Article pubs.acs.org/JAFC
Polyamine Triglycerides: Synthesis and Study of Their Potential in Lubrication, Neutralization, and Sequestration Rogers E. Harry-O’kuru,* Girma Biresaw, and Rex E. Murray Bio-Oils Research Unit, National Center for Agricultural Utilization Research, Agricultural Research Service, U.S. Department of Agriculture, 1815 North University Street, Peoria, Illinois 61604, United States ABSTRACT: Renewable resources have evoked a new awakening in both scientific and industrial circles in the past decade. Vegetable oil is one category of renewables that is amenable as a source of new industrial products. Because the source feedstock, seeds, are environmentally friendly, the derivatized products from these at the end of their lifetime could also be benign when designed appropriately. Bioethanol and biodiesel are examples of biobased industrial products currently in the market place and have become resources for uplifting the rural economy. Biolubricants also are playing a more prominent role because they have become closely competitive with petroleum-based lubricants. These products are renewable because the crops from which the feedstuff for the biofuels and biolubricants are produced are grown annually in contrast to nonrenewable mineral sources. Added to their renewability is the inherent biodegradability of their end-use products after their useful lifetime. In a recent study of the lubricity characteristics of peracylated polyhydroxy milkweed oil, the derivatives were found to exhibit good oxidative stability as well as excellent antiwear properties. To further explore an expansion in the properties of such materials in lubrication and other applications, in this study the polyhydroxy (OH) moieties of derivatized milkweed triglycerides were replaced with −NHR groupings in the oil. In this process novel polyketo triglyceride intermediates leading to polyamine derivatives of the vegetable oil have been synthesized. The polyamine triglyceride markedly improved the stability of the parent oil to oxidative stress. It has also attenuated the extreme viscosity of the starting polyhydroxy oil to a more useful product that could be amenable for use as a lubricating agent, for example, hydraulic fluid. Both the polyketone and polyimine intermediates of the polyamine have chelating properties. The intermediates and the polyamine were characterized spectroscopically, tribologically, and rheologically for their intrinsic properties. KEYWORDS: milkweed oil, polyhydroxy oil, oxidation, polyketone, polyamine triglyceride
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INTRODUCTION The character of amines as organic bases has allowed them to be used universally as neutralization agents, together with a host of other functions in biological systems. Polyamines in particular are known to be endogenously produced because they occur as cell components. They are also acquired from plant and animal sources of food; that is, polyamines are natural products from living systems.1,2 Many functions have been ascribed to polyamines. They are known to be promoters of metabolism and cell growth and to have protective and other health functions in the environments in which they are produced.2−4 Because of their cell proliferative properties, polyamines (putrescine, spermidin, and spermine) have been studied for years in the area of tumor growth or antineoplasticity.5−9 This work, however, explores a nonfood, nonbiological approach to novel types of polyamines generated as structural components of vegetable oils, specifically milkweed oil for industrial utilization. The polyhydroxy triglyceride of milkweed oil has earlier been generated via hydration of the olefinic groups of the native oil.10 The composition of milkweed oil is given in Table 1. It is a highly unsaturated oil. Conceptually, aminated vegetable oils could serve a variety of functions, namely, as neutralizers, buffers, pH adjusters, lubricants, chelating agents, in catalysis, etc. In this study, we have substituted nitrogen moieties in place of the OH units of the polyhydroxylated structure of the acyl groups of the derivatized This article not subject to U.S. Copyright. Published XXXX by the American Chemical Society
Table 1. Fatty Acid Composition of Milkweed (Asclepias syriaca) % C14 C16:0 C16:1, 16:2 C18:0 C18:1 C18:2 C18:3
0 5.7 9.6 2.5 31.0 50.5 1.2
(d9, d12) (d9, d11) (d9, d12) (d9,12,15)
intact triglyceride. In essence we have replaced each olefinic unit of the oil with vicinal diamine moieties. Although we had, in earlier work, reported syntheses of α-hydroxyamines of triglycerides via the oxiranes,10−12 the concept of complete replacement of the olefinic units of a vegetable oil with amine functionalities was attractive to us in terms of generating an oil of novel properties, especially in comparison to the polyhydroxy triglyceride starting material.11,12 We postulated that replacement of the hydroxyl groups of the starting material with amine groups would attenuate the extremely high viscosity of the polyhydroxy oil11−13 as well as vastly improve utilization Received: September 29, 2014 Revised: May 28, 2015 Accepted: June 26, 2015
A
DOI: 10.1021/acs.jafc.5b00598 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
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computer and software to allow for automatic data acquisition and analysis. All tests were conducted with the cell pressurized with pure oxygen to 500 ± 25 psig in dynamic mode, with a positive oxygen flow rate of 100 ± 10 mL/min. Details of the experimental procedure have been given previously.20 The resulting heat flow versus temperature data were analyzed using the instrument computer and software to determine onset temperature (OT) and peak temperature (PT). Duplicate runs were conducted on each sample, and average OT and PT values are reported. Four-Ball Tribometer and Test Specimen. The four-ball tests were conducted on a model KTR-30L four-ball tribometer (Koehler Instruments, Bohemia, NY, USA). The instrument comprises a mechanical unit, an electronic unit, and a computer with TriboDATA software (Koehler Instruments) that allows for setting and controlling test parameters as well as for automatic data acquisition. A detailed description and specification of the instrument can be found elsewhere.21 Test balls used in four-ball experiments were obtained from Falex Corp. (Aurora, IL, USA) and had the following specifications: material, chrome−steel alloy made from AISI E52100 standard steel; hardness, 64−66 Rc; diameter, 12.7 mm; finish, grade 25 extra polish. Test balls were degreased by two consecutive sonications in isopropyl alcohol and hexane solvents in an ultrasonic bath prior to use. The pot and spindle used for securing the balls were also thoroughly washed with isopropyl alcohol and hexane, wiped with Kimwipe (Kimberly-Clark Worldwide, Inc., Roswell, GA, USA), and allowed to air-dry prior to testing with a new lubricant. Four-Ball Anti-Wear (AW) Test. The four-ball AW tests were conducted according to the procedure outlined in ASTM D 4172-9422 under the following test conditions: load, 392 N; speed, 1200 rpm; lubricant temperature, 75 °C; test duration, 60 min. During the test, frictional torque and test conditions (load, speed, temperature) were automatically recorded as a function of time by the instrument computer at a rate up to 300 samples/min. The torque and load data were used to calculate the coefficient of friction (COF) for each test using the procedure outlined in ASTM D 5183.23 At the end of the AW test, the wear scar diameters (WSD) of each of the three balls in the ball pot were measured parallel and across the wear direction using a wear scar measurement system supplied by Koehler Instrument Co., Inc. The system comprises hardware (for taking a digital image of the wear scar of the three balls without disassembling the ball pot) and software (Scar View, Koehler Instrument Co., Inc.). The WSD along and across the wear direction of the three balls were used to calculate the average WSD for the test. Each test lubricant was used in at least two AW measurements, and average COF and WSD values were reported. Four-Ball Extreme Pressure (EP) Test. EP tests were conducted according to ASTM D 278324 under the following conditions: speed, 1760 ± 40 rpm; temperature, ambient; load, variable; test duration, 10 s. EP tests comprise a series of 10 s tests at increasing loads until welding of the four balls is observed. The load increments are selected on the basis of the expected weld point (WP) for the test lubricant, with finer increments being used close to the WP. The load at which welding is observed is the WP and is a characteristic EP property of the lubricant tested. Synthesis of Milkweed Polyketone from Polyhydroxy Milkweed Oil. In a typical reaction, polyhydroxy milkweed oil (61.0 g, 57.72 mmol) previously derived from milkweed oil10−13 was placed into a 500 mL dry round-bottom flask (RBF) containing a magnetic stir bar. Dry dichloromethane (150 mL) was then added and stirred to dissolve the oil. The stirred solution was cooled to −5 °C in an ice bath augmented with NaCl. Dess−Martin periodinane reagent (59.5 g, 140 mmol) was added quickly while stirring was continued. The reaction temperature was gradually allowed to warm to 0 °C over 3 h and then to room temperature overnight. A yellow solid formed, which later redissolved in the dichloromethane. The resulting mixture was then concentrated to dryness under reduced pressure, and the solid was triturated with anhydrous EtOEt (100 mL × 3) and filtered. The filtrate was washed with saturated NaHCO3. The organic layer was rewashed with a mixture of NaHCO3−Na2S2O3·5H2O. After separation of phases, the organic layer was rewashed with brine and
of the product. Each modified triglyceride molecule would now have two polar groups and two nonpolar groups, namely, the ester headgroup and the middle polar amino segment of each acyl chain compared to the virgin oil with a traditionally nonpolar tail. We envisioned this change would introduce better packing of the chains, leading to improved density with associated properties for the derivatives. The change in geometry and polarity of the chains would also enhance metal-binding capability in lubricating and chelating applications. It was also anticipated that such changes would find the product to be useful both as a neutralization agent and as a component in metal-working fluids application. Another potential application would be as an agent of sequestration of metal ions in waste effluents. Most amines currently in use for neutralizations or as pH adjusters are monoamines with shortto medium-sized alkyl chains and are generally milder than inorganic bases such as NaOH or KOH, which are at the top of the scale of neutralizing agents. However, the draw for amines as neutralizers is their less caustic nature on equipment, which leads to prolonged useful lifetimes for equipment. The polyamine triglyceride reported here is novel and initiates new characteristics and uses for the modified oil.
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MATERIALS AND METHODS
Materials. Polyhydroxy triglycerides of milkweed oil were obtained from our in-house supply from previous work.10 Initial Dess-Martin Periodinane (DMP), anhydrous MgSO4, sulfuric acid, and anhydrous diethyl ether were obtained from Sigma-Aldrich (St. Louis, MO, USA). Dichloromethane, ethyl acetate, NaHCO3, potassium bromate, 2iodobenzoic acid, sodium borohydride, and acetic anhydride were obtained from ACRO̅ S ORGANICS (Vineland, NJ, USA) whereas Na2S2O3.5H2O was from J.T. Baker Chemical Co. (Phillipsburg, NJ, USA). Methods. Instrumentation. (a) Fourier Transform Infrared (FTIR) Spectrometry. FT-IR spectra were measured on an Arid Zone FTIR spectrometer (ABB MB-Series, Houston, TX, USA) equipped with a DTGS detector. Liquid derivatives were pressed between two NaCl disks (25 mm × 5 mm) to give thin transparent oil films for analysis by FT-IR spectrometry. Absorbance spectra were acquired at 4 cm−1 resolution and signal-averaged over 32 scans. Interferograms were Fourier transformed using cosine apodization for optimum linear response. Spectra were baseline corrected, scaled for mass differences, and normalized to the methylene peak at 2927 cm−1. (b) NMR Spectroscopy. 1H and 13C NMR spectra were acquired on a Bruker AV-500 MHz spectrometer with a dual 5 mm proton/carbon probe (Bruker, Billerica, MA, USA). The internal standard used was tetramethylsilane. Refractive Index (RI). RI was measured as a function of temperature (30−80 °C) on a model Mark II Plus Abbe refractometer (Reichert Inc., Depew, NY, USA). The instrument provides data to four-digit precision. RI at 100 °C was obtained by extrapolation of data collected at ≤80 °C. Density, Viscosity, and Viscosity Index (VI). Density and dynamic viscosity were measured as a function of temperature on a Stabinger SVM3000/G2 viscometer (Anton Paar GmbH, Graz, Austria). The instrument also provides the kinematic viscosity as a function of temperature by automatically manipulating the corresponding measured density and dynamic viscosity data. VI was calculated from kinematic viscosity data at 40 and 100 °C following the procedure outlined in ASTM D 2270-93.17 Pour Point (PP) and Cloud Point (CP). PP and CP were determined on a model PSA-70S automatic analyzer from Phase Technology (Richmond, BC, Canada) according to ASTM standard test method D 594918 and D 5773,19 respectively. Pressurized Differential Scanning Calorimetry (PDSC). PDSC tests were conducted on a Q20P pressure differential scanning calorimeter (TA Instruments − Waters LLC, New Castle, DE, USA) fitted with a B
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dried over MgSO4 overnight.14 The filtrate was then concentrated under reduced pressure to give 60.7 g (99.0%) yield of the polyketone triglyceride. The FT-IR spectrum of the product obtained was νNCl cm−1 2927 vs (−CH2− asym stretch), 2858 s (−CH2− sym stretch), 1744 vs (second derivative CO) ester; 1720 s-m (ketone CO), 1463 m (CH2 def), 1370 m (CH3 def), 1235 m (COC−) stretch, 1166 s (−HCO) stretch, 1097 m (−HCO−) stretch, 1020 m (−CH2O−) stretch, 723 w (−CH2−) wag. 13C NMR: δ 213.6, 213.27, 209.7, 208.5, 206.6, 205.58, 205.52, 202.93, 202.8 (polyketone CO). Milkweed Schiff Base Formation from the Polyketone. Milkweed polyketone triglyceride (161.3 g, 156.5 mmol) was placed in a dry 1 L RBF) containing a magnetic stirrer. Dry dichloromethane (300 mL) was added and the mixture stirred and cooled to −1 °C in an ice bath; 2-propylamine (92.45 g, 1.564 mol) was then added dropwise to maintain the reaction temperature at 0 °C. After the amine had been added, the temperature was slowly allowed to warm to room temperature overnight. The solvent was removed under reduced pressure at 50 °C, and the crude product was diluted with EtOAc, dried over MgSO4, filtered, and concentrated under reduced pressure followed by pump vacuum drying to give a yield of 158.8 g (70.6%) of the Schiff base. FT-IR cm−1: 3366 w, 2959 sh, 2926 vs, 2874 sh, 2854s, 1741s, 1716 (CO), 1686, 1588 (CN of Schiff base), 1468 (−CH2− def), 1381 (−CH3 def), 1245, 1169,1018, 726. 13C NMR δ 173.19, 172.77, 172.46, 171.15, (CO ester), 160.69 (CO formyl ester), 68.88 (−CH− glyceride backbone), 63.75 methine C of isopropyl), 62.06, 60.36(−CH2O−glyceride backbone), 44.44, 43.72, 41.19, 40.30, 38.58 (−CH−isopropyl), 34.07− 21.71 (−CH2−), 20.98−13.89 (−CH3). Conversion of the Schiff base (Imine) Triglyceride to the Polyamine Triglyceride. In a 1 L RBF containing a magnetic stir bar, the intermediate imine triglyceride (155.7 g, 0.1084 mmol) dissolved in 50:50 EtOH (300 mL)/1,4-dioxane (300 mL) was placed. Acetic acid (3 mL) was added to the solution and the mixture vigorously stirred and cooled to 0 °C in an ice bath. Powdered NaBH4 (40.0 g) was added portion-wise to maintain the reaction temperature around 0 °C. At the end of the borohydride addition, the reaction was allowed to stir overnight, warming to room temperature. The mixture was then diluted to 1000 mL with 3% AcOH solution followed by extraction with dichloromethane. The extract was then washed with distilled water, dried over MgSO4, and concentrated under reduced pressure at 48 °C followed by further drying at the pump to yield the polyamine product (160.2 g). Density at 24 °C was 1.072 g cm−3; pH 8.09 (EtOH). FT-IR (NaCl film) ν cm−1: 3382 (−NH stretch), 2962 (−CH3 asym), 2929 (−CH2− asym) vs, 2873 (−CH3 sym), 2856 (−CH2− sym) s, 1741 vs (CO ester backbone), 1664 m, 1549 m-w (NC), 1470 s (−CH2− def), 1376 s (−CH3 def), 1256, 1240 s (−COCO), 1167 s, 1109 s, 875, 722 (−CH2−) wag. 13C NMR DEPT (CDCl3) δ 176.67 (CO ester), 160.96 (−HCO), 82.45, 81.54, 80.90, 80.11, 73.81, 71. 85, 68.88, 67.02, 65.80 (−CHN−), 62.05, 60.14 (−CH2O−), 57.25, 43.55, 41.19, 40.28, 35.94−22.63 (−CH2−), 21.08−14.19 (−CH3).
Figure 1. FT-IR spectrum of virgin milkweed oil.
Figure 2. FT-IR spectrum of milkweed polyhydroxy triglyceride generated from hydration of the olefinic units of the virgin oil.
shown here). Use of the Dess−Martin periodinane oxidation technique for the polyhydroxy oil, on the other hand, gave an excellent, almost quantitative, yield of the polyketone triglyceride. Figure 3 shows the synthetic procedure used. Spectroscopic characterization of the polyketone is shown in Figures 4−6. Figures 4 and 5 are the FT-IR spectra of the polyketone. The second derivative of this FT-IR spectrum (Figure 5) shows the carbonyl region of the spectrum with three bands indicating the keto carbonyl absorbance at 1724 cm−1, whereas the ester carbonyl absorption of the triglyceride is at 1746 cm−1 and there is a smaller higher frequency band at 1763 cm−1. The 13C NMR spectrum of this product (Figure 6) clearly shows the polyketo resonances (10 lines) from 202.8 to 213.6 ppm corresponding to about 10 keto groups expected in this intermediate from the 10 secondary hydroxyl groups of the polyhydroxy starting material. The arrangement of these carbonyl groups as shown in Figure 7 is such that on its own the polyketone would form a number of coordination spheres with metal ions. The operating principle is that when two keto groups are separated by a methylene species as in this structure (C10−C12 of the linoleyl chains), the two methylene protons at C-11 become more acidic by virtue of being sandwiched between C10 and C12 carbonyls. Enolization tends to be strong and is observed as shown in Figure 7 under appropriate conditions in the 13C spectrum. Evidence are absorbances around 91 and 191 ppm for the enolic carbons in addition to the intact keto forms (spectrum not included here). Because of this enolization tendency, this polyketone bears a strong resemblance to the molecular structure of 2,4-pentanedione (acetyl acetone), which is a known chelating agent. This
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RESULTS AND DISCUSSION Synthesis and Identification of Milkweed Polyamine. The FT-IR spectrum of the unmodified milkweed oil is shown in Figure 1. This spectrum displays the olefinic −CCH absorption mode at around 3009 cm−1 as well as the breathing or puckering mode of the CC bond at around 1658 cm−1. In contrast, the starting material, polyhydroxy oil Figure 2, exhibits the characteristic broad (O−H) stretching band centered around 3448 cm−1. This spectral region is transparent in the virgin oil. Substitution of the hydroxyl groups of polyhydroxy milkweed oil with nitrogen proceeded through an intermediate polyketone triglyceride of milkweed oil. An initial attempt at oxidation of the polyhydroxy oil (Figure 2) to the polyketone using the Jones reagent (chromic acid/H2SO4) resulted in an isolated product, which was characteristically not the desired material judging from the 13C NMR spectrum obtained (not C
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Figure 3. Generation of milkweed oil polyketone from Dess−Martin periodinane oxidation of the polyhydroxy milkweed oil.
Figure 4. FT-IR spectrum of milkweed oil polyketone.
Figure 6. 13C NMR spectrum of milkweed oil polyketone showing keto carbonyl resonances 202.63−213.56 ppm.
Conversion of the polyketone in dry dichloromethane at −1 °C with 2-propylamine as shown in Figure 8 gave the Schiff base (polyimines) in good yield. The polyimine adduct obtained was first characterized by FT-IR spectroscopy (Figure 9), followed by the 13C NMR spectrum. The obtained 13C NMR spectrum showed that the most downfield resonances observed were only those of the ester carbonyl absorbances at 173.19, 172.77, 172.46, and 171.15 ppm (CO ester), which correspond to the polar headgroup ester functionalities of the triglyceride. The indication is that all of the keto carbonyl groups have undergone reaction with 2-propylamine. Reduction of this polyimine was achieved with sodium borohydride utilizing the method of Haire.15,16 The polyamine isolated is a reddish viscous liquid. Its FT-IR spectrum (Figure 10) shows a prominent characteristic broad N−H stretching mode of secondary amines at 3384 cm−1 and C−N stretching bands at
Figure 5. FT-IR carbonyl spectral region second-derivative 1763− 1724 cm−1 showing keto carbonyl at 1724 and ester carbonyl absorption at 1746 cm−1, respectively.
skeletal arrangement gives the polyketone intermediate the properties of a powerful chelating agent, and it may be pertinent to note that this is the first reported polyketo vegetable oil. D
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Figure 7. Schiff base formation from milkweed oil polyketone followed by borohydride reduction to the polyamine triglyceride.
Figure 9. FT-IR spectrum of the polyimine (Schiff base).
1668 and 1551 cm−1. The band broadening arose from hydrogen bonding. Figure 11 is the second derivative of Figure 10 and allows us to conclude that the only carbonyl absorption bands present are those of the glyceride backbone ester functional groups at 1755 cm−1, the N−H stretch being around 3500 cm−1. Physical and Viscosity Properties of Milkweed Polyamine Oil. The effect of temperature on the density and RI of milkweed polyamine is summarized in Table 2. The polyamine is denser than water. Table 3 compares the kinematic viscosities of milkweed oil and its polyamine derivatives as a function of temperature. Also compared in Table 3 are the VIs of the two oils. As shown in this table, polyamination has considerably increased the viscosity of milkweed oil at all temperatures. At 40 °C, the viscosity displayed an almost 20-fold increase and an almost 6-fold increase at 100 °C. This means polyamination
Figure 10. FT-IT spectrum of milkweed polyamine following reduction of Schiff base.
Figure 8. Schiff base formation from milkweed oil polyketone followed by borohydride reduction to the polyamine triglyceride. E
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Cold flow properties are evaluated using pour point (PP) and cloud point (CP) data. As shown in Table 4, only PP data could be generated for milkweed polyamine oil because the oil was dark red and clouding could not be easily detected during the test. In general, oils with double-digit below zero PP values are preferred for lubrication applications. However, a PP of below freezing, such as that for milkweed polyamine oil, is not considered bad because it can be easily improved with the use of low concentrations of PP depressants. Table 4 also shows the results of oxidation stability tests by the PDSC method. In this method, the temperatures at which oxidation of the oil under oxygen atmosphere begins (OT) and peaks (PT) are reported. The higher the values of OT and PT, the higher the oxidation stability of the oil. Table 4 compares the OT and PT of milkweed polyamine oil with those of milkweed oil. As seen in Table 4, polyamination has resuted in considerable improvement in the oxidation stability of milkweed oil. Both OT and PT have increased by almost 40 °C after polyamination. There could be many reasons for this improvement in oxidation stability due to polyamination. A major contributor to such an improvement is the elimination of unsaturation in the oil structure, which eliminates reactive allylic and bis-allylic protons, which are considered major contributors to oil oxidation. Tribological Properties. Table 5 shows the results of AW and EP tests on milkweed polyamine oil. Also shown in Table 5 are similar AW data for three milkweed polyester derivatives from our previous work.26 The data in Table 5 indicate that milkweed polyamine oil displays the lowest average COF and the second lowest WSD among the four derivatives. No EP measurements were done on the milkweed polyesters in the study cited. Milkweed polyamine oil showed an EP WP of 140 kgf, which indicates that the oil lacks any significant EP properties. This is consonant with the fact that none of the elements comprising milkweed polyamine oil (C, H, O, N) are expected to display EP characteristics.26 Effect of Chemical Structure. Table 6 compares available properties of milkweed oil and several of its derivatives, namely, polyepoxy, polyamine, acetate, butyrate, and valerate polyesters and the polyhydroxy. Table 6 compares the kinematic viscosity, VI, cold flow (PP, CP), and oxidation stability (OS) determined using PDSC (OT and PT) of MWO and the listed derivatives. In Table 6, the oil derivatives are arranged in order of increasing kinematic viscosity from left to right. As can be seen in Table 6, the kinematic viscosity of MWO increases dramatically due to derivatization. Kinematic viscosity increases in the order MWO < polyepoxy < polyvalerate < polyamine < polybutyrate < polyacetate < polyhydroxy. This trend can be explained in terms of intermolecular hydrogen bonding, which will be greatest for the polyhydroxy derivative. The order for the polyesters and polyepoxide could be due to intermolecular polar interactions, which is expected to increase in the order polyepoxy < polyvalerate < polybutyrate < polyacetate. It appears there was no significant hydrogen bonding in the polyamine, and any polar interactions were slightly better than the polyepoxide. On the basis of structural considerations, the polyamine should have shown significantly higher intermolecular interaction and, therefore, significantly higher viscosity than the polyesters. However, the fact that the viscosity of the polyamine was below those of the polyesters implies a different mechanism is at play. A possible explanation for the observed data may lie in the contrast between the linearity of the polyester substituents as against the branched alkyl group on
Figure 11. Second-derivative spectrum of Figure 9 milkweed polyamine.
Table 2. Density and Refractive Index of Milkweed Polyamine Oila T, °C 25 30 40 75 100
RI 1.4694 1.4651 1.4530 1.4439
± ± ± ±
0.0002 (9.6 × 10−5) 0.0003 0.0017b
density, g/cm3 1.0209 1.0171 1.0093 0.9820 0.9642
± ± ± ± ±
0.0001 0.0001 0.0002 0.0003 0.0007
All data from this work unless noted. bRI data at 10 °C is extrapolated from measured data at ≤80 °C.
a
Table 3. Kinematic Viscosity and Viscosity Index (VI) of Milkweed Oil (MWO) and Its Polyamine Derivativea
a
kVis, mm2/s
MWO
MW polyamine
40 °C 75 °C 100 °C VI
33.8b
581.01 ± 16.69 95.80 ± 0.05 41.60 ± 0.60 116
7.3b 210b
All data from this work unless noted. bData from ref 26.
will increase the viscosity range for the application of milkweed oil in lubrication. As expected, Table 3 also shows that although polyamination results in considerable reduction of the VI of the parent milkweed oil to almost half its value, its VI is double that of the polyhydroxy starting oil. Even though the VI of milkweed polyamine is considerably below those of unmodified vegetable oils, it is still comparable to petroleum-based oils widely used in lubrication.25 Thus, milkweed polyamine can still be competitive with petroleum-based oils in lubrication applications. Oxidation Stability and Cold Flow Properties of Milkweed Polyamine Oil. Table 4 summarizes the cold flow and oxidation stability data for milkweed polyamine oil. Table 4. Cold Flow (Pour Point, Cloud Point) and Oxidation Stability (by PDSC) Properties of Milkweed Oil and Milkweed Polyaminea cold flow pour point, °C cloud point, °C oxidation stability PDSC − OT, °C PDSC − PT, °C a
milkweed oil
milkweed polyamine oil −4.3 ± 0.6 not discernible
136.78 ± 0.35 154.32 ± 0.47
173.49 ± 0.79 194.18 ± 0.07
All data from this work unless noted. F
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Table 5. Anti-wear (AW) and Extreme Pressure (EP) Properties of Milkweed Oil Derivativesa MW polyestersb
a
anti-wear
MW polyamine
valerate
butyrate
acetate
COF wsd, mm EP weld point, kgf
0.050 ± 0.001 0.593 ± 0.011
0.055 ± 0.005 0.778 ± 0.058
0.064 ± 0.014 0.622 ± 0.067
0.059 ± 0.008 0.576 ± 0.030
140
All data from this work unless noted. bData from ref 26.
Table 6. Selected Properties of Milkweed Oil (MWO) and Certain Derivativesa polyesters MWO kVis, mm2/s 40 °C 75 °C 100 °C VI cold flow PP, °C CP, °C OS by PDSC OT, °C PT, °C a
polyepoxy
polyamine
valerate
butyrate
acetate
polyhydoxy
33.8b
164.4b
489.4b
926.2b
1733b
2332b
7.3b 210b
19.22b 133b
581.01 ± 16.69 95.80 ± 0.05 41.60 ± 0.60 116
42.2b 136b
63.2b 131b
78b 105b
75.5b 85b
−18b −28b
−18b −30b
−3b −25b
−15b −7b
−4.3 ± 0.6
136.78 ± 0.35 154.32 ± 0.47
173.49 ± 0.79 194.18 ± 0.07
All data from this work unless noted. bData from ref 26.
information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. The authors declare no competing financial interest.
the nitrogen in the polyamine. It is well-known that increased branching tends to increase the bulkiness of a substrate rather than linearizing it, and this occurring in the middle aspect of the acyl chains of the triglyceride would make the molecule more spherical.27−29 Such induced sphericity naturally would cause attenuation of the apparent sweep-width of the molecule and hence lower its viscosity as observed for the polyamine triglyceride. Comparison of the VI of MWO and its derivatives in Table 6 shows an interesting correlation between the oil polarity and VI. MWO, which is the least polar of these substances, has the highest VI, whereas the polyhydroxy derivative, which is the most polar, has the lowest VI. The VI of the rest of the derivatives follows a similar trend and increases in the order polyacetate < polyamine < polybutyrate < polyvalerate < polyepoxy. This observation is similar to what is widely known about the effect of the polar hydroxyl groups on the VI of vegetable oils such as castor oil, which has a VI of ∼89, much lower than that for the nonpolar vegetable oils such as milkweed, soybean, etc., the VI of which is >200.26 Examination of Table 6 also shows that the VI was a better predictor of polarity than kinematic viscosity, because it predicts the polarity of polyamine to be closer to polyacetate than to polyvalerate. Table 6 also shows the derivatization of MWO to polyamine and polyacetate was not an effective method for improving the PP of MWO, whereas conversion to polybutyrate or polyvalerate was more effective.
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ACKNOWLEDGMENTS We are grateful to Dr. Karl Vermillion for the NMR spectra and Linda Cao and Mark Klokkenga for technical assistance.
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REFERENCES
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AUTHOR INFORMATION
Corresponding Author
*(R.E.H.-O.) Phone: (309) 681-6341. Fax: (309) 681-6524. Email:
[email protected]. Notes
Mention of trade names or commercial products in this publication is solely for the purpose of providing specific G
DOI: 10.1021/acs.jafc.5b00598 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
Journal of Agricultural and Food Chemistry
Article
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