Polyacids from Corn Oil as Curing Agents for Epoxy Resins - American

Chapter 14

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Polyacids from Corn Oil as Curing Agents for Epoxy Resins Jian Hong,1 Djavan Hairabedian,1,2 Zoran S Petrović,1 and Andrew Myers*,1 1Kansas

Polymer Research Center, Pittsburg State University, 1701 South Broadway Street, Pittsburg, Kansas, 66762 U.S.A. 2Department of Chemistry, Pittsburg State University, 1701 South Broadway Street, Pittsburg, Kansas, 66762 U.S.A. *E-mail: [email protected].

Vegetable oils are sustainable and environmentally benign sources for new monomers and polymers. Here we report the preparation and use of polyacids derived from corn oil as curing agents for commercial epoxy resins (DER332). First, epoxidized corn oil (ECO) was polymerized by ring-opening polymerization in the presence of boron trifluoride diethyl etherate as catalyst. The effect of catalyst concentration from 1 to 4 mol% was studied. Soluble fraction and thermal analysis showed that the product with highest crosslink density was with 2 mol %. Second, by hydrolysis, polymerized ECO was converted to a polyacid with acid value of 158 mgKOH/g and used for curing epoxy resins. DSC, TGA, DMA and tensile properties of cured epoxy resins were studied.

Introduction With increasing energy crises and environmental concerns, more and more researchers turn their interest from petroleum to sustainable sources, such as vegetable oils and animal fats (1–4). Vegetable oils are triglycerides consisting of five major fatty acids: palmitic (C16:0), stearic (C18:0), oleic (C18:1), linoleic (C18:2), and linolenic (C18:3) (5). The first two are saturated, while the other three contain some unsaturation. Typical corn oil composition is as follows: 13 % of palmitic and 3 % of stearic acids, 31 % of oleic, 52 % of linoleic, and 1 © 2015 American Chemical Society In Green Polymer Chemistry: Biobased Materials and Biocatalysis; Smith, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


% linolenic acids (5). According to a report from the National Corn Growers Association released in 2013, the U.S. is the largest producer of corn in the world with 10,780 million bushels produced annually, which accounts for 32 % of the world’s corn supply (6). Corn contains around 3.8 % of corn oil which is an abundant resource for valuable chemicals. Additionally, wate materials from the corn oil to ethanol process, specifically distillers dried grains with soluble (DDGS), contains up to 15 % corn oil and is widely available for inexpensive markets like animal feed. As we know, triglycerides contain two kinds of functional groups: carbon-carbon double bonds in the fatty acid chain and the ester group in the triglyceride linkage. With different chemical modifications, various useful chemicals can be obtained from vegetable oils. However, aside from methanolysis of triglycerides at the ester group to produce biodiesel (7, 8), most chemicals derived from vegetable oils are through one or multi step reactions of double bonds. Epoxidized vegetable oils are prepared by epoxidation of double bonds, and polyols or polyacids can be obtained by ring-opening of epoxy rings with proper nucleophiles (9–12). Alternatively, polyols can also be produced by ozonolysis and reduction of double bonds (13), while ozonolysis and oxidation of double bonds produces polyacids (14). Erhan et al. (15, 16) reported one method to obtain polyacids involving the reaction of two functional groups: epoxidation of double bonds, epoxy ring-opening polymerization, and hydrolysis of ester groups (Scheme 1).

Scheme 1. Synthesis of polyacids from corn oil

Polyacids can be converted to polysoaps by neutralizing the polyacid with an appropriate base and using it as a surfactant. Furthermore, in the epoxy resins industry, carboxylic acids are one of most popular hardening reagents. Epoxy 224 In Green Polymer Chemistry: Biobased Materials and Biocatalysis; Smith, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

resins cured by polyacids have good flexibility and weatherability. Here we report polyacids prepared from corn oil and use them to cure commercial epoxy resin, DER 332 which is a bisphenol A diglycidylether based epoxy resin.

Experimental Part


Materials Commercial-grade refined, bleached and deodorized (RBD) corn oil was purchased from Wal-Mart (Pittsburg, KS, USA) with the “best used by” date of 1/25/2013. Fluoroboric acid (HBF4, 48 wt % in water), Amberlite IR 120H ion exchange resin, benzimidazole, and boron trifluoride diethyl etherate (BF3·OEt2) were purchased from Sigma-Aldrich. DER 332 epoxy resin was obtained from the Dow Chemical Company. Hydrogen peroxide (H2O2, 30 wt% in water), acetic acid (CH3COOH), and other reagents were purchased from Fisher Scientific. All reagents were used as received. Methods Iodine value (IV) and acid value (AV) were determined by the Hanus method according to IUPAC 2.205 and the indicator method following IUPAC 2.201, respectively. The epoxy-group oxygen content (EOC) was determined by direct titration of epoxy groups with HBr according to the standard method for oils and fats. FT-IR spectra were obtained from Shimadzu IR Affinity-1. A liquid film spread on one KBr plate technique was used. Gel permeation chromatography (GPC) was performed with a Waters gel permeation chromatograph (Milford, MA, USA), consisting of a 515 pump, a 410 differential refractometer, four phenogel 5μ columns (50, 102, 103 and 104 Å) and Millennium software. The flow rate of tetrahydrofuran (THF) eluent was 1 mL/min at 30 °C. Differential scanning calorimetry (DSC) was performed on TA DSC Q100 at a heating and cooling rate of 10 °C/min under nitrogen. Glass transition temperature (Tg) was determined from the second scanning curves. Thermogravimetric analysis (TGA) was performed on a TA TGA Q500. Samples were heated under nitrogen using a heating ramp of 10 °C/min from 25 to 600 °C. Dynamic mechanical analysis (DMA) was carried out on TA 2980 from TA Instruments. The testing was performed in the tension mode under nitrogen at a heating rate of 3 °C/min and a mechanical vibration frequency of 1 Hz. Synthesis of Polyacids Epoxidation of Corn Oil Epoxidation was performed via the in situ peroxyacetic acid method to epoxidize corn oil (9). Corn oil, acetic acid, Amberlite IR-120 H ion exchange resin (25 wt% of oil) and toluene (50 wt% of oil) were added into a 3-neck round bottom flask equipped with a condenser, thermometer, magnetic stir bar and an 225 In Green Polymer Chemistry: Biobased Materials and Biocatalysis; Smith, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


addition funnel (apparatus shown in Figure 1a). The molar ratio of double bonds to acetic acid is 1:0.5. Hydrogen peroxide (molar ratio to double bond 1.5:1) was added drop-wise into the reaction mixture via addition funnel when the reaction temperature reached 60 °C. After all hydrogen peroxide was added, the reaction temperature was increased to 70 °C and maintained for 7 hours. The mixture was cooled to room temperature and ion exchange resin was separated by vacuum filtration. The water phase was separated via a separatory funnel. The organic phase was washed with hot water and hot 5 wt% sodium chloride solution until the pH reached approximately 7. Toluene in the organic phase was removed by rotary evaporation under low pressure (50-70 mmHg) followed by high vacuum (6-10 mmHg) at 70 °C. Quantitative pale yellowish epoxidized corn oil (>98 %) was obtained with EOC of 6.8 % (theoretical EOC was 7.0 % calculated from corn oil IV of 120 g I2/100g).

Ring-Opening Polymerization of Epoxidized Corn Oil (PECO) Epoxidized corn oil was polymerized via ring-opening polymerization according to the literature (15, 16). 40 g epoxidized oil and 400 mL CH2Cl2 were added to a 500-mL round bottom flask fitted with a magnetic stirrer, condenser and dropping funnel. Then appropriate weight of BF3·OEt2 was added drop-wise over 2 min under N2 condition. The solution was stirred at room temperature under N2 condition for 3 hours and 3 mL ethanol was added to the mixture to deactivate the catalyst. The solvent was removed using a rotary evaporator at low pressure (50-70 mmHg) and the polymer was washed twice with hexane. The pale yellowish solid polymer was obtained after drying under high vacuum (6-10 mmHg) at 70 °C.

Hydrolysis of Polymerized Epoxidized Corn Oil Hydrolysis was conducted by following a literature method (16). 10 g PECO was cut into pieces and put into 200 mL of 0.4M NaOH solution. The solution was refluxed for 24 hours. Then the solid was filtered and the solution was cooled to room temperature. The solution was poured into 320 mL of 1.0M HCl and the gel was precipitated. The gel was washed with water three times and 10 % (v/v) aqueous acetic acid two times. 8.0 g (80 % yield) yellowish polyacid with AV of 158 mg KOH/g was obtained after drying under high vacuum (6-10 mmHg) at 70 °C. Polyacid Curing Epoxy Resin (DER 332) Polyacids and DER 332 were mixed with different molar ratio of [Acid] to [Epoxy]. 2 wt% of benzimidazole was used as catalyst. The mixture was poured into a steel mold (100ï¿½100ï¿½1 mm3) and heated at 100 °C for 24 hours. The cured films were characterized with DSC, TGA, DMA, and tested mechanical properties. 226 In Green Polymer Chemistry: Biobased Materials and Biocatalysis; Smith, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

Results and Discussion


Synthesis of Polyacids Polyacids were prepared from corn oil using a three-step procedure. The first step was epoxidation of corn oil. The second step was ring-opening polymerization of epoxidized corn oil with a cationic initiator. The last step was hydrolysis of polymerized epoxidized corn oil. As for second step, the effect of catalyst concentration on crosslink density was studied. Four catalyst concentrations were used: 1, 2, 3, and 4 mol%. It was found that with 2 mol % of catalyst, the product had the lowest hexane soluble fraction (4.3 %) and the highest Tg (-18.8 °C), indicating 2 mol % of catalyt gave product the highest crosslink density. Then, polymerized epoxidized corn oil with 2 mol% catalyst was hydrolyzed to produce polyacid.

Figure 1. GPC curves of corn oil and polyacid.

From GPC profile (Figure 1) it can be seen that the polyacid consists of oligomers. The peak at 37.2 min represents mono-acid. From the peaks’ area, the content of mono-acid can be roughly calculated, which was 20 %. It is higher than common saturated fatty acid content of corn oil (~16 %), which means besides saturated fatty acids, some epoxidized fatty acids did not undergo ring-opening polymerization. TGA curve of polyacid showed an initinal thermal 227 In Green Polymer Chemistry: Biobased Materials and Biocatalysis; Smith, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


decomposition stage under 300 °C, which can be attributed to the mono-acid. The IR spectrum of polyacid (Figure 2) showed the characteristic acid C=O stretch peak at 1711 cm-1 and O-H stretch peak in the range of 3000-3500 cm-1, in addition of strong peak of ether at 1075 cm-1 and weak peak of epoxy at 811 cm-1. At the same time, corn oil’s double bond (3010 cm-1) disappeared, indicating double bonds were converted to epoxy or ether groups. However, IR spectrum of polyacid showed a side peak of ester C=O stretching (1745 cm-1), indicating that part of the triglycerides were not hydrolyzed. This was confirmed by AV of polyacid (158 mg KOH/g), which is lower than theoretical (198 mg KOH/g).

Figure 2. FT-IR spectra of corn oil and polyacid.

Epoxy Resins Cured by Polyacid In order to investigate the effect of polyacid content on properties of final cured resin, three different ratios of [acid] to [epoxy] were used: 1:0.8, 1:1, and 1:1.2. Due to polyacid as kind of weak acid and to accelerate the reaction of acid and epoxy, benzimidazole was used as catalyst. Mixtures of polyacid, DER 332, and catalyst were placed in the mold and kept at 100 °C for 24 hours. The cured resins are yellowish and transparent. Polymers thermal and mechanical properties are listed in Table 1. 228 In Green Polymer Chemistry: Biobased Materials and Biocatalysis; Smith, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

Table 1. Thermal and Mechanical Properties of Polyacids Cured Epoxy Resins Tg (°C) Resin

TGA (°C) Tmaxc

Tensile strength (MPa)

Elongation (%)

334

417

1.25±0.09

61.7±1.8

39

344

417

1.65±0.04

74.8±3.4

44

364

416

3.26±0.20

74.9±4.6

DSC

DMAa

T5%b

Polymer-0.8

11.9

35

Polymer-1.0

17.0

Polymer-1.2

19.3

Tg was read from the peak of Tanδ curve. Temperature at 5 % weight loss. Temperature at maximum degradation rate (read from the peak of Deriv. weight curve).


a

b

c

The Tg of polymers were determined by DSC (Figure 3) and DMA (Figure 4), and found to range from 11.9 to 19.3 °C and 35 to 44 °C (Table 1), respectively. Regardless of the method used, polymer-1.2 exhibited the highest Tg, while polymer-0.8 exhibited the lowest, which can be explained by polymer-1.2 having the highest cross-linking density. As shown in Scheme 2, acid attacks the epoxy ring and forms a hydroxyl group (Step 1 in Scheme 2). The hydroxyl group can also attack another epoxy ring to get polyether (Step 2 in Scheme 2). Due to Step 2, the cured system with higher content of polyacid will result in some pendant acid groups, and further decrease cross-link density.

Figure 3. DSC curves of polyacid and cured epoxy resins. 229 In Green Polymer Chemistry: Biobased Materials and Biocatalysis; Smith, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Figure 4. DMA curves of polyacid and cured epoxy resins.

Scheme 2. Reaction of acid with epoxy

Unlike polyacid, three polymers did not have significant decomposition under 300 °C and all three exhibited similar decomposition behavior (Figure 5). The TGA curves of polymers indicated similar maximum weight loss temperature (~417 °C). It can be explained that most of the mono-acid reacted with epoxy. The decomposition process resulted in the formation of a small char at ~500 °C in all cases. However, polymer-0.8 which has the highest content of polyacid showed the lowest initial decomposition temperature (334 °C at 5 % weight loss), and 30 °C lower than that of polymer-1.2 (364 °C) which has the lowest content of polyacid, This is because polymer-1.2 has the highest cross-linking density. 230 In Green Polymer Chemistry: Biobased Materials and Biocatalysis; Smith, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Figure 5. TGA curves of polyacid and cured epoxy resins. Typical stress-strain relationships of polymers are provided in Figure 6 and mechanical data are listed in Table 1. Polymers behaved as elastomers. Their tensile strengths at break increased as polyacid content decreased. Polymer-1.2 has the highest tensile strength (3.26 MPa) and the highest elongation at break (74.9 %) because of the highest cross-linking density.

Figure 6. Typical stress-strain relationships of polyacid cured epoxy resins. 231 In Green Polymer Chemistry: Biobased Materials and Biocatalysis; Smith, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

Conclusions


Polyacid with AV of 158 mg KOH/g was successfully obtained from corn oil by epoxidation, ring-opening polymerization, and hydrolysis. The epoxy resins cured with this polyacid were transparent, had Tg in the range of 12-19 °C, and exhibited good thermal stability with 5 % weight loss up to 334 °C. With increasing ratio of epoxy from 0.8 to 1.2, the cured epoxy resin’s tensile strength at breaking point increased from 1.25 to 3.26 MPa, while breaking elongation from 58.7 to 74.9 %. The cured epoxy resins are potential packaging materials.

Acknowledgments Financial support by the United States Department of Agriculture (USDA) is gratefully acknowledged (Award No. 2008-38924-19200).

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233 In Green Polymer Chemistry: Biobased Materials and Biocatalysis; Smith, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

Polyacids from Corn Oil as Curing Agents for Epoxy Resins - American

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