Elastomers from Renewable Metathesized Palm Oil Polyols - ACS

May 18, 2017 - Allen (Editor-in-Chief), Carrier (Associate Editor), Hwang (Associate Editor), Licence (Associate Editor), Moores (Associate Editor), P...
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Research Article pubs.acs.org/journal/ascecg

Elastomers from Renewable Metathesized Palm Oil Polyols Prasanth K. S. Pillai, Michael C. Floros, and Suresh S. Narine* Trent Centre for Biomaterials Research, Departments of Physics & Astronomy and Chemistry, Trent University, 1600 West Bank Drive, Peterborough, Ontario K9J 7B8, Canada S Supporting Information *

ABSTRACT: Polyurethane (PU) elastomers were prepared from 1-butene metathesized palm oil (PMTAG) polyols in order to study the effect of their molecular structure on the properties of polyurethanes. The chemical structure, thermal degradation, thermal properties, and tensile strength of the PUs were determined using Fourier transform infrared (FTIR), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and texture analysis, respectively. Depending on the crosslinking density, which increases with increasing hydroxyl values of the polyols, the polyurethanes produced behave as rigid plastic or elastomeric rubbers. The highest hydroxyl polyol (OH value: 184 mg KOH/g) produced polyurethanes displaying a tensile strength of ∼18 MPa and displayed characteristics of a rigid plastic. On the other hand, the polyurethane prepared from the low hydroxyl polyol (OH value: 83 mg KOH/g) displayed more elastomeric characteristics with a lower tensile strength of ∼4.2 MPa and ∼120% elongation at break. All of the PUs prepared in this study display high thermal stability (Ton of degradation of ∼280 °C), comparable to commercial PUs. The thermal transition behavior of the PUs from DSC and DMA indicates that the glass transition temperature of the PUs increased with an increase in OH value of the polyols. Polyurethanes prepared from metathesized palm oil polyols display superior performance when compared with more expensive bio based PUs derived from canola and soybean oils and represent potential for replacing current commercially available petrochemically derived polyols. KEYWORDS: Metathesized palm oil, Polyol, Elastomers, Vegetable oil, Polyurethane, Renewable

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quantities, and are priced at relatively low costs.11 Vegetable oils are naturally occurring triesters of fatty acids, and some of the most common fatty acids are palmitic (C16:0), stearic (C 18:0), oleic (C 18:1), linoleic (C18:2), and linolenic (C18:3) acids.12 Polyols can be synthesized from vegetable oils by suitable functionalization of the unsaturations naturally present in the triacylglycerol structure.11 A number of studies on the synthesis of polyols and PU elastomers from natural oil such as soybean oil, canola oil, sunflower oil, linseed oil, grape seed oil, and castor oil have been reported.7 All of these vegetable oil feedstocks featured internal double bonds, and the functionalization of these internal double bonds creates polyols with significant amounts of long dangling chains (Scheme S1 in the Supporting Information). The dangling chains result from both unmodified saturated fatty acids present in the feedstock as well as in the hydroxylated fatty acids present following double bond functionalization. These dangling chains act as plasticizers, decreasing the mechanical properties of the polymers.5 Palm oil is one of the most inexpensive and widely available oils (64.5 53 million metric tons were produced in 2016/17

INTRODUCTION Polyurethanes (PUs) are one of the most versatile classes of polymers having urethane linkages (R−NH−COOR′) as the backbone chain. They can be manufactured with properties ranging from rubbery elastomers to rigid plastics and foams.1 Polyurethanes are prepared by polyaddition reactions between polyols and polyisocyanates and frequently utilize additives such as chain extenders, blowing agents, and catalysts.1 By varying the reactants and reaction conditions, polyurethanes can be produced with properties that are suitable for applications including elastomers,2 adhesives,3 coatings,4 and foams.5 Polyurethane elastomers are in high demand for structural and biomedical applications including automotive seats, household furniture, cardiac-assist pumps, blood bags, and medical implants.1,6 The properties of polyurethane elastomers are highly dependent on the structure and functionality of the substrates used for their preparation.7 Renewable materials such as cellulose, chitosan, lignin, starch, etc. have been employed in the preparation of polyurethanes.8−10 Recent interest in renewable vegetable oil feedstocks have demonstrated their value in the synthesis of a variety of sustainable materials including polyurethanes. Sustainably derived materials exhibit improved biodegradability, are readily availability in large © 2017 American Chemical Society

Received: February 18, 2017 Revised: May 14, 2017 Published: May 18, 2017 5793

DOI: 10.1021/acssuschemeng.7b00517 ACS Sustainable Chem. Eng. 2017, 5, 5793−5799

Research Article

ACS Sustainable Chemistry & Engineering

Table 1. Detailed Characteristics of Metathesized Palm Oil Polyols (GP83, GP119, PP155, and LP184) Used for the Fabrication of Elastomers polyols

viscosity @ 40 °C (mPa·s)

molecular weight (calculated from functionality) g/mol

OH value (mg KOH/g)

acid value (mg KOH/g)

functionality calculated from 1H NMR (±0.1)

elastomer produced

GP83 GP119 PP155 LP184

355 411 330 440

1891 1508 1483 1493

83 119 155 184

1.3 1.3 2 0.8

∼3.0 ∼3.2 ∼4.1 ∼4.9

GP83E GP119E PP155E LP184E

reacting these prepared PMTAG polyols with diphenylmethane diisocyanate (MDI, NCO content: 31.5%/wt) which was purchased from Bayer Materials Science (Pittsburgh, PA) and used as received. Preparation of Polyols and PU Elastomers. Polyols. Polyols were synthesized from PMTAG and LF-PMTAG (Liquid Fraction PMTAG) in a two-step reaction; epoxidation by formic acid and hydrogen peroxide (H2O2), followed by hydroxylation using water and perchloric acid (HClO4) as the catalyst following a previously reported method.19 This is a well-established economical route to produce polyols with maximum hydroxyls. In this method, the double bonds are converted into oxirane moieties, and the epoxy groups are converted into hydroxyl groups by ring opening reaction with suitable reagents like HClO4 and H2O to give the polyol. The polyols produced are labeled as PMTAG-Polyol (PP155), green PMTAGpolyol (GP83, GP119), and liquid fraction polyol (LP184), respectively. The PP155 and LP184 polyols were synthesized by solvent (dichloromethane) assisted epoxidation and hydroxylation procedure. On the other hand, green PMTAG polyols, GP83 and GP119, were synthesized by a solvent free epoxidation and hydroxylation strategy. The structure of the polyols were confirmed by the proton nuclear magnetic resonance spectroscopy (1H NMR) and the corresponding spectra are provided in Figure S1a−d in the Supporting Information. The synthetic strategy to produce polyols are listed in the Supporting Information, Schemes S3 and S4. The general structure of the polyols is produced in the Supporting Information, Schemes S5 and S6. The gel permeation chromatography (GPC) of the polyols (the GPC data are provided in the Supporting Information in Figure S2 and Table S1) revealed the presence of relatively important levels of oligomers in the polyols. These include high molecular weight (Mw: 7030 g/mol) oligomers as well as low molecular weight (Mw: 1463 g/mol) oligomers. GP83 and GP119 comprised 37% and 45% oligomers, respectively, compared to 13% oligomers in PP155 and 15% in LP184. The higher oligomerization during the solvent free reaction was due to its higher reaction temperature.21 The characteristics of the polyols such as hydroxyl value, acid value, functionality and molecular weight and viscosities are provided in Table 1. Polyurethanes. Polyurethane elastomers were produced from PMTAG polyols (see Table 1) and MDI using a modified method.2 The code for the elastomers produced from the metathesized polyols and the characteristics of MDI are provided in Table 1 and Table S2, respectively. The mixing ratios used to prepare each of the elastomers are presented in Table S3. The amount of each component of the polymerization mixture was based on 100 parts by weight of total polyol (see Table S3). The amount of MDI used for the polymerization was determined in order to achieve an isocyanate index of 120 (an NCO to OH ratio of 1.2:1, respectively). A calculated amount of polyol and MDI were added to a plastic container, and they were mechanically mixed vigorously for 20 s. The mixture was then poured into a plastic Petri dish previously treated with a silicone release agent (SH70-250D). The sample was cured for 48 h at 55 °C, and postcured for an additional 24 h at room temperature. Note that no catalyst was used in the polymerization process. Chemical and Physical Characterization. Gel Permeation Chromatography (GPC). Molecular weights and distribution were determined by gel permeation chromatography (GPC). The measurements were carried out on an e2695 GPC instrument equipped with a Waters e2695 pump, Waters 2414 refractive index detector, and a 5μm Styragel HR5E column (Waters Alliance, Milford, MA).

according to the United States Department of Agriculture (USDA)). Palm oil consists of approximately 50% saturated fatty acids and 50% unsaturated fatty acids containing internal double bonds, the majority of which are monounsaturated. This low degree of unsaturation has limited its use for many polymer applications.13 Olefin cross-metathesis is a very potent transformation pathway adopted to resolve the problems associated with large quantities of dangling chains in TAG molecules to produce fine chemicals, substrates, and materials, many of which serve as or are potential petrochemical replacements.14−18 At Trent Center of Biomaterial Research (TCBR), a series of investigations have already been reported in the conversion of 1-butene cross-metathesized palm oil (PMTAG), an industrial byproduct from alpha-olefin manufacturing, into useful polyols for the preparation of rigid and flexible polyurethane foams.19,20 The different terminal hydroxyl 1-butene metathesized palm polyols display favorable properties such as high compressive strength and good flexibility in rigid and flexible foams applications, respectively. This work sets out to investigate the transformation of different metathesized polyols into polyurethane counterparts for use in applications including, elastomers, coatings, and adhesives. These applications can significantly increase the commercial value of palm oil derived polyols and produce value added materials from PMTAG, which is currently an underutilized byproduct with limited applications. Finding new applications for underutilized biorefinery byproducts will also increase their profitability. The present work reports the preparation of polyurethane elastomers from metathesized palm oil polyols. A series of polyols created from different reaction conditions and fractionation can produce PMTAG polyols with varying OH values. Elastomers were produced from these different metathesized palm oil polyols, in order to show that the polyols are suitable for elastomeric and rigid plastic applications and study the effect of OH value and dangling chains on the physical, chemical, and mechanical properties of the resulting polyurethanes. The produced elastomers were characterized by Fourier transform infrared (FTIR), thermal stability (TGA), mechanical analysis (tensile analysis and dynamic mechanical analysis (DMA)), and differential scanning calorimetry (DSC).

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MATERIALS AND METHODS

Materials. PMTAG is one of the products of the cross-metathesis of palm oil and 1-butene which was stripped of olefins and provided by Elevance Renewable Sciences (ERS, Bolingbrook, IL). The 1-butene cross-metathesis reaction of palm oil results modified TAG molecules with shortened and terminal double bonded structures in the molecule, which can result polyols with terminal and internal hydroxyls (having shorter chain) on complete hydroxylation. Scheme S2 in the Supporting Information represents the structure of PMTAG. Metathesized palm oil polyols with different OH values were produced in our laboratory from this PMTAG following our previously reported procedures.19,20 Elastomers were prepared by 5794

DOI: 10.1021/acssuschemeng.7b00517 ACS Sustainable Chem. Eng. 2017, 5, 5793−5799

Research Article

ACS Sustainable Chemistry & Engineering Chloroform was used as eluent with a flow rate of 0.5 mL/min. The concentration of the sample was 1 mg/mL, and the injection volume was 10 μL. Polystyrene (PS) standards and pure TAG-oligomers (synthesized previously22) were used for calibration. Waters Empower Version 2 software was used for data collection and data analysis. Fourier Transform Infrared Spectroscopy (FTIR). FTIR spectra were obtained with a Thermo Scientific Nicolet 380 FTIR spectrometer (Thermo Electron Scientific Instruments, LLC, USA) equipped with a PIKE MIRacle attenuated total reflectance (ATR) system (PIKE Technologies, Madison, WI, USA.). Solid samples were positioned over the ATR crystal, and spectra were acquired over a scanning range of 400−4000 cm−1 for 32 repeated scans at a spectral resolution of 4 cm−1. Thermogravimetric Analysis (TGA). The thermogravimetric analysis (TGA) was carried out on a TGA Q500 (TA Instruments, DE, USA) equipped with heat exchanger (P/N 953160.901). Between 8.0−15.0 mg of sample was loaded in an open platinum TGA pan. The sample was then heated from 25 to 600 °C under a dry nitrogen atmosphere at a heating rate of 10 °C/min. Differential Scanning Calorimetry (DSC). The thermal characteristics of the elastomers were investigated using a Q200 DSC (TA Instruments, New Castle, DE) following the ASTM E1356-03 standard. The sample (8−10 mg) was hermetically sealed in an aluminum DSC pan and was equilibrated at 25 °C. The sample was then heated to 120 °C at 10 °C/min (first heating cycle). The sample was held isothermally at this temperature for 5 min to erase thermal memory and then cooled down to −90 °C at 10 °C/min, where it was again held isothermally for 5 min. Finally, the sample was again heated to 120 °C at a rate of 10 °C/min (second heating cycle). The heating and cooling cycles were modulation with an amplitude of 1 °C over a period of 60 s. The “TA Universal Analysis” software (V. 4.5A) was used to analyze the TGA and DSC thermograms. The characteristics of nonresolved peaks were obtained using the first and second derivatives of the differential thermogravimetry (DTG) and differential heat flow. Mechanical Properties. Cured PU sheets of uniform thickness (1.0 mm) were cut into dumbbell shapes using an ASTM D638 type V cutter. The tests were performed at room temperature using a texture analyzer (TA-TX HD, Texture Technologies Corp, NJ, USA) equipped with a 50 kgf load cell and activated grips which prevented slippage of the sample during stretching. The cross-head speed was 50 mm/min. At least five identical dumbbell-shaped specimens for each sample were tested, and mechanical properties are reported as averages from these repeated measurements. The reported errors are the subsequent standard deviations of at least five replicates. Dynamic Mechanical Analysis (DMA). DMA measurements were carried out on a DMA Q800 (TA Instruments, DE, USA) equipped with a liquid nitrogen cooling apparatus. Polymers were cured in a rectangular Teflon mold 16.0 mm × 7.0 mm × 1.0 mm in dimension. DMA measurements were performed following the ASTM E1640-99 standard at a fixed frequency of 1 Hz and a fixed oscillation displacement of 0.015 mm in single cantilever mode. The sample was initially cooled to −90 °C and held at this temperature isothermally for 5 min. It was then heated at a rate of 1 °C/min from −90 to 60 °C. In the case of multiple isothermal oscillation experiments, the isothermal evolution of rheological parameters was recorded as a function of frequency ranging from 0.1 to 100 Hz. The isothermal oscillation was made every 5 °C, beginning 30 °C below and above the glass transition temperature (Tg). Stress relaxation experiments were performed at 50 °C above the Tg with a shear clamp. A 0.2% strain was applied to each sample immediately prior to the collection of the relaxation spectrum. The relaxation modulus was monitored for approximately 10 min. All the analyses were repeated in triplicate. The reported values represent averages and the reported errors represent standard deviations.

and LP184E, respectively. As shown in Figure S3 the elastomers were smooth and transparent. The GP83E and GP119E derived elastomers were significantly softer compared to the PP155E and LP184E derived elastomers. FTIR Characterization. The FTIR spectra of the elastomers are shown in Figure 1. The characteristic absorption

Figure 1. FTIR spectra of the PMTAG polyol derived polyurethane elastomers.

band of NH centered at 3322 cm−1 confirms the formation of urethane linkage.23 The characteristic C−N absorption band at 1520 cm−1 also confirmed the formation of urethane bonds in the elastomers.24 The weak band at 2270 cm−1 for GP83E and GP119E indicates that small quantities of unreacted NCO is still present.23,25 The presence of urea in the elastomers was confirmed by the overlapping peaks between 1710 and 1725 cm−1. The peak at 1418 cm−1 reveals the presence of small amount of isocyanurate trimers, indicating the occurrence of the trimerization reaction of diisocyanates during the polymerization process. The stretching bands of the ester groups are particularly visible at 1744 cm−1 (CO), 1150−1160 cm−1 (O−C−C), and 1108−1110 cm−1 (C−C(O)−O). The stretching vibration of −C−H in −CH3 and −CH2 groups in the aliphatic chains were also visible at 2923 and 2853 cm−1, respectively.26 Thermal Stability. Figure 2 shows the DTG profiles of the elastomers produced from the metathesized palm oil polyols. The corresponding characteristic degradation temperatures are provided in Table S4. The onset and the degradation temperature determined at 5 and 10% weight loss of all the

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RESULTS AND DISCUSSION Figure S3a, b, c, and d, in the Supporting Information, represents the pictures of elastomers GP119E, GP83E, PP155E,

Figure 2. DTG profile of elastomers: GP83E, GP119E, PP155E, and LP184E. 5795

DOI: 10.1021/acssuschemeng.7b00517 ACS Sustainable Chem. Eng. 2017, 5, 5793−5799

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elastomers GP83E, GP119E, PP155E, and LP184E. All the elastomers displayed two inflection points indicating a glass transition temperature at Tg1 and Tg2 (see Figure 3). The elastomers show two inflection points indicating two glass transitions (Tg1 at −5.9 °C and Tg2 at 29.2 °C of GP83E, Tg1 at −1.1 °C and Tg2 at 33.2 °C of GP119E, Tg1 at −6.7 °C and Tg2 at 36.5 °C of PP155E, and Tg1 at −27.5 °C and Tg2 at 46.7 °C of LP184E). No melting peaks were observed for any of the elastomers, indicating their high cross-linking density which is characteristic of thermosets. The first glass transition, Tg1, occurred between −1.1 and −27.5 °C for all of the elastomers and is attributable to the molecular motion of the soft segments related to the polyols.32 The second glass transition temperature, Tg2, is attributable to the relaxation of the urethane segments, occurring between 29.2 and 46.7 °C.32 Note that the elastomers produced with polyols with higher OH values displayed a higher Tg2 value. LP184E produced from the polyol with an OH value of 184 mg KOH/g displayed significantly higher Tg values compared to the other elastomers and is due to its higher cross-link density.33 The jump in heat capacity (ΔCp) was ∼0.4 J/(g K) at Tg1, and 0.7 J/(g K) at Tg2, suggesting that a large number of molecular chains were associated with the relaxation of the molecular urethane segment.32 Figure 4 shows the storage and loss moduli measured at a frequency of 1 Hz for PU elastomers with different polyols. As can be seen in Figure 4a, a β transition occurs around −40 °C, which is followed by a drop in the storage modulus, indicative of a glass transition.2 Also, one can note that, in the glassy state (−40 to 0 °C), the storage moduli of the elastomers decreased with decreases in the OH value of the polyols used for the fabrication of the elastomers. LP184E, prepared from LP184 having OH value 184 mg KOH/g, displayed the highest storage moduli and loss moduli compared to the rest of the elastomers, which decrease in the order of PP155E, GP119E, and GP83E, respectively. Thus, the marginal increases as indicated in both the storage modulus and loss modulus curves are attributed to the increase in cross-link density of the elastomers prepared from the different polyols.7 Also the maxima on loss modulus− temperature curves are associated with the glass transition temperature of the elastomers.2 Figure 5 shows the tan δ value of the different elastomers as a function of temperature. The glass transition (Tg) temperature obtained from DMA for the elastomers GP83E, GP119E, PP155E, and LP184E are 33 ± 2, 36 ± 2, 40 ± 4, and 50 ± 5

elastomers (GP83E, GP119E, PP155E, and LP184E) fall over similar temperature ranges, and occurring between 280 and 300 °C for all of the polyurethanes. The DTG curves of the elastomers showed three prominent and distinct regions (TD1, TD2, and TD3), indicating a multistage decomposition process. The first peak TD1 displays a sharp peak at approximately 300 °C, involving a total weight loss of ∼10% and is related to the dissociation of urethane bonds. Urethane bond breakdown takes place either through its dissociation into isocyanate and alcohol or through the formation of primary or secondary amines, olefins, and/or carbon dioxide.27,28 The decomposition step at TD2 occurring between 360 and 430 °C is associated with the degradation of the soft segments (polyol backbone).28 The soft segments degrade into carbon monoxide, carbon dioxide, carbonyls (aldehydes and acids), olefins, and alkenes.28,29 This decomposition step involved the largest weight loss, with approximately 50% of the total weight loss occurring. The last decomposition step, TD3, displays a peak at approximately 455 °C and is related to the decomposition of strongly bonded fragments and remaining carbonaceous materials remaining.30,31 The rates of urethane degradation in the elastomers (GP83E: ∼0.36%/°C, GP119E: ∼0.42%/°C, PP155E: ∼0.49%/°C, LP184E: ∼0.56%/°C) increased in the order of the OH value of the polyol used, corresponding with higher rates of loss for polymers containing higher concentrations of urethane groups. Thermal Characteristics. Figures 3 shows the DSC profiles obtained during the second heating cycle of the

Figure 3. DSC thermogram of elastomers: GP083E, GP119E, PP155E, and LP184E.

Figure 4. (a) Storage and (b) loss moduli of the elastomers GP83E, GP119E, PP155E, and LP184E. 5796

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in cross-linked urethanes.2,7 As seen in Figure 6, the tensile strength of the elastomers increased with increase in the OH value of the polyol that was incorporated for elastomer fabrication. On the other hand, the elongation at break of the elastomers LP184E, PP155E, GP119E, and GP83E increased respectively with decreases in the OH value of the polyol used. This is another effect of the cross-link density with respect to increase in the OH values of the polyols used.7 The GP83E produced from GP83, displayed the lowest strength tensile at 4.2 MPa, but the highest elongation at break at 117%, demonstrating behavior characteristic of a rubbery material. Inversely, LP184E displayed the highest tensile strength at 18.3 MPa, and the lowest elongation at break of 24%, behavior more characteristic of a rigid plastic. The high tensile strength of LP184E is also due to the lower amount of dangling chains in the LP184 polyol. This polyol had the lowest concentration of dangling chains followed by PP155, GP119, and GP83, respectively, as it was subjected to both the metathesis (resulting in more terminal double bonds) and fractionation (resulting in less saturated fatty acids) procedures and resulted in the highest OH value and greatest cross-linking density.19 On the other hand, the rest of the polyols (GP83, GP119, and PP155) possess more dangling chains due to a higher saturated fatty acid content, and the plasticizing effect of these chains reduce the rigidity of the corresponding elastomers.19,20 Of all the polyols, GP119 and 83 has more dangling chains compared to the other polyols, since it has fatty acid moieties with unreacted terminal double bonds. Table 3 shows the tensile strength and elongation at break of some of the PU elastomers produced from the soybean oil and

Figure 5. Tan δ of the elastomers GP83E, GP119E, PP155E, and LP184E.

°C, respectively. The Tg values determined by DMA were slightly higher those values obtained by DSC, a phenomena consistent with findings for other biobased polymers and urethane elastomers.2,34 As seen in Figure 5, LP184E displayed the highest glass transition temperature, followed by PP155E, GP119E, and GP83E, respectively. This is again attributed to the higher cross-link density of LP184E due to the higher OH value.7 The increase in the intensity of the tan δ peak from GP83E to LP184E may be due to the increase in the number of relaxation of urethane segments, which is present in a greater percentage in the latter than former.35 Tensile Properties. Figure 6 shows the stress versus strain curves of the polyurethane elastomers. Table 2 gives the

Table 3. Tensile Strength and Elongation at Break Percent of Elastomers from Standard Vegetable Oils PU elastomers soybean oil PU canola oil PU

characteristic tensile strength, Young’s modulus, and corresponding elongation at break of each of the respective films. Young’s modulus and tensile strength at break increased with increasing OH value of the polyol, which is a common feature Table 2. Tensile Strength and Elongation at Break Percent of Elastomers from Metathesized Palm Oil Polyols Prepared in This Study

LP184E PP155E GP119E GP83E

tensile strength (MPa) 18.3 13.7 5.8 4.2

± ± ± ±

1.50 1.50 0.80 0.60

elongation at break (%) 24 42 85 117

± ± ± ±

4.0 7.0 4.0 6.0

Young’s modulus (MPa) 3.77 1.57 0.22 0.14

± ± ± ±

11 3.7

elongation at break (%) 93 118

characteristics OH value: 160 mg KOH/g used MDI (diphenylmethane diisocyanate)7 OH value: 132 mg KOH/g used IPDI (isophorone diisocyanate)33

canola oil polyol using different diisocyanates.7,33 As can be seen from Tables 2 and 3, the PU elastomers from the metathesized palm oil polyols, especially GP83E and GP119E, display better elastomeric properties than the canola oil. Even though the elongation at break of PP155E and LP184E are not as high as GP083E, their higher tensile strength shows suitability for rigid plastics. The terminal hydroxyl polyols, PP155 and LP184 obtained via metathesis platform contains diols, tetrols and hexols, which can amplify the cross-link density of the polyurethanes to a greater extent and increase the tensile strength.19 Interestingly, the tensile strength of the palm MTAG derived LP184E is significantly higher (18.3 vs 11 MPa) than that of a soybean oil polyurethane with a similar OH value (160 KOH/g). This demonstrates quite well the effect dangling chains after the OH groups have on the mechanical properties of a highly unsaturated vegetable oil (represented in Scheme S1a). Another factor which effects the properties of the PMTAG derived polymers is the degree of oligomerization present in the polyols as a result of the metathesis procedure. The terminal hydroxyl polyols, GP83 and GP119, are composed of approximately 45% oligomers, mostly as diols

Figure 6. Stress (MPa) strain (%) curves of elastomers prepared in this study.

elastomeric films

tensile strength (MPa)

0.11 0.08 0.03 0.01 5797

DOI: 10.1021/acssuschemeng.7b00517 ACS Sustainable Chem. Eng. 2017, 5, 5793−5799

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ACS Sustainable Chemistry & Engineering and tetrols, significantly enhancing the flexibility and resulting elastomeric properties of the corresponding PUs.19 Thus, the lower hydroxyl value and higher percentage of oligomeric structures in green polyols (GP83 and GP119) suits it for the preparation of flexible plastics compared to their counterparts, such as PP155 and LP184. An interesting follow-up to the current study concerns the possibility of either making rigid polyurethane plastics or rubbery elastomers by blending two or more of these polyols together, such as GP83 and LP184, and may result in polymers with improved elastomeric properties.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank the Grain Farmers of Ontario, Elevance Renewable Sciences, Trent University, the GPA-EDC, Ontario Ministry of Agriculture, Food and Rural Affairs, Industry Canada, and NSERC for financial support.

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CONCLUSIONS Metathesized palm oil polyols produced with terminal hydroxyl groups were investigated for applications in elastomeric polyurethane systems. They were found to be an ideal material for the preparation of sustainable polyurethane elastomers. The PU elastomers were successfully prepared by catalyst and solvent free polymerization of 4 different polyols with varying OH values (GP83, GP119, PP155, and LP184) with MDI. PU elastomers were fully characterized by FTIR, TGA, DSC, texture analysis, and DMA. The increase in the OH value of the polyols resulted in an increase in the cross-linking density of the PU elastomers, directly increasing the tensile strengths and Tg values. On the other hand, the elongation at break of the elastomers decreased with increasing OH values of the polyols. The polyurethane created from the highest hydroxyl value polyol (LP184 OH value: 184 mg KOH/g) displayed a tensile strength of ∼18 MPa and behavior characteristic of a rigid plastic. The tensile strength of this polymer was considerably higher than that of a comparable soybean oil derived polyol, demonstrating the effect of dangling chains on the mechanical properties. The PU (GP83E) prepared from the lowest hydroxyl value polyol (GP83 OH value: 83 mg KOH/g) behaved as a rubbery material with a significantly lower tensile strength (4.2 MPa) and high elongation at break (120%). All of the PUs prepared in this study displayed high thermal stability (Ton of degradation of ∼280 °C), which are comparable to commercially available PUs. The structure and composition of the metathesized polyols (oligomers, terminal diols, tetrols, and hexols) further suggest the possibility of making materials with intermediate properties by blending two or more of these polyols together. This work demonstrates that polyurethanes synthesized from metathesized vegetable oils compare favorably with commercial biobased polyurethanes, as well as with petroleum derived polyols currently used commercially.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00517.

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REFERENCES

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Schemes S1−S6, Figures S1−S3, and Tables S1−S4 (PDF)

AUTHOR INFORMATION

Corresponding Author

*Tel.: 1-705-748-1011. Fax: 1-705-748-1652. E-mail: [email protected]. ORCID

Suresh S. Narine: 0000-0001-7217-3830 5798

DOI: 10.1021/acssuschemeng.7b00517 ACS Sustainable Chem. Eng. 2017, 5, 5793−5799

Research Article

ACS Sustainable Chemistry & Engineering lipid-based polyols and derived polyurethane foams. Ind. Crops Prod. 2016, 84, 273−283. (21) Guo, A.; Cho, Y.; Petrović, Z. S. Structure and properties of halogenated and nonhalogenated soy-based polyols. J. Polym. Sci., Part A: Polym. Chem. 2000, 38 (21), 3900−3910. (22) Li, S.; Bouzidi, L.; Narine, S. S. Synthesis and Physical Properties of Triacylglycerol Oligomers: Examining the Physical Functionality Potential of Self-Metathesized Highly Unsaturated Vegetable Oils. Ind. Eng. Chem. Res. 2013, 52 (6), 2209−2219. (23) Chuayjuljit, S.; Sangpakdee, T.; Saravari, O. Processing and properties of palm oil-based rigid polyurethane foam. J. Metals Mater. Miner 2007, 17, 7−23. (24) Piszczyk, Ł.; Strankowski, M.; Danowska, M.; Hejna, A.; Haponiuk, J. T. Rigid polyurethane foams from a polyglycerol-based polyol. Eur. Polym. J. 2014, 57, 143−150. (25) Narine, S. S.; Yue, J.; Kong, X. Production of polyols from canola oil and their chemical identification and physical properties. J. Am. Oil Chem. Soc. 2007, 84 (2), 173−179. (26) Gu, R.; Konar, S.; Sain, M. Preparation and characterization of sustainable polyurethane foams from soybean oils. J. Am. Oil Chem. Soc. 2012, 89 (11), 2103−2111. (27) Javni, I.; Petrović, Z. S.; Guo, A.; Fuller, R. Thermal stability of polyurethanes based on vegetable oils. J. Appl. Polym. Sci. 2000, 77 (8), 1723−1734. (28) Shufen, L.; Zhi, J.; Kaijun, Y.; Shuqin, Y.; Chow, W. Studies on the thermal behavior of polyurethanes. Polym.-Plast. Technol. Eng. 2006, 45 (1), 95−108. (29) Gryglewicz, S.; Piechocki, W.; Gryglewicz, G. Preparation of polyol esters based on vegetable and animal fats. Bioresour. Technol. 2003, 87 (1), 35−39. (30) Lin, B.; Yang, L.; Dai, H.; Hou, Q.; Zhang, L. Thermal analysis of soybean oil based polyols. J. Therm. Anal. Calorim. 2009, 95 (3), 977−983. (31) Prasanth, K. S.; Pillai, S. L.; Laziz, B.; Narine, S. S. Water-Blown Bio-Based Polyurethane Foams from 1-Butene Metathesized Palm oil Polyol, not published. (32) Tanaka, R.; Hirose, S.; Hatakeyama, H. Preparation and characterization of polyurethane foams using a palm oil-based polyol. Bioresour. Technol. 2008, 99 (9), 3810−3816. (33) Guo, A.; Demydov, D.; Zhang, W.; Petrovic, Z. S. Polyols and Polyurethanes from Hydroformylation of Soybean Oil. J. Polym. Environ. 2002, 10 (1), 49−52. (34) Floros, M. C.; Leão, A. L.; Narine, S. S. Vegetable Oil Derived Solvent, and Catalyst Free “Click Chemistry” Thermoplastic Polytriazoles. BioMed Res. Int. 2014, 2014, 1. (35) Liu, H.; Zheng, S. Polyurethane networks nanoreinforced by polyhedral oligomeric silsesquioxane. Macromol. Rapid Commun. 2005, 26 (3), 196−200.

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DOI: 10.1021/acssuschemeng.7b00517 ACS Sustainable Chem. Eng. 2017, 5, 5793−5799