versión impresa ISSN 0327-0793
Lat. Am. appl. res. vol.41 no.1 Bahía Blanca ene. 2011
Preparation of oleochemical polyols derived from soybean oil
A.A. Beltrán and L.A. Boyacá
Abstract - The hydroxylation of epoxidized soybean oil was performed in a well-mixed agitated reactor under isothermal operation. Two agents for hydroxylation were evaluated using a homogeneous catalyst. The tests were positive and the best results are achieved with molar relations of alcohol to epoxide of 4:1 for ethanol (T=70°C) and 6:1 for ethylene glycol (T=80°C), using sulfuric acid (2 % p/p) as a catalyst. The hydroxyl numbers of the oleochemical polyols obtained are 120 and 331 mg KOH/g, with theoretical functionalities of 2 and 6 for ethanol and ethylene glycol soy-based polyols, respectively.
Keywords - Soybean Epoxide; Oleochemical Polyester Polyols; Hydroxylation with Ethylene Glycol.
In the last decades there has been a growing concern about the industry dependence on petroleum and its derivates. The stability of that market is under analysis, taking into account the price of crude oil (around 18¢/lb), more than a 150% increase from 1985 to 2007 (Zhang et al., 2007). New green technologies have to be developed assuring the use of renewable resources as an alternative to petrochemical products. The epoxidation of oils is a well known technique used in the production of binders, coatings, adhesives and sealants. It is also possible to hydroxylate the epoxidized oil resulting in a polyol structure, a process that has been recently introduced for use in polyurethane foams, reducing the environmental impact (Paster et al., 2003).
In this context, bio-polyols can be obtained from agricultural products like vegetable oils, wood, carbohydrates (cellulose and starch) and lignine (Latere et al., 2005). Oleochemical polyols are a great alternative for the polyurethane industry in applications where hydrofobicity, hardness, flexibility, and mechanical and chemical resistance are needed: foams, coatings and floorings (Höfer et al., 1997). Although most triglycerides contain unsaturations, few oils naturally contain other groups. Therefore, it is necessary to perform the hydroxylation of double bounds through one of four main approaches (Guo and Petrović, 2005; Latere et al., 2005): a) epoxidation followed by the ring-opening, almost secondary hydroxyl groups are generated; b) Hydroformylation and reduction of aldehydes oils; c) Transesterification with different polyols; d) Microbial or enzymatic conversion. It is always desirable to obtain the highest conversion in the preparation of polyols because of the requirements for polyurethane rigid foams (hydroxyl numbers above 300 mg KOH/g) (Guo and Petrović, 2005; Vilar, 2004); for that reason most investigations are focused to increase hydroxyl numbers to improve functionality values. The average molecular weight of oleochemical polyols obtained by this way is between 250 and 2500; due to the low viscosity and good compatibility with methyl-di(phenyl isocyanate) (MDI), these polyols are particularly useful to produce PU rigid foams (Hill, 2000). The value has increased more than twice since the price of soybean crude oil is 28 ¢/lb (Zhang et al., 2007) while epoxidized soybean oil is about 48 - 1 US$/lb (Paster et al., 2003) with a growing market of ~70 000 ton/year (Rangarajan et al., 1995). The aggregate value of PU foams is even higher, reaching prices up to 3 US$ (Paster et al., 2003; Burridge, 2003).
About the conditions of hydroxylation, Petrović et al. (2003) patented the alcoholysis of epoxidized oils using fluoboric acid as a catalyst (48%, < 2%p/p) with different hydroxylated molecules like water, monoalcohols (methanol, ethanol, propanol and butanol) and their mixtures, using molar excess to avoid polymerization and products of higher viscosities. They obtained yields of 85-95% and hydroxyl numbers (OH numbers) of 110-213 mg KOH at 25-50°C, using molar relations between 1:1 and 10:1 of a mixture of alcohols (methanol and isopropyl alcohol with water) to epoxide group.
Other authors (Zlatanić et al., 2004) reported conversions of 76-84% for the hydroxylation of different epoxidized oils using boiling methanol (in the presence of fluoboric acid) as a catalyst. They obtained a soybean oil polyol with functionality of 4.5, molecular weight of 1249, and hydroxyl number of 179.3 mg KOH/g. For the hydroformylation of epoxidized soybean oil (Guo et al., 2002), they reported OH number of 230 and 160 mg KOH/g with Rh and Co catalysts, functionalities of 4.1 and 2.7, and molecular weights of 1006 y 962, respectively. Guo et al. (2000b) studied other catalysts such as clorhidric and bromhidric acids, hydrogen or methanol reaching functionalities between 3.5 and 4.1, with OH numbers 182-212 mg KOH/g. For example, using a 6:1 molar relation of methanol: epoxide, and a catalyst concentration of 1.7% p/p the reaction time is just 1 hour at 50ºC. Other works of the same authors (Guo et al., 2000a, 2006) present OH number of 215 and 228 mg KOH/g for polyols obtained from methanolysis and hydroformylation processes over epoxidized soybean oil.
In this research, we obtained an oleochemical polyol from soybean oil through the in situ epoxidation of the oil with acetic acid. Then, we made an opening oxirane ring to introduce two hydroxyl groups in each unsaturation (Fig. 1), a well-known and used method. This hydroxylation reaction is catalyzed in acid medium with non solvent required. (Höfer et al., 1997). The catalysts can be mineral acids (sulfuric, clorhidric or phosphoric) and even some organics, with molar excesses of 1:1-1:10 between the agent of hydroxylation and the epoxide group, using temperatures of 20-100ºC, better at 50ºC (Kluth et al., 1988; Petrović et al., 2003).
Figure 1. Hydroxylation reaction of soybean epoxide with a. Ethanol, b. Ethylene glycol.
Soybean oil (iodine value of 130g I2/100 g oil) was used for the epoxidation with sulfuric acid (96% p/p), hydrogen peroxide (50% p/p) and acetic acid (98% p/p), all reagents provided by Merck S.A. For the hydroxylation step, two types of alcohol were used: ethylene glycol (99.9% p/p) and ethanol (99.9% v/v), both from Panreac S.A.
The reaction system (Figure 2) is formed by a jacket glass vessel of 500 mL with a mechanical stirred system connected to a thermostated bath (to control the temperature of the jacket). Other equipments are: a copper coil for water-cooling purposes, and a peristaltic pump for the addition of the catalyst solution.
Figure 2. Reaction system used for the epoxidation and the hydroxylation of soybean oil.
The soybean oil was placed into a glass reactor and the temperature of the system was raised until 80°C. The catalyst solution was prepared dissolving the sulfuric acid into the hydrogen peroxide. The reaction time is started with the addition of that solution into the system reactor (8.8 mL/min) with the acetic acid. The reaction took place for about 2.5 h to obtain an oxirane oxygen content (OOC) of 6.4% using concentrations of H2O2 (25% of molar excess), CH3COOH (5% p/p) and H2SO4 (2% p/p). The epoxidized oil was purified with repeated washes with water until it reached an acid value of <0.70 mg KOH/g epoxide.
Twelve hydroxylation test were performed varying temperature and molar relation values (Table 1) using ethanol and ethylene glycol. First, the epoxide was placed into the glass vessel and the catalyst (sulfuric acid, 2% p/p) dissolved into the alcohol and aggregated into the reactor when the system had reached the required temperature. The reaction took place between 2 and 6 hours to obtain a white mass that might have to be purified with water washes and finally dried in a vacuum system (22 inch Hg) until it obtained a transparent product with 0.14 % p/p of moisture.
C. Analytical Determinations
The reactions were followed by the determination of oxirane oxygen content (OOC) according to international standards, AOCS CD9-57 and hydroxyl number determination from ASTM D4274-94. The acid value of the epoxide was determined using NTC 2366 technical norm and polyol humidity was measured using ASTM D44672-00. All samples (from soybean oil, epoxide and polyols) were characterized with infrared spectroscopy (FT_IR) using KBr cells in a Paragon 500 device, series 1000, Perkin Elmer (Software Spectra for Windows).
III. RESULTS AND ANALYSIS
A. Hydroxylation of epoxidized soybean oil
The reactions were followed through the decreasing of the oxirane oxygen content. Table 1 shows the minimum OOC obtained in each case as well as the time
Table 1. Conditions used for hydroxylation tests
Figure 3. Hydroxylation with ethanol varying alcohol:epoxide molar relation (M.R.) at: a). T=60; b). T= 70ºC.
Figure 4. Hydroxylation with ethylene glycol varying alcohol:epoxide molar relation (M.R.) at: a). T=60 ºC; b). T=70ºC.
At 60ºC (Fig. 3) it can be suggested that an induction period required for test E2 that is not observed for Rx E1 because the higher molar excess used dilutes the system and it can diminish the reaction rate. Even a higher relation molar (4:1) generates a lower chemical kinetic (test E3) so any induction period is evident. This period could happen due the mass transfer phenomena that occurred at the interphase reaction and later studies are needed for a full understanding.
According to E4-E6 tests (Fig. 3b) hydroxylations with ethanol at 70ºC required appreciable low reaction times (<3h), showing the influence of temperature on the kinetic rate. These reactions also present little induction times that get higher increasing the molar relation used (test E5 and E6). The best result (E6) is obtained with the highest ethanol:epoxide molar relation (4:1) in a convenient reaction time (45 minutes).
In the case of ethylene glycol (Fig. 4) there is a similar behavior between temperature and molar relations but the minimum COO values are obtained using the lesser molar excess (6:1). Temperature positively influence the reaction time, it decreases from 4 h (at T=60ºC) to short periods (t<1.7h at T=80ºC).
The minimum molar relation (6:1) obtained the best result (G4) in a reaction time of 1hour. As expected, these hydroxylations needed more time than those with ethanol, due the steric limitations of ethylene glycol molecule.
B. Infrared spectroscopy
IR spectroscopy allows us to corroborate the chemical nature of the products and also to demonstrate that the reaction went from epoxidation to ring-opening to form the OH groups. For soybean oil, the principal function is the ester group due to the large bonds of triglycerides that compounds it. Esters have two strong characteristic absorption bands arising from C=O and C-O stretching. In this case (Fig. 5a), carbonyl group appears in the 1750-1735 cm-1 region while the second one appears in the 1210-1163 region as a quite strong and wide band.
Figure 5. IR spectra for: a) soybean oil; b) epoxidized oil; c) E6 ethanol polyol; d) G4 ethylene glycol polyol.
On the other hand, the alkene function is also very important due to the high unsaturation (iodine value) of the soybean oil according to its average fatty acids composition: oleic (C18:1) 23%, linoleic (C18:2) 54% y linolenic (C18:3) 8% (Zlatanić et al., 2004). The sp2 hybridation of alkene function (C-H bond) is waited just above of 3000 and it is clearly observed in 3009 such a high intense band. Also, unsaturation absorption (C=C bond) appears in 1640-1680 as expected (when it is not conjugated and not corresponds to any aromatic group).
Comparing with Fig. 5b, epoxidation reaction is evident due to the disappearance of the bands for the alkene group (3009 and 1654 in Fig. 5a) along with a new band that shows up in 823 for this spectrum, expected in 822-833 as a split signal (Vlček and Petrović, 2006). A main bands of the spectrum, called fingerprint region, are located between 1500 and 500. The superposition of this region between both, oil and epoxide spectra, allow us to analyze a possible chemical degradation of triglycerides chains, which in this case does not happen (Fig. 5a and 5b).
The analysis for the obtained polyols, based on ethanol and ethylene glycol, is very similar. The epoxide band in 823 disappears for both cases, and two new bands have to appear due to alcohol function. First, O-H stretching absorption usually occurs near 3333 as a broad and rounded band. The second absorption, which corresponds to C-O bond, indicates the type of alcohol depending on where it appears: 9.5 primary, 9.0 secondary and 8.5 for tertiary alcohols. Ethanol-based polyol (Fig. 5c) shows two signals in 1097 and 1163 for secondary and tertiary alcohols while (ethylene glycol)-based polyol (Fig. 5d) only present a quite intense band in 1110 for secondary alcohols. So it can be implied the presence of residual alcohol (as a reagent) that does not react during hydroxylation. This can be establish taking into account the kind of carbons next to hydroxyl chemical function before (raw material) and after (those formed over triglycerides chains), according to the expected chemical structures (Fig. 1).
Through IR spectroscopy it is not possible to differentiate between mono alcohols, diols and polyols, merely it could be waiting a deeper and intensified band in 3000 as it can be observed in 3411 for ethylene glycol polyols that gets the higher hydroxyl number.C. Characterization of polyols
The percentage yield for the hydroxylation reaction was 70-75%. The properties obtained for both polyols, E6 and G4 are listed in the Table 2. The number-average functionality for a polyol (Zlatanić et al., 2004) is:
|:||Number-average functionality for polyol|
|:||Theoretical molecular weight for polyol|
|OH#||:||Hydroxyl number (mg KOH/g polyol)|
|MWSBO||:||Molecular weight of soybean oil = 872|
|fSBO||:||Functionality of soybean oil = 4.49|
Table 2. Comparison of oleochemical polyols properties obtained from soybean oil*.
Accepting the proposed structure for both polyols (Fig. 2), the functionality of each polyol can be determined by using Eq. 1 and 2. Note that these values are theoretical functionalities calculated on the assumption of the molecular weights for soybean oil and both polyols:
a) Polyol E6: OH number = 120 mg KOH/g
Polyol G4: OH number = 331 mg KOH/g
We obtained a hydroxyl number of 120 and 331 mg KOH/g for oleochemical polyols using ethanol and ethylene glycol respectively. The properties of the soybean polyol from ethanol are comparable with patented results (Kluth et al., 1988) reporting similar alcohol:epoxide molar relations (3.3:1) but with higher reaction times: 3.3 h (OH number of 105.6 mg KOH/g) and 5 h (OH number of 134.8 mg KOH/g); although with a lower catalyst concentration (H2SO4, 0.4% p/p). These polyols are widely used as prepolymers, not only for adhesives and sealants, but also as a base component for polyurethane foam formulations (Ramirez et al., 2003; Kluth et al., 1988). The hydroxyl number achieved using ethylene glycol, 331 mg KOH/g, is almost superior to among all references, giving a very competitive biopolyol, taking into account the maximum value reported in the literature, Sovermol® POL 912 (345-360 mg KOH/g) (Höfer et al., 1997). The polyol obtained from ethylene glycol is suitable to use directly in a rigid PU foam formulation, where high functionality is required ( = 4 or 6) (Guo and Petrović, 2005; Vilar W., 2004).
A successful production of polyester polyols was achieved through epoxidation and subsequent hydroxylation of soybean oil for polyurethane applications. Two agents of hydroxylation were used in twelve hydroxylation tests and the best results were obtained using alcohol:epoxide molar relations of 4:1 and 6:1 with ethanol (T=70ºC) and ethylene glycol (T=80ºC), respectively, with sulfuric acid (2% p/p) as a homogeneous catalyst. The physicochemical properties of the soybean polyols are similar to the commercial products and, remarkably, the (ethylene glycol)-based polyol obtained is appropriate for the production of rigid PU foam, due to its high theoretical functionality (6.0).
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Received: August 25, 2009.
Accepted: April 5, 2010.
Recommended by Subject Editor Ricardo Gómez.