH CH− group on the CO molecule was epoxidized.

Figure 6. 1H NMR spectra of raw materials and prepared reactive monomers.

characteristic peaks at 3.6 and 2.1 ppm corresponding to the protons adjacent to and on the hydroxyl groups, respectively. The overlapped peaks at about 5.5 ppm are assigned to the methine protons of the −CHCH− group. The peaks at 6887


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ACS Sustainable Chemistry & Engineering The RA 1H NMR spectrum (Figure 6b) shows weak characteristic peaks at 7.0 ppm corresponding to the proton on the carboxyl group. The RAE curve has characteristic peaks at 3.8 and 4.4 ppm corresponding to the methylene proton next to the ester group. Two more peaks, at 2.5 and 2.8 ppm, correspond to the methylene proton on the epoxy group. The peak at 3.1 ppm corresponds to the methine proton from the epichlorohydrin moiety. Finally, the peak at 7.0 ppm corresponds to the proton on the carboxyl group on the RA 1 H NMR spectrum that appeared then disappeared. The above findings indicate a successful synthesis of the RAE reactive monomer. The 1H NMR analysis demonstrates that the two target products, ECO and RAE, have been successfully synthesized. The obvious characteristic peaks indicate high product purity. FTIR Analysis of Different Copolymerized Systems. In all of the samples, the FTIR spectra (Figure 7) and the

antisymmetric and symmetric stretching vibrations of the C− H bonds of −CH3 and −CH2−, respectively. In the unreacted CAE/TDI and ECO/TDI mixing systems, a strong peak at 2260 cm−1 was attributed to the stretching vibration of the −NCO group. A wide, weak absorption peak at 3400 cm−1 was assigned to hydroxyl stretching absorption. In the FTIR spectra of the DCPNs (CAE/TDI and ECO/TDI), the stretching vibration peak of the −NC O group at 2260 cm−1 had almost disappeared, whereas the intensity of the characteristic peak at 1666 cm−1, corresponding to the CO of the ester groups, had obviously increased. It was demonstrated that nearly all of the −NCO groups reacted with epoxy groups. Furthermore, the weak peak at 3300 cm−1 was assigned primarily to the newly formed hydrogen bond system produced by the −NH groups, indicating the formation of a urethane molecular structure. A characteristic peak appeared at 870 cm−1 and then disappeared, this peak corresponded to vibrations of the epoxy group that appeared on the spectra of the unreacted CAE/TDI and ECO/TDI mixing systems. Overlapping peaks at 1158 and 1082 cm−1 corresponding to the antisymmetric and symmetric stretching vibration absorptions of the COC of ester groups further confirmed ring-opening and esterification. The above findings indicate the formation of DCPNs containing urethane and oxazolidinone groups in these two copolymerized systems. Morphologies of the Different Copolymerized Systems. Microphotographs of the fractured surfaces of the copolymerized systems photographed by SEM at 1000, 2000, and 5000 times magnification are shown in Figure 8. No obvious holes or cracks were observed on the fractured surfaces, indicating that the copolymerized systems formed perfect crosslinked structures. Microphotographs of liquid nitrogen fracture surface of the CO/TDI curing systems showed relatively glossy and smooth surfaces with distinctive fast brittle fractures, indicating poor mechanical properties.56 However, in the CAE/ TDI and ECO/TDI DCPN curing systems, plastic deformation was observed under the condition of liquid nitrogen, indicating these two DCPN curing systems have excellent mechanical properties, in accordance with the analysis of mechanical properties. Under the conditions of necessary high temperature, the epoxy and hydroxyl groups of RAE or ECO both reacted with the −NCO group of TDI. The molecular chains in each of the reactive monomers, thus, could covalently link with one another at the cross-linking point to form two cross-linking systems. Compared with the pure CO/TDI copolymerized system, the CAE/TDI and ECO/TDI DCPNs systems both contained polyurethane and polyoxazolidinone two cross-

Figure 7. FTIR spectra of unreacted mixing systems and fabricated DCPNs.

corresponding representative absorption bands (Table 4) of the unreacted CAE/TDI and ECO/TDI mixtures and the CAE/ TDI and ECO/TDI DCPNs clearly show four absorption peaks at 1602, 1583, 1498, and 1452 cm−1 corresponding to the vibrations of the outer benzene ring hydrocarbon plane. Strong absorption peaks at 758 and 704 cm−1 verify the hydrocarbon plane bending vibration of the benzene ring. Intense absorption peaks at about 2900 and 2850 cm −1 correspond to

Table 4. Representative IR Absorption Band Values for of the Unreacted CAE/TDI and ECO/TDI Mixtures and the CAE/TDI and ECO/TDI DCPNs Absorption bands (cm−1) All sample

Unreacted mixtures

CAE/TDI and ECO/TDI DCPNs

1602, 1583, 1498, 1452 758, 704 2900 2850 2260 3400 870 1666 3300 1158, 1082

Functionality The The The The The The The The The The 6888

vibrations of the outer benzene ring hydrocarbon plane hydrocarbon plane bending vibration of the benzene ring antisymmetric stretching vibration absorptions of CH in CH3 and CH2 symmetric stretching vibration absorptions of CH in CH3 and CH2 vibrations of epoxy group hydroxyl stretching absorption vibrations of the epoxy group CO of the ester groups newly formed hydrogen bond system produced by the − NH groups stretching vibration absorptions of the COC of ester groups DOI: 10.1021/acssuschemeng.7b01103 ACS Sustainable Chem. Eng. 2017, 5, 6883−6893

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Table 5. Detailed Data of Gel Content and Shore Hardness in Different Copolymerized Systems Sample

Gel content (%)

Shore hardness (HD)


92.96 ± 0.51 94.67 ± 0.94 97.11 ± 0.81

10.2 ± 0.2 38.1 ± 0.4 69.3 ± 1.1

ethanes and polyoxazolidinones greatly increase GC and the density of cross-linking. Increased cross-linking density improved shore hardness in the copolymerized systems. In the same way, the introduction of a rigid benzene ring and oxazolidinone groups also improved hardness. The ECO/TDI DCPN has a bigger ECO molecule polymerized segment with higher functionality and greater cross-linking density than the RAE/TDI DCPN, and thus ECO/TDI DCPN had higher shore hardness. This is in agreement with the tensile properties analysis. Tensile Properties of Copolymerized Systems. Figure 10 diplays the tensile stress−strain testing curves in different

Figure 10. Tensile stress−strain curves of different copolymerized systems. Figure 8. SEM microphotographs of liquid nitrogen fracture surface of different copolymerized systems.

copolymerized system. The stress−strain curve of the ECO/ TDI copolymerized system (Figure 10) shows rigid tensile behavior with no yield stress point, a characteristic of rigid materials. The copolymerized system also had the highest tensile strength (22.87 MPa) and the lowest elongation at break(6.04%). In the CO/TDI copolymerized system, the tensile stress−strain curve indicated the lowest tensile strength (0.26 MPa) and a median elongation at break (35.13%). The tensile strength of thermosetting materials is determined by the cross-linked state and the chemical structure of copolymer monomers.56 The cross-linked state and chemical structures in the different copolymerized systems are varied. A more rigid bridge−ring chemical structure and a higher cross-linking density yield higher tensile strength and lower breaking elongation in the copolymers. The two factors jointly influence the tensile properties of the copolymers. The RAE/TDI system had relatively higher tensile strength (12.47 MPa) and the highest elongation at break (154.99%), indicating its excellent toughness. Compared with ECO/TDI copolymerized system, the RAE/TDI copolymerized system had an equivalent content of tough fatty chains, rigid benzene rings, and oxazolidinone groups, in agreement with the relevant data in Table 1. The

linking structures. A possible cross-linked diagram is shown in Figure 9. Shore Hardness and GC Analysis. The shore hardness and GC of RAE/TDI and ECO/TDI are higher than that of the pure CO/TDI copolymerized system (Table 5). For these DCPN copolymerized systems, the combination of polyur-

Figure 9. Possible cross-linked diagram of different copolymerized systems. 6889


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structure begins to relax gradually and above 50 °C, all E′ values of the copolymerized systems are close to constantbelow 100 MPa. ECO/TDI had the highest E′ of the three copolymerized systems, whereas CO/TDI copolymerized system had the lowest. It is well-known that the mechanical and thermal properties of thermosetting materials are sensitive to the chemical structure and nature of copolymerized systems. As the number of rigid groups and the density of cross-linking increase in copolymerized systems, so does rigidity. The CO/TDI copolymerized system had fewer rigid benzene rings and oxazolidinone groups than the ECO/TDI and RAE/ TDI DCPNs. In addition, the ECO/TDI and RAE/TDI DCPNs had more functional groups, which increase the density of cross-linking (Table 6). Thus, the ECO/TDI and RAE/TDI DCPNs had relatively high E′ values. Furthermore, compared with the RAE/TDI DCPN, the ECO/TDI DCPN had greater cross-linking due to the higher molecular weight and polyfunctionality of the ECO molecule. Thus, the ECO/TDI DCPN had a relatively higher E′, which is in keeping with the copolymers’ mechanical properties. The glass transition temperatures (Tg) corresponding to the tan δ peak on loss factor (tan δ) curves for all copolymers are shown in Figure 11b. All tan δ curves in Figure 11b display one peak, indicating that all three copolymerized systems had only one phase. Similar to the changing tendency of E′, ECO/TDI DCPN had the highest Tg of the copolymers whereas CO/TDI copolymer had the lowest. It is known that Tg depends on several factors such as chemical structure, and cross-linked state. Compared with ECO/TDI DCPN, the RAE/TDI DCPN system had a relatively equivalent content of rigid benzene ring and oxazolidinone groups. However, the ECO/TDI DCPN had the highest density of cross-linking. With greater rigid benzene ring and oxazolidinone content, the formed cross-linking structures could more effectively hinder the chain mobility of molecular chain terminals, thus the tan δ curves would show wider and lower peaks at high temperature.60 A great deal of energy was needed to relax the molecular chains of the copolymerized systems, resulting in increased temperature corresponding to the tan δ peak (Tg) at high temperature.

perfect combination of these characteristic groups gives RAE/ TDI DCPN excellent comprehensive mechanical properties. Although the CO/TDI copolymerized system has some characteristics of tough materials, it also has fewer functional groups and a lower density of cross-linking, leading to poor mechanical properties in this system. The ECO/TDI copolymerized system had the most rigid benzene ring groups and excellent cross-linking systems, characteristics of rigid materials. This result was in agreement with data on the crosslinking density (Table 6). Table 6. Detailed Data of Tg, Calculated Cross-Linking Density in Different Curing Systems Sample

Tg (°C)

Tg + 20 °C (K)

E′ at Tg + 20 °C (MPa)

Ve (mol m−3)


−3.98 67.04 73.69

289.17 360.19 366.84

1.96 4.89 30.99

271 544 3387

Dynamic Mechanical Thermal Analysis of the Different Copolymerized Systems. The dynamic mechanical behaviors of the copolymerized systems were studied and summarized (Figure 11). Cross-linking density (Ve) can be calculated according to the rubber elasticity theory utilizing storage modulus (E′) data in the rubbery state (eq 2):57−59 Ve = 3RT /E′

(2)

where E′ is the storage modulus of the cured sample in the rubber state, R is the gas constant (8.314 J/mol·K), and T is the absolute temperature of Tg+20 °C.59 In this study, the E′ at Tg + 20 °C was used, which ensured that the cured resins were in the rubber state. The Tg data, and calculated cross-linking densities in the different copolymerized systems are shown in Table 6. Figure 11a depicts the curves of the storage modulus (E′) of the different copolymerized systems. In all of the curves, E′ decreases sharply in a broad range. At low temperatures, the copolymers exhibit a glassy character. In this case, the molecular segmental motions are frozen and E′ remained high. With increasing temperature, the frozen segmental

Figure 11. DMA curves of different copolymerized systems. 6890


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Figure 12. TG analysis curves of different copolymerized systems.

TGA, there was very little solid residual carbon. Each of the copolymers had low solid residue rates (

Preparation and Properties of Castor Oil-Based Dual Cross-Linked

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