Fatty Acid-Derived Diisocyanate and Biobased Polyurethane

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Biomacromolecules 2009, 10, 884–891

Fatty Acid-Derived Diisocyanate and Biobased Polyurethane Produced from Vegetable Oil: Synthesis, Polymerization, and Characterization Leila Hojabri, Xiaohua Kong, and Suresh S. Narine* Alberta Lipid Utilization Program, Department of Agricultural Food and Nutritional Science, 4-10 Agriculture/ Forestry Centre, University of Alberta, Edmonton, Alberta T6G 2P5, Canada Received December 5, 2008; Revised Manuscript Received January 29, 2009

A new linear saturated terminal diisocyanate was synthesized from oleic acid via Curtius rearrangement, and its chemical structure was identified by FTIR, 1H and 13C NMR, and MS. The feasibility of utilizing this new diisocyanate for the production of polyurethanes (PUs) was demonstrated by reacting it with commercial petroleumderived polyols and canola oil-derived polyols, respectively. The physical properties of the PUs prepared from fatty acid-derived diisocyanate were compared to those prepared from the same polyols with a similar but petroleumderived commercially available diisocyanate: 1,6-hexamethylene diisocyanate. It was found that the fatty acidderived diisocyanate was capable of producing PUs with comparable properties within acceptable tolerances. This work is the first that establishes the production of linear saturated terminal diisocyanate derived from fatty acids and corresponding PUs mostly from lipid feedstock.

Introduction Fatty acids and their derivatives are an important renewable resource for the production of fine chemicals as well as for the preparation of monomers and polymers.1 Potential for replacing petroleum derived raw materials with renewable plant-based materials for the production of polymeric materials is significant due to social, environmental, and economic concerns. Using plant-based raw materials contributes to global sustainability without depletion of scarce resources. Furthermore, these materials are increasingly economical compared to petrochemicals, as the price of crude oil tends to rise in a long-term trend.2 Furthermore, the utilization of plant based feedstock for polymer synthesis is one way in which the urgent need to reduce carbon footprints and shorten life cycles can be addressed. Recently, increased attention has been paid to obtaining polymers such as polyurethanes (PUs) from renewable resources because of their potential use as substitutes for petrochemical derivatives.3-7 PUs are materials widely used in coatings, medical applications, construction engineering applications, automotive and consumers goods industries in the form of rigid and flexible plastics, elastomers, protective coatings, lacquers, and adhesives.8,9 PUs are currently produced using polyols, such as alkanediols and glycerol, with toxic diisocyanate, which is derived from the even more toxic phosgene. Health and safety concerns associated with isocyanate chemistry motivates the search for alternative routes for PU synthesis. There have been many attempts to develop nonphosgene and nonisocyanate routes for the preparation of PUs.10 However, none of these have been commercially established. Based on numerous scientific papers,11-20 almost all biobased PUs have been prepared using polyols derived from vegetable oil in combination with petrochemical-based diisocyanates. These biobased PUs can compete in many aspects with PUs derived from petrochemical polyols.19 Over the past few years, * To whom correspondence should be addressed. Tel.: 1-780-492-9081. Fax: 1-780-492-7174. E-mail: [email protected].

our research group has been contributing to the effort to develop methodologies for the synthesis of vegetable oil based PUs.11-16,18 The diisocyanates normally used in PU synthesis are petroleumderived, but if they were derived from vegetable oil sources then this would lead to an increased amount of renewable carbon in such materials. Difurfuryl diisocyanate, as an example was produced from furfural derived furfurylamine. It is reported that furfural is potentially available from a variety of renewable, biomass-based feedstocks.21 To the best of our knowledge, the one instance of diisocyanate production from fatty acids is reported in a U.S. patent22 that describes the synthesis of diisocyanates from diamine precursors prepared from the hydrogenation of dinitrile compounds. Ultimately, these dinitrile compounds were obtained from hydroxyl-substituted fatty nitrile and unsaturated nitrile starting materials. Recently, isocyanatecontaining soybean oil has been synthesized in two steps, which involved substitution of the allylic bromides of plant oil triacylglycerols (TAGs) with AgNCO.23 In addition, a commercially available fatty acid-based diisocyanate known as dimer acid diisocyanate supplied by Henkel Corportion company or General Mills Co.24-26 has been utilized for the preparation of PUs. This diisocyanate is based on a dimer of a fatty acid and has 36 carbon atoms in the chain. In all the cases mentioned above, dangling chains are presented in the diisocyanate or polyisocyanate structure, as the isocyanate functional groups are located in the center of the diisocyanate molecules, which leads to steric hindrances. Upon cross-linking these disocyanates with polyols, dangling chains in the diisocyanate molecules are unsupported, which results in significant steric hindrance to further cross-linking and limits to stress transduction. In consideration of such limited literature available concerning the synthesis of diisocyanate from vegetable oils, we report on our efforts in this work as the first example of the preparation of linear saturated terminal aliphatic diisocyanate from fatty acid via Curtius rearrangement. To our best knowledge, this is the first time that linear saturated terminal aliphatic diisocyanate was synthesized using a cheap and short procedure from fatty acid. We also demonstrate the feasibility of this new type of

10.1021/bm801411w CCC: $40.75  2009 American Chemical Society Published on Web 03/12/2009

Diisocyanate and Polyurethane from Vegetable Oil

Biomacromolecules, Vol. 10, No. 4, 2009

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Scheme 1. Chemical Reaction Procedure of Polyols from Triolein

Figure 1. GC-FID chromatogram of fatty acid-derived diisocyanate.

fatty acid-derived diisocyanate in the preparation of PUs, and compare physical properties of these PUs to their counterparts produced from the same polyols with a similar but petroleumderived commercially available diisocyanate: 1,6-hexamethylene diisocyanate (HDI).

Experimental Section Materials. Oleic acid (90% purity), dimethyl sulfide, tert-butyl hydroperoxide solution 70 wt % in H2O, triethylamine, copper (I) chloride, sodium azide, dibutin dilaurate (DBTDL), acetonitrile, anhydrous tetrahydrofuran (THF), and dichloromethane (DCM) were purchased from Sigma-Aldrich. Ethylchloroformate and sodium azide were obtained from BDH Ltd. and Fluka, respectively. Ethyl acetate, hexane, and diethyl ether were obtained from Fisher. Canola polyols were synthesized using an ozonolysis and hydrogenation technology developed by our group; the detailed procedure was reported elsewhere.14,18 The chemical reaction procedure is illustrated in Scheme 1, as an example. The hydroxyl number of the canola polyols was 237 mg KOH/g, as determined according to the ASTM D1957-86. The polyols contained 60.18 ( 1.16 wt %, 26.00 ( 0.48 wt %, and 4.72 ( 0.03 wt % of triol, diol, and mono-ol, respectively.13 The remainder consists of approximately 9 wt % saturated TAGs. Commercial polyols (Desmophen 800) was donated by Bayer Corporation, Pittsburgh, PA. Its functionality is over 4 and equivalent weight is 193 g/mol as provided by the supplier. Aliphatic 1,6-hexamethylene diisocyanate (HDI, Desmodur H) was also sourced from Bayer Corporation. Fatty Acid-Derived Diisocyanate Synthesis. 9-Oxononanoic Acid 2. Oleic acid [40.0 g (90%), 0.142 mol] was dissolved in dry CH2Cl2 (400 mL) in an oven-dried flask. The flask was open to the atmosphere and was cooled to -78 °C with stirring. Ozone was then bubbled into the solution until a faint blue color was observed (90 min), after which the solution was purged with N2 for 20 min. Dimethylsulfide (14.6 mL, 0.2 mol) was then added via syringe and the solution was stirred for 3 h. Excess Me2S and solvent were then removed under reduced pressure and the mixture was dissolved in EtOAc (200 mL). The solution was then washed three times with brine and dried over MgSO4, and the solvent was removed under reduced pressure. The residue was purified by column chromatography (1:1 hexane/ethyl acetate) and 22.5 g of compound 2 (84%) was obtained as a white solid.27 Azelaic Acid 3. To a solution of CuCl (0.495 mg, 5 mmol) in 200 mL of CH3CN and aldehyde 2 (17.2 g, 0.1 mol), an aqueous solution

of tBuOOH was added dropwise (14.2 mL, 0.1 mol, 70% in water) over 5 min. The resulting reaction mixture was stirred at room temperature for 24 h. After completion of the reaction, the solvent was evaporated and to the resulting crude reaction mixture, water was added. The pH was adjusted to 8.0-8.5 with saturated NaHCO3, and then the reaction mixture was extracted with ethyl acetate. The aqueous layer was acidified to pH 2.0 using 2 N HCl and extracted with ethyl acetate. The organic layer was concentrated and purified by silica gel column chromatography to give 16.3 g of the azaleic acid 3 (86%). The spectroscopic data of this compound is in accordance with previously reported work.28 1,7-Heptamethylene Diisocyanate 4 (HPMDI). A suspension of compound 3 (9.4 g, 0.05 mol) and triethylamine (16.72 mL, 0.12 mol) in anhydrous THF was cooled to 0 °C and ethylchloroformate (10.48 mL, 0.11 mol) was added dropwise. The resulting mixture was stirred for 3 h at 0 °C. Then it was added dropwise to a solution of sodium azide (13 g, 0.2 mol) in water (140 mL) that was cooled to 0 °C and kept for 1 h, followed by another hour at 5 °C. To a separatory funnel containing 100 mL of cold water, the reaction mixture was added, and the organic layer was separated and dried over MgSO4 and stirred for 10 min at room temperature. After filtering, the solvent was removed under reduced pressure with a room temperature water bath. Anhydrous THF was then added to the flask and the resulting solution was heated to reflux for 3 h under a nitrogen atmosphere, then solvent was removed under reduced pressure. A total of 10 mL of hexane was added to the flask and passed through a very short silica gel column under pressure. After removing the solvent under reduced pressure, 6.2 g of diisocyanate 4 was obtained as a pale yellow oil (68% yield and 97% purity). Purity of compound was determined by GC-FID (Figure 1). 1H NMR δ 1.34-1.42 (m, 6H, 3CH2), 1.57-1.65 (m, 4H, 2CH2), 3.31 (t, 3JHH ) 6.5 Hz, 4H, 2CH2); 13C NMR δ 26.4 (CH2), 28.3 (CH2), 31.1 (CH2), 42.9 (CH2), 121.9 (NCO); IR (cm-1) 863, 1355, 1465 (CH2 Scissoring), 2274 (NdCdO), 2859, 2934 (C-H); LRMS (EI) calcd for C9H14O2N2 ([M - H]-), 181.2; found m/z, 181.0. Polyurethane Preparation. PU sheets were prepared by reacting polyols with diisocyanates at molar ratios of the OH group to the NCO group, that is, OH/NCO of 1.0/1.1. The desired OH/NCO molar ratio satisfies the equation

Mratio )

Wpolyol /EWpolyol (WPU - Wpolyol)/EWisocyanate

(1)

where Wpolyol is the weight of the polyols, EWpolyol is the equivalent weights of polyols, WPU is the total weight of PU to produce, and EWisocyanate is the equivalent weight of the isocyanate. The equivalent weights of both diisocyanates were calculated based on their molar mass. The equivalent weight of commercial polyols was

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Scheme 2. Synthesis of Saturated Diisocyanate from Oleic Acid

provided by the supplier. The equivalent weight of canola polyols was determined using the equation: EWpolyol ) (molecular weight of KOH × 1000)/(OH number) ) (56110)/(OH number) g per mole of hydroxyl group. Canola polyolHPMDI and canola polyol-HDI represent PU produced from canola oil based polyols with HPMDI and HDI, respectively. Com polyolHPMDI and Com polyol-HDI represent PU produced from commercial polyols with HPMDI and HDI, respectively. A suitable amount of polyols and diisocyanate were weighed in a plastic container and stirred for 2 min before trace amounts of DBTDL was added. The mixture was further mixed for 5 min, poured in another container, and placed in a vacuum oven at 40 °C for 5-8 min to degas the CO2 released during the side reaction of diisocyanate with moisture or carboxylic acids and the air trapped during mixing. Air was then introduced to the oven to avoid the deformation of the sample under vacuum and the sample was postcured for about 24 h at 70 °C and for 10 h at 110 °C. Characterization. Gas Chromatograph-Flame Ionization Detector (GC-FID). Gas chromatograms were obtained on an Agilent 6890N capillary GC (Santa Clara, CA) equipped with a flame ionization detector and Agilent 7683B auto sampler. The 30 m × 0.32 mm × 0.1 µm DB-5HT column was used for the determination of diisocyanate compound purity. The temperature of the column was initially set at 85 °C and then increased to 250 °C at a rate of 10 °C/min and held for 5 min. FTIR, NMR, and Mass. FTIR spectrum of diisocyanate was measured with a Mattson Galaxy series 3000 FTIR spectrophotometer. The analysis of polymer specimens was performed using a Bruker Vertex 70 FTIR main bench with an attached Hyperion FTIR microscope using OPUS software. The spectra were obtained using a micro-ATR objective with an analysis area approximately 100 µm in diameter. The spectra were acquired using 128 scans at a resolution of four wavenumbers. 1H and 13C NMR were recorded at larmor frequencies of 500 and 125 MHz, respectively, using a Varian Unity 500 NMR spectrometer (Varian, Inc., CA). Deuterated chloroform (CDCl3) was used as solvent. Mass spectrum was acquired on a Kratos Analytical MS-50 (Kratos Analytical Ltd., Manchester, U.K.) EI low resolution spectrometer. Wide Angle X-ray Diffraction (WAXD). A Bruker AXS X-ray diffractometer (Madison, WI) equipped with a filtered Cu KR radiation source (λ ) 0.1542 nm) and a 2D detector was used to record the WAXD patterns. The procedure was automated and controlled by the Bruker AXS’s “GADDs V 4.1.08” software. The frames were processed using GADDS software and the resulting spectra were analyzed using Bruker AXS’s “Topas V 2.1” software. Thermal Properties. MDSC measurements were carried out on a DSC Q100 (TA Instruments, DE, U.S.A.) equipped with a refrigerated cooling system. All the DSC measurements were performed following the ASTM E1356-03 standard procedure under a dry nitrogen gas atmosphere. The samples were heated at a rate of 10 °C/min from 25 to 140 °C to erase thermal history and cooled down to -50 °C at a cooling rate of 5 °C/min. MDSC measurements were performed with a modulation amplitude of 1 °C/min and a modulation period of 60 s at a rate of 3 °C/min to 140 °C. The second heating stage was selected for the analyzing of heating data.

DMA measurements were carried out on a DMA Q800 (TA Instruments, DE, U.S.A.) equipped with a liquid nitrogen cooling apparatus in the single cantilever mode, over a temperature range of -80 to 100 °C with a constant heating rate of 1 °C/min and a fixed frequency of 1 Hz. The dimension of the samples was 18 × 7 × 2 mm. The measurements were performed following ASTM E1640-99 standard at a fixed oscillation displacement of 0.015 mm. TGA was carried out on a TGA Q50 (TA Instruments, DE, U.S.A.) following the ASTM D3850-94 standard. The sample was ground to a powder after chilling with liquid nitrogen and approximately 20 mg of the specimen was loaded in the open platinum pan. The samples were heated from 25 to 600 °C under dry nitrogen at constant heating rates of 10 °C/min. All the samples were run in triplicate for thermal property measurements. The reported errors are the subsequent standard deviations. Mechanical Properties. Mechanical properties were determined using an Instron (MA, U.S.A.) tensile testing machine (model 4202) equipped with a 50 Kgf load cell and activated grips that prevented slippage of the sample before break. Specimens for tensile tests were cut out from the PUs sheets using an ASTM D638 type V cutter. The measurements were performed at room temperature with cross-head speed of 50 mm/ min, as suggested by the above-mentioned ASTM standard. The data presented were average of five different measurements. The reported errors are the subsequent standard deviations.

Results and Discussion Aliphatic diisocyanates are major building blocks for valueadded PU products most commonly used in the coatings industry. According to the work presented here, unsaturated fatty acids could be an excellent resource for the preparation of linear saturated diacid compounds that are precursors for the synthesis of diisocyanates via a Curtius rearrangement. The Curtius rearrangement is a thermal decomposition of an acyl azide that has been employed for the preparation of isocyanates only as intermediates that are then converted in situ into PUs;29-31 however, there are no reports on the separation of diisocyanate from the reaction mixture, which is then used to produce PUs when combined with polyols. A major difficulty with this approach is the hydrolysis of the acyl azide and any isocyanate that might have formed.32 This problem is greatly alleviated by using anhydrous reagents. Despite this requirement for dry conditions, we believe that the production of diisocyanate via Curtius rearrangement is worthwhile because of the short and relatively inexpensive procedure for diisocyanate production from diacids. Of note, diacids such as azealic acid are easily produced from oleic acid.33 As illustrated in Scheme 2, the saturated diacids 3 were conveniently prepared by ozonolysis of oleic acid to the corresponding aldehyde product,27 followed by purification and oxidation of the aldehyde to desired acid product.34 Synthesis of the diacid via the aldehyde was advantageous because of the ease of purification of two aldehyde compounds over two acid compounds. Saturated diacyl azide was prepared from the


Figure 2. FTIR spectra of (a) fatty acid-derived diisocyanate, (b) canola polyol-HPMDI, (c) canola polyol-HDI, (d) Com polyol-HPMDI, and (e) Com polyol-HDI polyurethane.

diacid 3 via the preferred Weinstock modification35 of the Curtius reaction route. This produced the desired diacyl azide in a pure fashion and with a higher yield in comparison to the diacylchloride route.29 The FTIR spectrum (Figure 2, curve a) of the diisocyanate obtained showed strong characteristic bands at 2274 cm-1, which corresponded to the NdCdO stretching vibration and the very small bands observed at 1717 cm-1 corresponded to the stretching vibration of carbonyl group of urethane. This indicates that there is a very small amount of diisocyanate already converted into urethane in the diisocyanate synthesis process. The mass spectra (Figure 3) and the 1H NMR spectra (Figure 4) of the diisocyanate were in accordance with the structure proposed. Also, 13C NMR (Figure 5) showed the signal at δ 121.9, which is related to isocyanate group. The FTIR spectra for canola polyol-HPMDI, canola polyolHDI, Com polyol-HPMDI, and Com polyol-HDI are shown in Figure 2. A strong stretching band located at 3340 cm-1 characteristic of the N-H group and a stretching vibration band centered around 1700 cm-1 characteristic of the CdO group are present in all the FTIR spectra. Meanwhile, the NdCdO stretching vibration band located at 2270 cm-1 is clearly missing, which confirmed that all of the diisocyanate groups reacted during polymerization. From Figure 2 it is noticed that the spectra are similar for the PUs prepared from the same polyols but different diisocyanate sources. However, there are distinct differences between the spectra in the N-H stretching and CdO stretching regions for the PUs prepared from the same diisocyanate but different polyol sources. In the case of both canola polyol-based PUs, the band centered around 1700 cm-1 split into two resolved bands, namely, hydrogen bonded urethane CdO groups in ordered (crystalline) domains (1680-1685 cm-1) and free nonhydrogen bonded CdO groups (1740 cm-1).36 However, in the case of both commercial polyol-based PUs, only one band, that is, hydrogen bonded urethane CdO groups in disordered (amorphous) domains (1720 cm-1) was observed. The other evidence showing the difference between the PUs prepared from two types of polyols is the distribution of N-H stretching band. It is obvious that the N-H stretching bands (3340 cm-1) of PUs resourced from commercial polyols are much broader than those of PUs resourced from canola polyols. The narrowing of N-H stretching bands in both of canola polyol-based PUs is due to the crystallization of the sample.37-39 The existence of crystals in canola polyol-based PUs was further confirmed by WAXD and MDSC and will be discussed in detail later. The above results indicated that the fatty acid-derived diisocyanate,


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similar to petroleum-derived commercial available aliphatic diisocyanate, are successfully reacted with polyols, formed PUs with comparable chemical structures when the same polyols source was applied. The WAXD patterns of canola polyol-HPMDI, canola polyolHDI, Com polyol-HPMDI, and Com polyol-HDI are shown in Figure 6. In the case of the two commercial polyol-based PUs, a single broad peak at 2θ ≈ 20° was observed. This broad peak is a typical characteristic of amorphous polymers, which confirms that there are no crystals in Com polyol-HPMDI and Com polyol-HDI. In the case of canola polyol-based PUs, features different from those of the commercial polyol-based PUs were observed. The main diffraction peak was shifted to 2θ ≈ 21.2° for canola polyol-HPMDI and split to 20.8° and 22.2° for canola polyol-HDI, which indicated the existence of crystals in canola polyol-based PUs. Nevertheless, the crystalline structures of canola polyol-HPMDI and canola polyol-HDI PUs are different, which might be due to odd-even effects. For canola polyol-HPMDI, the numbers of methylene groups of polyol and diisocyanate monomers are 8 and 7, respectively; therefore, this specimen is classified as even-odd PUs. In contrast, for canola polyol-HDI, the numbers of methylene groups of polyol and diisocyanate monomers are 8 and 6, respectively; thus, this specimen is classified as even-even PUs. These X-ray data are in good agreement with the results reported for PUs with similar structures by Tasaka et al.40 Figure 7 shows the reversing heat flow versus temperature curves obtained for all the PU samples with different composites. The melting temperature (Tm) of canola polyol-based PUs were observed from MDSC, which complement the FTIR and WAXD results. The existence of crystal in both canola polyol-based PU is due to the fact that canola polyols is a mixture of triol, diol, and mono-ol.13 The production of mono-ol and diol is unavoidable because the starting oil contains TAGs (6%)41 that have a mixture of saturated and unsaturated fatty acids. Those diols (26% on a mass basis) might act as a chain extender and react with diisocyanate forming crystalline phase in the resulted PUs. The commercial polyols (Desmophen 800) used in this study is a highly branched polyol with functionality over 4; therefore, the resultant PUs have a network structure with high crosslinking density. Compared to commercial polyol-based PU networks, the formation of canola polyol-based PU networks (from three or more reactants) is more complicated because the relative reactivity of functional groups are not equal and may change during the reaction, which leads to structurally inhomogeneous systems on a nanoscopic length scale. Consequently, both of the canola polyol-based PUs contained various domains with different sizes, consisting of alternate polyols (triol or diol)isocyanate sequences. Part of the domain is chemically crosslinked controlled by the reactions of diisocyanate with triol, diol, and mono-ol through covalent bonds, and part of it results from physical cross-linking (hydrogen bonding) between segments. The crystals are functioning as physical cross-links as well. The glass transition temperature (Tg) and Tm values determined from Figure 7 are summarized in Table 1. The Tgs of both PUs made from commercial polyols are about 40 °C higher than those corresponding PUs made from the same diisocyanate but canola polyols. This is due to the functionality of commercial polyols being over 4, hence, the chemical cross-linking of the resultant PUs is much higher than the canola polyol based PUs. The increasing of chemical cross-linking would cause a restriction of segment mobility, therefore, an increasing of Tg values. On the other hand, the

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Figure 3. LRMS spectrum of fatty acid-derived diisocyanate.

Figure 4. 1H NMR of fatty acid-derived diisocyanate.

Figure 5.

13

C NMR of fatty acid-derived diisocyanate.

Tgs of PU samples made from HPMDI are about 3-5 °C lower than those made from the same polyols but commercial HDI. The Tm value of canola polyol-HPMDI PUs is about 25 °C lower than that of canola polyol-HDI. This is probably

due to the fact that the former material is an even-odd polymer, while the latter one is an even-even polymer. It has been reported42 that the even-odd PUs tended to have lower melting points than even-even PUs.


Figure 6. Wide-angle X-ray diffraction patterns of canola polyolHPMDI, canola polyol-HDI, Com polyol HPMDI, and Com polyol-HDI polyurethane.

Figure 7. MDSC curves of canola polyol-HPMDI, canola polyol-HDI, Com polyol HPMDI, and Com polyol-HDI polyurethane. Table 1. Thermal Properties of Polyurethane Made from Different Raw Materials Obtained by MDSC and DMA Tg (°C) MDSC Tg (°C) DMA Tm (°C) MDSC canola polyol-HPMDI canola polyol-HDI Com polyol-HPMDI Com polyol-HDI

-16.0 ( 1.0 -11.5 ( 0.5 27.5 ( 0.5 32.0 ( 1.0

-11.5 ( 0.5 -6.0 ( 1.0 30.0 ( 0.5 33.5 ( 0.5

61.5 ( 0.5 84.1 ( 0.1 NA NA

It is worth mentioning that the Tm values of both canola polyol-based PUs are in good agreement with the Tms (in the range of 43-52 °C) of PUs prepared from castor oil with similar structures reported by Yeganeh et al.43 They synthesized PUs through the epoxy-terminated PU prepolymer (prepared from reaction between castor oil and HDI) with 1,6-hexamethylene diamine as curing agent. Nevertheless, it is also noticed that these Tm values are much lower than that of the PUs (ca. 200 °C) produced from HDI and macrodiol, such as polycaprolactone diol, with 1,4-butanediol as chain extender.44 This results from the effect of domain structures as well as from defects in the crystals of canola polyol-based PUs due to the presenting of dangling chains in canola polyols as illustrated in Scheme 1. These dangling chains would cause significant entropic barriers to nucleation and, therefore, restrict the effective packing of the polymer chains into crystals by creating steric hindrance. Additionally, the chemical cross-links would reduce the mobility of molecular chains and cause a reduction of these chains to orient to form crystals.


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Figure 8. Storage moduli vs temperature, obtained from DMA of canola polyol-HPMDI, canola polyol-HDI, Com polyol-HPMDI, and Com polyol-HDI polyurethane.

Figure 8 shows the changes in the storage moduli (E′) of canola polyol-HPMDI, canola polyol-HDI, Com polyol-HPMDI, and Com polyol-HDI samples with temperature, obtained from DMA carried out at frequency of 1 Hz. BelowTg, the storage moduli of both PUs made from commercial polyols are much higher (more stiff) than those corresponding PUs made from the same diisocyanate but canola polyols. This might be caused by the different molecular structures of these two types of polyols. Additionally, the storage moduli of the PUs made from petroleum-derived HDI are much higher than those PUs made from the same polyols but fatty acid-derived HPMDI. Again, this is due to the fact that HPMDI is one carbon longer than HDI, which increase the flexibility of polymer chains. Tg values as determined from the inflection point of E′ versus temperature are higher than those determined by DSC by about 5-10 °C (see Table 1) as generally found in the literature.20 The trend is however the same: using the same diisocyanate, higher Tg was recorded for the PUs made from commercial polyols than the PUs made from canola polyols. Meanwhile, using the same polyols, lower Tg was recorded for the PUs made from HPMDI than the PUs made from HDI. TGA curves of canola polyol-HPMDI, canola polyol-HDI, Com polyol-HPMDI, and Com polyol-HDI samples at 10 °C/ min heating rate and their derivatives (DTGA) are shown in Figure 9a and b, respectively. For all the formulations, the decomposition started at approximately 200 °C. In the case of canola polyol-based PUs, DTGA curves revealed three distinct stages of degradation. In the first step, the sample lost 30% of its weight up to 350 °C, in the second step it lost 30-80% up to 450 °C, followed by a third step during which the remaining weight was lost. In the case of commercial polyol-based PUs, DTGA curves only revealed two stages of degradation. In the first step, the sample lost 60% of its weight, and in the second step, it lost the remaining weight. Thermal stabilities of PUs depend mainly on the equilibra between polymerization and depolymerization of the functional groups or linkages present in the polymer chains. It is known that the first stage of degradation is related to urethane bond decomposition,45 which takes place through the dissociation to isocyanate and alcohol, the formation of primary amines, a terminal olefinic group on the polyester chain and CO2, or the formation of secondary amines.46 The second stage of degradation is attributed to ester bond decomposition through chain scission.47-49 The appearance of a third stage of degradation at considerably higher temperature (ca. 470 °C), in the case of

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Figure 10. Nominal stress vs strain curves of canola polyol-HPMDI, canola polyol-HDI, Com polyol-HPMDI, and Com polyol-HDI polyurethane.

Figure 9. (a) TGA curves of canola polyol-HPMDI, canola polyolHDI, Com polyol-HPMDI, and Com polyol-HDI polyurethane. (b) DTGA curves of canola polyol-HPMDI, canola polyol-HDI, Com polyolHPMDI, and Com polyol-HDI polyurethane.

canola polyol based PUs, might be due to a probable C-C bond cleavage.18 Note that canola oil offers sites for functionality mainly located at carbon 9 and, hence, produce canola polyols with functional groups (i.e., hydroxyl groups) located at carbon 9. The molecular structure of the commercial polyols (Desmophen 800) could not be found anywhere. However, based on the raw material (adipic acid, phthalic acid anhydride, trimethylolprpopane, and propylenglycol-1,2) used for its synthesis, the length between hydroxyl and ester group for this polyols should be less than nine carbons. It has been reported that the PUs prepared from polyols with shorter chain lengths have different degradation products compared to those produced from polyols with longer chain lengths.50 For example, Grassie et al.50 found that ethylene appeared in the residue of PUs prepared from high molecular-weight poly(ethylene glycols), which was attributed to the cleavage of C-C bond. This mechanism might be applied to the canola polyol-based PUs as well, which gives the third step of degradation. Further studies on inquiring into the discrepancy between these two series of PU based on evolved gas analysis are underway in our laboratory. The stress versus strain curves for canola polyol-HPMDI, canola polyol-HDI, Com polyol-HPMDI, and Com polyol-HDI samples are shown in Figure 10. Com polyol-HPMDI PU behaves as a rubber with elongation at break of 75 ( 6% and strength of 38 ( 2 MPa, while Com polyol-HDI PU has a typical mechanical property of plastics, which is more rigid but

brittle with tensile strength at yield 32 ( 2 MPa and elongation at break of 14 ( 2%. This is not surprising, as Tg (27 °C as exhibited in Table 1) of the Com polyol-HPMDI coincides with the temperature (room temperature) at which the tensile property was carried out, whereas Tg (32 °C) of the Com polyol-HDI is higher than room temperature. The plastic nature of Com polyolHDI PU is due to the excessive density of cross-links which restricted molecular mobility as discussed previously. For canola polyol-HPMDI PU, the tensile strength at break is 15 ( 2 MPa and maximum elongation is 133 ( 13%, whereas for canola polyol-HDI PU, the tensile strength at break is 23 ( 5 MPa and maximum elongation is 105 ( 22%. The poor performances of the biobased PUs can mainly be explained by the composition of the canola polyols. The mono-ol and saturated TAGs contents of the canola polyols resulted in relatively small cross-linking densities and low Tgs. The monool in the canola polyols acted as a chain terminator when polyols are cross-linked with diisocyanate, contributing to the poor performances of the resulting PUs. Additionally, because Tg of the PU samples involving one or both biosourced ingredients are lower than the measuring temperature (25 °C), these polymers behave as elastomers with lower Young’s modulus and tensile strength but higher elongation at break. If these products would have been measured significantly below their Tgs, they would have typical mechanical properties of rigid plastics as the other two PUs with Tg around 30 °C. These tensile results implied that PUs made from HPMDI are slightly weaker, but more flexible than the PU made from the same polyols but commercial HDI. It is reasonable to believe that the fatty acidderived diisocyanate should be capable of producing PUs with comparable properties. It is worth pointing out that the successful synthesis of fatty acid-derived diisocyanate provides the opportunity of producing PUs from vegetable oil sources, which increases significantly the amount of renewable carbon in such materials. These new PUs produced mostly from renewable resources could be potential candidates to replace or partially replace PUs produced from petroleum-derivatives in the current market. While this work suggests that acceptable PUs with properties comparable to those made from vegetable oil polyols and commercial diisocyanates and commercially available polyols and commercial diisocyanates can be produced from these polyols and fatty acid-derived diisocyanates, there are still some inherent limitations that must be addressed before this endeavor is


considered an industrial solution. Note that the inherent limitations to producing polyols from vegetable oils have been discussed in great detail before in a variety of references13,14,51 so that only those which pertain to the production of diisocyanate from fatty acids using the synthesis method presented, are discussed here. The diisocyanate material produced using the procedure described in this publication is difficult to scale-up, and would require significant process development focus before it becomes commercially relevant. Furthermore, organic azides can be explosive and requires extreme care to be taken to work at lower temperatures, which can be a barrier to larger scale batch type processes, but which would lend itself well to continuous processing. It should be noted that in the laboratory we did not encounter any incidents with no more than a normal attention to caution. The synthesis of diisocyanate from a fatty acid-derived diacid such as azelaic acid involves the use of a fairly hazardous solvent, THF. However, it is felt that this solvent can be replaced by a more benign solvent with additional research, and compared to the current industrial process to produce commercial HDI, which utilizes phosgene, it is still comparatively safer. Azelaic acid is commercially available in relatively pure form, which is important, as the purity of the corresponding diisocyanate is very important and its purity is determined by that of the azelaic acid utilized. The variability of the fatty acid profiles in canola oil, soybean oil, and olive oil, the primary sources of oleic acid, is minimal, and the industry produces large amounts of pure azelaic acid from these sources, so that bioresource variability does not present a problem in this endeavor. Although these challenges exist with this approach, the route to production of diisocyanates described here is simple and comparatively inexpensive. Furthermore, this route produces acceptable diisocyanate materials for use in the PU industry, which are sourced mostly from renewable sources.

Conclusions A novel linear saturated terminal diisocyanate has been successfully synthesized from oleic acids via Curtius rearrangement. This diisocyanate has then been used as a starting material for the preparation of PU products. It has been found that the produced diisocyanate is suitable for the preparation of PUs and can be manipulated to create PUs with desirable properties. The physical properties of these PUs are compared to their counterparts made from similar but petroleum-derived commercially available diisocyanate, such as 1,6-hexamethylene diisocyanate. Compared to petroleum-derived diisocyanate, the fatty acid-derived diisocyanate synthesized here was capable of producing PUs with comparable properties. Therefore, this new PU material, prepared mostly from renewable resources, can prove to be a valuable substitute for existing materials in various applications. Acknowledgment. The financial support of Bunge Oils, NSERC, Alberta Canola Producers Commission, Alberta Agricultural Research Institute, and Alberta Crop Industry Development Fund are gratefully acknowledged. Authors also would like to acknowledge Mr. Robert Jacksteit from Bayer Corporation who kindly provided HDI.

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BM801411W

Fatty Acid-Derived Diisocyanate and Biobased Polyurethane

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