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Use of hempseed oil-derived polyacid and rosin-derived anhydride acid as co-curing agents for epoxy materials Ran Li, Pei Zhang, Tuan Liu, Balasingam Muhunthan, Junna Xin, and Jinwen Zhang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04399 • Publication Date (Web): 07 Feb 2018 Downloaded from http://pubs.acs.org on February 9, 2018

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ACS Sustainable Chemistry & Engineering

Use of hempseed oil-derived polyacid and rosin-derived anhydride acid as co-curing agents for epoxy materials Ran Li,1 Pei Zhang,1 Tuan Liu,1 Balasingam Muhunthan,2 Junna Xin,1,* Jinwen Zhang1,*

1. School of Mechanical and Materials Engineering, Composite Materials and Engineering Center, 2001 E Grimes Way, Washington State University, Pullman, WA 99164, USA 2. Department of Civil and Environmental Engineering, Sloan Hall 102, 305 NE Spokane St, Washington State University, Pullman, WA 99164, USA * Correspondence to: JN Xin ([email protected]), JW Zhang ([email protected]).

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Abstract A polycarboxylic acid (HO-polyacid) was prepared from the fatty acids of hempseed oil and used as a curing agent for epoxies. Polyunsaturated fatty acid (C18:2 and C18:3) that composed ~ 77% of the hempseed oil fatty acid (HOFA) were introduced more carboxylic groups by conjugation followed by Diels-Alder addition with maleic anhydride and hydrolysis. This conversion process was further confirmed using pure methyl linolenate as a model compound. In addition, a rosin-derived anhydride acid, maleopimaric acid (MPA) that has a rigid molecular structure, was used as a co-curing agent. Effects of the HO-polyacid/MPA co-curing agents in different ratios on the curing behavior, dynamic mechanical properties, thermal stability and flexural properties of a commercial bisphenol A epoxy were studied. The results suggest the balance of stiffness/flexibility of the cured resins was effectively regulated by varying the cocuring agent ratio.

Keywords: biobased curing agent; hempseed oil; epoxy；maleated linoleic acid; maleopimaric acid.

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Introduction In recent years, an array of vegetable oil (VO)-based polymers including polyurethanes,1-2 unsaturated polyesters,3-4 polyamides5-7 and epoxies8-11 has been studied. For the attempt of epoxy application, epoxidized VOs (EVOs) are the simplest type of derivatives and the most often used in the literature.12-18 However, the internal epoxides of EVOs exhibit low curing reactivity16-17 and the cured materials with EVOs alone as epoxy monomers generally exhibit low performance in terms of glass transition temperature, strengths and moduli owing to the long fatty chains.14,18-19 On the other hand, modification of commercial epoxies with EVO can purposefully adjust the rigidity/flexibility balance of the cured material and result in the versatile applications.12,17,20-21 Recently, we introduced VO fatty acid-derived epoxies with terminal epoxy groups. In one work,11 a diglycidyl ester of dimer fatty acid with two terminal epoxy groups was prepared and used in combination with a rosin-derived epoxy for balanced performance of epoxy materials. In another work,10 a tung oil fatty acid-derived epoxy (TGEC22) with three terminal oxiranes was prepared. The TGEC22 exhibited significantly higher mechanical and thermal properties than the ESO for epoxy application. There are also a few attempts in the literature to prepare curing agents from VOs.22 Warth et al.

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prepared maleated soybean oil and linseed oil by directly reacting triglyceride and maleic

anhydride (MA) under nitrogen atmosphere at 200 °C for 7 h and used the maleated VOs to curing EVO to receive crosslinked polyester products. Tran et al. prepared a maleated soybean oil through a free radical-initiated maleation in a Parr reactor.24 However, only one mole of MA was introduced into the one mole of triglyceride. Jaillet et al. prepared a soybean oil-based polyacid 3



hardener through the thiol-ene reaction with thioglycolic acid.25 Although the conversion of the thiol-ene reaction was about 89%, the Tg of that polyacid cured bisphenol A diglycidyl ether epoxy resin was only -3 °C by DSC or 7 °C by DMA. The low Tg is similarly attributed to the long fatty chains and low functionality in the structures of the VO derivatives. So far, the exploration of VO-based polymer materials has heavily relied on traditional food oils such as soybean oil, sunflower oil and castor oil. It is desirable to select alternative VOs for future development of industrial products. In recent years, as natural fibers are considered alternative reinforcements for polymer composite applications, interest in hemp that was once banned for growing in U.S. but now is relegalized by a number of states, is rekindled. Besides providing the quality bast fibers, hemp also yields hempseed that is a valuable source for protein and oil. Hempseed contains 25 - 35% oil by weight. Hempseed oil (HO) has a significantly higher iodine value than soybean oil (140 - 175 vs. 128 - 143 g/100 g), meaning that the fatty acids of hempseed oil (HOFA) has a higher degree of unsaturation. HOFA contains approximately 80% polyunsaturated fatty acids including linoleic acid (C18:2, 55-60%), linolenic acid (C18:3, 1735%) and stearidonic acid (C18:4, 0-2 %).26 The high degree of unsaturation makes HO a valuable feedstock for chemicals and polymers but less desirable for biodiesel production. However, research of HO for industrial products are scarce. Manthey et al. compared the effects of epoxidized hempseed oil (EHO) and epoxidized soybean oil (ESO) co-monomers, respectively, in combination with a bisphenol A diglycidyl ether epoxy (Kinetix R246TX) as matrix resins for jute biocomposites.27 Mixed triethylenetetramine and isophorone diamine curing agents were employed in that study. The resulting composites with the EHO co-monomer exhibited better properties in terms of mechanical and water absorption properties. However, the incorporation of 4


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either EHO or ESO in the matrix resulted in reductions in stiffness and interfacial adhesion of fiber-matrix for the biocomposites. The same group also investigated the cure kinetics of EHO/triethylenetetramine (TETA) using DSC.28-29 In this work, a new polycarboxylic acid-type curing agent (HO-polyacid) was prepared from the HOFA by introducing more carboxylic acid groups to the fatty chains. The HO-polyacid in combination with maleopimaric acid (MPA) was used as a mixed curing agent for a bisphenol A diglycidyl ether epoxy (DER 332). MPA is the adduct of rosin acid and maleic anhydride and possesses the similar rigidity and chemical stability to many petroleum-based cyclic anhydride type curing agents, due to the hydrophenanthrene structure of the rosin.30-32 It is understood that the properties of the cured epoxy resin can be greatly manipulated by blending the epoxy monomers with rigid and flexible structures to achieve balanced mechanical and thermal properties. The same approach can also be applied to the selection of curing agents. Therefore, blending the rigid rosin-derived and flexible fatty acid-derived curing agents is a way to balance the performance of the epoxy resin system. The curing behaviors, thermal stabilities, flexural properties and mechanical properties of the MPA/HO-polyacid cured epoxy resins were compared with only HO-polyacid cured epoxy resin. Experimental Materials Methyl linolenate (ML, >95.0%) was obtained from TCI America. Potassium hydroxide (90%), maleic anhydride (MA, 99%) and 2-ethyl-4-methylimidazole (95%) were obtained from SigmaAldrich. Ethylene glycol was obtained from Macron Fine Chemicals. Acetic acid, hydroquinone 5



and sodium hydroxide were purchased from Fisher Scientific. Magnesium sulfate anhydrous and hydrochloric acid were obtained from EMD Millipore. Tridecanoic acid was purchased from NuChek Prep, Inc. Hempseed oil (HO) was obtained from Hemp oil Canada Inc. A commercial epoxy resin, DER 332 with an epoxide equivalent weight of 171–175 g eq.−1 was obtained from Dow Chemical Company. Maleopimaric acid (MPA) was synthesized by using a procedure similar to a previous method reported by our group.31 Briefly, 100 g abietic acid is isomerized at 180 °C for 34 h to form levopimaric acid. After the temperature is lowered to 120-140 °C, 40 mL acetic acid and 35 g maleic anhydride are added to the reactor and the reaction is continued at this temperature for 4 h. After cooled down to room temperature, the reaction mixture is added to 100 g acetic acid to receive the MPA precipitate. The acid value of MPA is ~420 mg g-1. Modification reaction of model compound The polyunsaturated FAs constitute up to ~80% of HOFA, and particularly C18:2 takes ~ 60 % of it. The C18:3 only composes a minor part of the HOFA. Therefore, in this study, methyl linoleate (ML) was logically selected as a representative model compound for the investigations of alkali conjugation and subsequent Diels-Alder addition. Scheme 1 illustrates the synthesis route of triacid from ML. As aforementioned in the introduction section, maleated plant oils have been claimed in some literatures.23-24 However, in this study, we noted that the maleated linoleic acid (LA-MA) was partially hydrolyzed from washing of unreacted MA, and purification of it by silica gel chromatograph column would cause further hydrolysis of the introduced maleic anhydride groups. Therefore, after maleation reaction, DI water was added to completely convert the maleated product (LA-MA) to the carboxylic acid form (triacid). 6


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Scheme 1. Synthetic route of triacid from C18:2 through conjugation and maleation reactions. Conjugation of ML Methyl ester of the linolenic acid (C18:2), ML, was used as a model compound to evaluate the conjugation of the double bonds of polyunsaturated fatty acids in HOFA and the subsequent DielsAlder addition with maleic anhydride. The conjugation reaction of ML was performed by following an alkaline method in the literature.33 In a 25 mL flask, 10.0 g ML, 10.0 g potassium hydroxide and 40 mL ethylene glycol were refluxed at 200 °C for 10 h under stirring and nitrogen gas atmosphere. After the reaction temperature was cooled down to 100 °C, 10 mL water was added and the reaction was continued at 100 °C for 30 min. The temperature was cooled and then the reaction mixture was acidified with dilute HCl. The product was diluted with ethyl ether, and the organic layer was washed with DI water for three times and dried over anhydrous magnesium sulfate. After evaporation of the solvent, the conjugated linoleic acid (CLA) was obtained (yield, 92 %) and directly used for the subsequent maleation reaction without further purification. The MS spectrum of CLA exhibited the two major mass species of m/z 303 (M + Na) and m/z 319 (M + K) (Fig. S1). 1H NMR (CDCl3, δ ppm) 6.33-6.25 (m, 1H), 6.04-5.90 (m, 1H), 5.68-5.54 (m, 1H), 7



5.34-5.28 (m, 1H), 2.37-2.32 (t, 2H), 2.16-2.01 (m, 4H), 1.65-1.60 (m, 2H), 1.43-1.27 (m, 14H), 0.94-0.86 (m, 3H).

13C

NMR (CDCl3, δ ppm) 180.84, 134.98, 134.77, 130.34, 130.08, 128.93,

128.80, 125.88, 125.77, 34.35, 33.14, 33.10, 31.98, 31.72, 29.88, 29.66, 29.62, 29.60, 29.52, 29.42, 29.39, 29.35, 29.27, 29.25, 29.24, 29.16, 27.90, 27.86, 24.87, 22.87, 22.80, 14.34, 14.31. Maleation of CLA CLA (10.0 g, 36 mmol), maleic anhydride (10.6 g, 108 mmol), hydroquinone (200.6 mg) and acetic acid (30 mL) were charged into a flask and the reaction was refluxed at 120 °C for 5 h under stirring and nitrogen atmosphere. After reaction, the maleated linoleic acid (LA-MA) was obtained. To prepare the carboxylated linoleic acid, 10 mL DI water was then added into the flask and the reaction was refluxed at 120 °C for another 30 min. After reaction, the mixture was diluted using ethyl ether and the organic layer was washed using DI water for three times to remove any unreacted maleic anhydride and solvent acetic acid. The organic layer was dried over magnesium sulfate, filtered and then concentrated to receive the crude carboxylated linoleic acid (Triacid). The raw product was purified via silica column chromatography (ethyl acetate/ hexane=1:3 v/v) to yield pure triacid (yield, 51%). The MS spectrum of the triacid exhibited the two major masses of m/z 419 (M + Na) and m/z 435 (M + K) (Fig. S2). 1H NMR (CDCl3, δ ppm) 5.93-5.81 (m, 2H), 3.37-3.31 (m, 1H), 3.15-3.10 (m, 1H), 2.73-2.67 (m, 1H), 2.61-2.54 (m, 1H), 2.37-2.32 (m, 2H), 1.69-1.52 (m, 2H), 1.46-1.25 (m, 20H), 0.95-0.82 (t, 3H).

13C

NMR (CDCl3, δ ppm) 180.28,

174.66, 172.04, 131.71, 131.61, 131.49, 45.57, 44.34, 35.32, 35.24, 34.87, 34.23, 33.84, 31.91, 31.85, 31.51, 29.64, 29.47, 29.41, 29.34, 29.28, 29.20, 29.11, 28.01, 27.43, 27.24, 24.85, 24.77, 22.81, 22.78, 14.29, 14.25. 8


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Preparation of hempseed oil-derived polyacid (HO-polyacid) Hydrolysis of hempseed oil In a 1-L three-necked round-bottom flask equipped with reflux condenser, magnetic stirrer, and thermometer, sodium hydroxide (16 g) solution in 140 mL ethanol-H2O (1:1, V/V) was added and heated to 70 °C. After HO (100 g) was dropwise added to the above reaction solution using a dropping funnel, the reaction was continued at 70 °C for 2 h. Next, the reaction system was adjusted to pH 2-3 using 1 M hydrochloric acid, and then the reaction was continued at 70 °C for another hour. After the reaction was cooled down, the organic layer was diluted with ethyl ether, washed with DI water for three times and dried over magnesium sulfate. Finally, ethyl ether was evaporated to giving a viscous mixture of hempseed oil fatty acids (HOFA). The fatty acid composition of HO was determined using GC-FID by a fatty acid methyl esters (FAME) procedure 34.

The acid value of HOFA was 199 mg g-1 (theoretical value: 201 mg g-1). Table 1. Fatty Acid compositions of hempseed oil

HO

C16:0 (%)

C18:0 (%)

C18:1 (%)

C18:2 (%)

C18:3 (%)

others

6.19

2.87

11.54

58.10

19.27

2.03

Preparation of HO-polyacid The conjugation and maleation of HOFA followed the same procedure as aforementioned for ML (section 2.2). For the conjugation reaction, 50 g HOFA, 50 g potassium hydroxide, and 200 mL ethylene glycol were refluxed at 200 °C for 10 h under stirring and nitrogen atmosphere. The 9



product (yield 92 %) was directly used for maleation without purification. For the maleation reaction, 46 g conjugated fatty acids from hempseed oil, 38.6 g maleic anhydride, 138 mL acetic acid and 843.6 mg hydroquinone were refluxed at 120 °C for 9 h under stirring and nitrogen atmosphere. Next, the 20 mL DI water was added into the flask and the reaction was refluxed at 120 °C for 30 min. After reaction, the mixture was diluted using ethyl ether and the organic layer was washed using DI water for three times to remove the unreacted maleic anhydride and the acetic acid solvent. The organic layer was dried over anhydrous magnesium sulfate, filtered and then concentrated. The resulting carboxylated fatty acids were in a brownish liquid form and were named as HO-polyacid. The acid value, density and viscosity of HO-polyacid were 362 mg g-1 (theoretical value: 423 mg g-1), 1.03 g/cm3 at 25 °C and 14.83 Pa.s at 25 °C, respectively. The NMR results of HO, HOFA, Conjugated HOFA (CHOFA), HO-polyacid are discussed in the latter section. Preparation of epoxy samples and test specimens In this study, the HO-polyacid acted as a curing agent was evaluated by curing with a commercial epoxy product, DER332. MPA as a rigid co-curing agent was blended with HOpolyacid in different molar ratios in the study as shown in Scheme 2. The test samples for determinations of dynamic mechanical and flexural properties were prepared with the following procedure. HO-polyacid and MPA were mixed by molar ratios of 6:0, 5:1, 4:2 and 3:3, respectively. The molar ratio of epoxy group to the total of carboxylic and anhydride groups were kept consistent at 1:0.9 in all the formulations. 2-ethyl-4-methylimidazole (1 wt% on the basis of the total weight of the curing agent and epoxy) was used as the catalyst for preparations of all samples. Next, the mixture was mixed at ~50 °C and degassed in a vacuum oven at 50 °C for an hour and then 10


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transferred to a mold that had five cavities. The dimensions of the specimens for DMA and threepoint bending tests were 50 mm × 18 mm × 5 mm and 130 mm × 13 mm × 3 mm, respectively. Curing was performed in a convection oven at 150 °C for 2 h and then at 200 °C for 1 h. The cured specimens were carefully removed from the mold.

Scheme 2. Curing system for epoxy sample preparation. Characterizations 1H

NMR and

13C

NMR spectra of the compounds in deuterated chloroform (CDCl3) were

recorded using a Bruker 400 MHz spectrometer at room temperature. Mass spectra of CLA and triacid were obtained on a Micromass LCTTM mass spectrometer. Fourier transform infrared (FTIR) spectra were performed on a NEXUS 670 FT-IR spectrometer (Nicolet) with a resolution of 4 cm-1 and 32 scans. The FT-IR samples were prepared by dissolving a small amount of compound in acetone, followed by dropping the solution onto a KBr crystal disc and evaporating the solvent completely to form a uniform film. The viscosity of the HO-polyaicd was determined using a Discovery HR-2 rheometer. The fatty acid composition of HO was determined by an Agilent 7890A capillary gas chromatograph equipped with a flame ionization detector and Agilent 7683B auto sampler (GC11



FID). A 0.5 FameWax capillary column with 30 m x 320 um x 0.25 um from Restek was used. The experiment for determination of the fatty acid percentage in hempseed oil was performed following a standard fatty acid methyl esters (FAME) procedure:34 the sample (~200 mg) was dissolved in 5.3 mL methanol, then 0.58 mL 24 N sulfuric acid and 1.0 mL internal standard solution (C13:0, 0.5 mg tridecanoic acid in 1.0 mL methanol) was added. The sample was shook and placed in water bath at 85 °C for 15 min. After cooling for 15 min to room temperature, 2.0 mL of DI water and 2.0 mL of hexane were added before centrifuging for 7 min. The upper hexane layer, containing FAME, was transferred to vials for GC-FID analysis. The injector temperature of GC was set at 280 °C and the oven temperature was initially set at 50 °C and then increased to 240 °C at a rate of 5 °C/min and held for 5 min. Each analysis was repeated in triplicate. For the investigation of curing kinetics, non-isothermal curing of the epoxy resin was carried out using differential scanning calorimetry (DSC) on a DSC 1. STARe System instrument from Mettler Toledo Company under a nitrogen ambiance. Each scan was performed from 30 to 250 °C at heating rates of 2.5, 5, 7.5, 10, 15 °C/min, respectively. Approximately 10 mg of each sample was weighed and sealed in a 40-μL aluminum crucible, and the DSC curing was conducted immediately. Three replicates were performed for each sample. Dynamic mechanical analysis (DMA) of each specimen was implemented by a TA Q800 analyzer from TA on the singlecantilever mode. All samples were tested from -40 to 180 °C at a heating rate of 3 °C/min. Two replicates of the DMA test were performed for each sample. Thermogravimetric analysis (TGA) was performed on a TGA/DSC 1. STARe System instrument from Mettler Toledo Company. Each sample was tested from 30 to 700 °C at a heating rate of 5 °C/min under a nitrogen ambiance. Flexural properties of reinforced epoxy resins were determined by a screw-driven 12


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universal testing machine (Instron 4466) equipped with a 10 kN electronic load cell according to ASTM D 790 at 25 °C. The tests were conducted at a crosshead speed of 10 mm/min with a support span of 48 mm. Five replicates were performed for each sample to obtain an average value. Results and discussion Investigation of the modification reactions via model compound 1H

NMR characterization Fig. 1 shows the 1H NMR spectra of the ML, CLA and triacid. The peak attributed to the

methylene protons (e) between the two double bonds at 2.80-2.73 ppm in the spectrum of ML completely disappeared in the spectrum of CLA. Furthermore, the protons of conjugated double bonds were noted at 6.33-6.25 (m, 1H), 6.04-5.90 (m, 1H), 5.68-5.54 (m, 1H), 5.34-5.28 (m, 1H) ppm (a, b, c, d) in the spectrum of CLA in contrast to the protons of double bonds appeared at 5.31-5.43 ppm (a, b, c, d) in the spectrum of ML. These results indicate that ML was effectively conjugated through the alkali method. The presence of the conjugated double bonds in four clusters of peaks suggests that there may be two types of isomers (trans10, cis12-CLA and cis9, trans11CLA)35 in the product. The new peaks at 3.37-3.10 ppm in the spectrum of triacid were attributed to the protons of the two methylenes (f, e) originated from maleic anhydride. Additionally, the protons (a, d) from the conjugated double bonds were shifted from 6.33-5.28 ppm in the spectrum of CLA to 2.73-2.54 ppm in the spectrum of triacid. All these evidences suggest that the CLA was successfully turned to triacid via Diels-Alder addition under optimal reaction conditions.

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h,i,k-r

s g

b,c

Triacid

j

f,e a,d

CDCl3

k-q,s

g

r CLA CDCl3

h,i

a,b,c,d

j

k-q

ML

r

f

a,b,c,d

g e

CDCl3

8

7

6

5

4

3

h,i

2

j

1

0

ppm

Fig. 1. 1H NMR spectra of methyl linolenate (ML), conjugated linoleic acid (CLA) and triacid. 13C

NMR characterization The chemical structures of ML, CLA and triacid were further verified by 13C NMR (Fig. 2).

The chemical shifts of carbons on double bonds appeared at around 132-129 ppm (a, b, c, d) in the spectrum of ML, while the chemical shifts of carbons on double bonds in the spectrum of CLA appeared at 135-126 ppm (a, b, c, d). In addition, the absence of carbon peaks between the two double bonds at 37 ppm (e) and methyl ester at 52 ppm (f) in the spectrum of CLA compared with the spectrum of ML, indicating the ML can be fully hydrolyzed and conjugated through the alkali method. Compared with the spectrum of CLA, the chemical shifts of carbons on double bonds were shifted to 132-131 ppm (b, c) in the spectrum of triacid. The new peaks at 175 and 172 ppm (t, u) appeared in the spectrum of triacid, which were attributed to the carbons on carbonyl derived 14


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from maleic anhydride. Furthermore, the methylene carbons on maleic anhydride were noted at 46 and 44 ppm (f, e) in the spectrum of triacid. All these results suggested that the triacid was formed.

a,d g n,o,q

35

Triacid

i,j,k,l m,p

30

b,c

r

h

25

20

f,e

CDCl3

s

t,u v t

CLA

r

a,b,c,d

v

CDCl3

a,b,c,d

ML

e

CDCl3

f

r

v

180

Fig. 2.

13C

120

ppm

60

0

NMR spectra of methyl linolenate (ML), conjugated linoleic acid (CLA) and triacid.

The structures of products were further verified by FT-IR and MS. According to the results obtained from FT-IR (Fig. S3), the LA-MA was converted to the triacid. In addition, we used NMR in deuterated dimethyl sulfoxide (DMSO-d6) to determine the proton on the carboxylic acids in the triacid (Fig. S4). Furthermore, the MS (Fig. S2) analysis of final product (triacid) exhibited the two major peaks of m/z 419 (M + Na) and m/z 435 (M + K), which also confirmed the formation of the desired triacid in the maleation reaction. Preparation of polycarboxylic acid curing agent from HO 15



Fig. 3 shows 1H NMR spectra of the hempseed oil (HO), hempseed oil fatty acid (HOFA), conjugated HOFA (CHOFA) and HO-polyacid. The peak at 4.36-4.10 ppm attributed to the methylene protons (f) of the glycerol unit in triglyceride in the spectrum of HO completely disappeared in the spectrum of HOFA. This result suggests that the triglycerides were completely hydrolyzed to receive the fatty acids. The protons of conjugated double bonds were noted at 6.335.28 ppm (a, b, c, d) in the spectrum of CHOFA, in contrast to the protons of double bonds appeared at 5.31-5.43 ppm (a, b, c, d) in the spectrum of HOFA. The disappearance of the peak at 2.80-2.73 ppm (e) attributed to the methylene protons between two double bonds in the spectrum of CHOFA indicate that the C18:2 and C18:3 in the HOFA were effectively conjugated through the alkali method. The new peaks at 3.37-3.10 ppm in the spectrum of HO-polyacid were attributed to the protons of the two methylenes (f, e) originated from maleic anhydride. Additionally, the peaks at 6.33-5.28 ppm attributed to the protons (a, d) of the conjugated double bonds in the spectrum of CHOFA shifted to 2.73-2.54 ppm in the spectrum of HO-polyacid. All these evidences suggest that the CHOFA was successfully turned into polyacid via the Diels-Alder addition.

Scheme 3. Illustration of conjugation isomerization of C18:3 and subsequent Diels-alder 16


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addition with maleic anhydride (the conjugated C18:3 may present in a series of isomer forms, and only two are listed here). It should be mentioned that HOFA is a mixture of different fatty acids (Table 1) with ~ 89% being unsaturated fatty acids, and especially, C18:2 is the dominant one (~58%) followed by C18:3 (~19%). Therefore, the chemical shifts of HOFA in Figure 3 were mainly assigned against C18:2 for the simplicity. In fact, C18:3 would similarly undergo the conjugation conversion and DielsAlder addition like C18:2, except that the triacid from C18:3 still has a single ethylene bond intact in the structure (Ⅰ, Ⅱ in Scheme 3). Because the single double bonds in the triacid from C18:3 and in C18:1 were retained in the HO-polyacid, the chemical shift peaks of double bond protons at 5.73-5.34 ppm were noted as two groups of peaks in the 1H NMR spectrum of HO-polyacid. Moreover, the allyl protons in C18:1 and triacid from C18:3 (Ⅲ, Ⅳ in Scheme 3) were also seen at 2.16-2.01 ppm in the spectrum of HO-polyacid. The acid value of HOFA was 199 mg g-1, while the acid value of HO-polyacid (362 mg g-1) was close to the theoretical value of 423 mg g -1 based on the total conversion of C18:2 and C18:3 to triacids. This result also suggests that the maleic anhydride moiety was successfully introduced to the molecular structures of the conjugated C18:2 and C18:3 via Diels-Alder reaction and then further hydrolyzed to form the polyacid.

17



Fig. 3. 1H NMR spectra of hempseed oil (HO), hempseed oil fatty acids (HOFA), conjugated HOFA (CHOFA) and HO-polyacid. Curing behaviors of the mixed HO-polyacid/MPA curing system The HO-polyacid as curing agent was evaluated by reacting with a commercial epoxy DER332. Furthermore, the acid value of the HO-polyacid is very close to the theoretical value, meaning the composition of HO-polyacid is close to that fatty acid composition of HO. HO-polyacid is a 18


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Page 19 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


mixture in which the triacids derived from C18:2 and C18:3 (~77.4 % in the HOFA, Table 1) are the effective curing agent components. The rest monocarboxylic acids (C16:0, C18:0 and C18:1) though do not form the crosslinks but can still react with epoxies, so they remain in the network structure. Figure 4a shows the exothermic curves of non-isothermal DSC curing of DER 332 with HO-polyacid alone. A smooth single exothermic peak is noted at each heating rate, suggesting that the all carboxylic acid groups on the fatty chains of HO-polyaicd have similar reactivities. On the other hand, some subtle difference in reactivity still exists between the terminal and mid-chain carboxylic acid groups as it is reflected in the relatively broad peak. Because the monocarboxylic acids result in dangling chains in the network structure, in order to achieve the satisfactory properties for the cured epoxy materials, MPA was used as a rigid cocuring agent. Figs. 4b-d show the DSC non-isothermal thermograms of DER332 curing with HOpolyacid/MPA in different molar ratios. For all samples, increasing the heating rate resulted in shifting of the exothermic peak to higher temperature, which is a typical methodological phenomenon. As shown in Figs. 4(b) through (d), addition of the MPA co-curing agent resulted in a new exothermic peak at higher temperature in the DSC curves, which can be ascribed to the reaction of epoxy and anhydride groups.31 Clearly, carboxylic acid is more reactive than anhydride in curing with epoxy.

19



o

o

(a) HO-polyacid/ MPA=6/0

157.7 C

(b) HO-polyacid/ MPA= 5/1

166.5 C

EXO

EXO

o

o

174.4 C

149.6 C

o

Heat Flow (w/g)

Heat Flow (w/g)

158.2 C

o

153.2 C

o

145.9 C

o

15 C/min o

10 C/min

o

o

144.2 C

o

160.5 C 137.4 C

o

15 C/min

o

o

154.9 C

10 C/min

o

7.5 C/min

o

124.6 C

o

126.7 C

o

o

5 C/min

o

5 C/min

166.5 C

o

o

7.5 C/min

143.0 C

o

2.5 C/min

o

2.5 C/min 100

150

200

100

o

(d) HO-polyacid/ MPA= 3/3

154.6 C o

170.3 C

EXO o

o

Heat Flow (w/g)

o 145.4 C 161.6 C

o

o

157.6 C

o

134.9 C

o

15 C/min

o

153.2 C 170.0 oC

EXO

147.2 C o 163.1 C 141.7 C

200

Temperature ( C)

o

(c) HO-polyacid/ MPA= 4/2

150 o

Temperature ( C)

Heat Flow (w/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 33

o

150.2 C

o

o

o

140.5 C 156.5 C o

o 133.2 C 148.9 C

o

15 C/min o

10 C/min

10 C/min

o

o

o

7.5 C/min o 5 C/min

123.9 C

o

o 122.7 C 136.8 C

7.5 C/min o 5 C/min

o

138.6 C

o

o

2.5 C/min

2.5 C/min

100

150

200

100

150

200 o

o

Temperature ( C)

Temperature ( C)

Fig. 4. DSC thermograms of non-isothermal curing of DER 332 cured with HO-polyacid and MPA in different molar ratios (a) 6:0, (b) 5:1, (c) 4:2 and (d) 3:3. Dynamic mechanical properties Fig. 5 shows the changes of storage modulus (E) and damping (tan δ) of the cured epoxy resins with temperature. All compositions exhibited a drastic decrease (from 103 to 10 MPa) in E and one relatively narrow damping peak during the transition from glassy state to rubbery state, suggesting that no phase separation occurred during mixing and curing. As expected, both the E 20


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and Tg (measured from the tan δ peak) of the epoxy resins increased continuously with MPA content in the co-curing agent system. It is well known that the E and Tg of a cured epoxy resin correspond to the stiffness of the crosslinked network structure and chain segment mobility. MPA contains 22 carbon atoms in the molecule and has a similar molecular weight to that of the triacids which are derived from C18:2 and C18:3 FAs and contain 20-carbon atoms in structures. However, the bulky fused ring structure between the carboxylic group and anhydride group in MPA is much shorter and more rigid in comparison with the flexible hydrocarbon chain between the adjacent carboxylic groups and the end carboxylic group in the HO-polyacid. Further, the HO-polyacid mixture still has a significant amount (~23%) of monoacids (including C18:1 and saturated FAs) which do not contribute to the crosslinking but instead plasticize the network structure. Therefore, it is obvious that partially replacing HO-polyacid by MPA would increase crosslink density and reduce the plasticization effect, both decreasing segment mobility of the network structure. As a result, addition of the MPA co-curing agent greatly increased the E and Tg of the cured epoxy product. Fig. 6 shows that the Tg of the co-curing agent cured epoxy increased almost linearly with MPA molar fraction. This result further supports the cured epoxy product was a homogenous single-phase material. On the other hand, the slightly broadening of the tan δ peak (Table 2) associated with glass transition by addition of MPA was probably due to the rigid MPA whose introduction into the network structure complicated the overall relaxation process.

21



HO-polyacid/MPA=6:0, 5:1, 4:2, 3:3 (b)

HO-polyacid/MPA=6:0, 5:1, 4:2, 3:3 (a)

6:0

1000

5:1

3:3 1

100

5:1

4:2 3:3

Tan Delta

Storage modulus (MPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 33

4:2

6:0 10

0 1 40

80

120

160

40

o

Temperature ( C)

80

120

160

o

Temperature ( C)

Fig. 5. DER 332 cured with HO-polyacid/MPA in different molar ratios: (a) storage modulus versus temperature and (b) tan δ versus temperature.

Fig. 6. Change of Tg with MPA content in the co-curing agent system.

Table 2. Effects of co-curing agent on thermal properties of the epoxy resins 22


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HO-polyacid/MPA (mol/mol) 6:0 5:1 4:2 3:3

Tg (C) 60.5 78.3 101.9 135.0

DMA Half-peak width of tan δ (C) 18.5 21.5 21.7 23.6

TGA T5% (C)

T10% (C)

337.6 350.8 357.0 360.1

360.0 370.1 374.4 375.7

Thermal stability Fig. 7 shows the TGA results of the DER332 cured with HO-polyacid/MPA in different molar ratios. The temperature of 5% (T5%) and 10% weight loss (T10%) are given in Table 2. In the initial stage, the weight loss of all cured epoxy resins showed very similar values, which indicate the thermal stabilities of these cured epoxy resins were similar. In addition, increasing the MPA cocuring agent content resulted in the enhancement of T5% and T10% of the epoxy resins. These results suggest that the addition of the rigid MPA co-curing agent improved thermal stability of the epoxy resins.

23



HO-polyacid/MPA=6/0 HO-polyacid/MPA=5/1 HO-polyacid/MPA=4/2 HO-polyacid/MPA=3/3

100

0.00

Driv. Weight (mg/s)

80

Weight %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 33

60

40

-0.01

-0.02 20


0 300

400

500

600

300

o

400

500 o

Temperature ( C)

Temperature ( C)

Fig. 7. TGA curves of the DER 332 cured with HO-polyacid/MPA in different molar ratios. Flexural properties Fig. 8 and Table 3 show the effects of addition of the MPA co-curing agent on flexural properties of the cured DER332 samples. Modulus of the cured resin increased continuously from 0.84 to 1.49 GPa as the HO-polyacid/MPA molar ratio varied from 6/0 to 3/3. On the other hand, flexural strength increased rapidly with MPA content and reached a maximum at the HOpolyacid/MPA molar ratio of 4/2, while the yield strain showed little change with MPA addition. In fact, the two epoxy samples cured with HO-polyacid/MPA (= 5/1 and 4/2) not only exhibited significant improvement in strength (being 71.36 and 74.44 MPa) but also retained the good toughness of the neat HO-polyacid cured epoxy sample as all three samples did not break during the bending testing. However, both strength and yield strain exhibited great drops as the MPA content increased further to a HO-polyacid/MPA ratio of 3/3. At the higher level of MPA, i.e., with a HO-polyacid/MPA ratio of 3/3, the cured epoxy displayed a brittle behavior (Fig. 8) as it broke at a strain of 4.72% without obvious yielding and exhibited a reduced flexural strength 24


Page 25 of 33

(66.91 MPa). The SEM images (Fig. S6) of the fracture surface of DER 332 cured with HOpolyacid/MPA=3:3 also show a brittle failure with a homogeneous appearance. The variations of flexural properties with the addition of MPA were attributed to the same reasons as the aforementioned ones influencing dynamic mechanical properties, i.e., increased crosslink density and reduced plasticization with MPA addition.

75

Break

50

Stress(Mpa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


25


0 0

2

4

6

8

10

12

14

Strain(%) Fig. 8. Flexural stress-strain curves of the cured DER 332 epoxy samples of different compositions (the HO-polyacid/MPA=3/3 (mol/mol) sample broke but others did not break during testing).

25



Page 26 of 33

Table 3. Effects of co-curing agent on flexural properties of the cured DER 332 resins HO-polyacid/MPA (mol/mol) 6:0 5:1 4:2 3:3 *Data

Flexural strength (MPa) 57.75±2.25 71.36±4.90 74.44±0.28 66.91±3.32

Elastic modulus (GPa) 0.84±0.02 1.05±0.05 1.14±0.02 1.49±0.12

Yield strain (%) 9.35±0.02 9.62±0.62 8.86±0.38 4.72±0.05*

was obtained at break point, others were obtained at yield point.

Conclusions A new polycarboxylic acid-type (HO-polyacid) curing agent was successfully synthesized from hempseed oil fatty acids (HOFA). The polyunsataured fatty acids (C18:2 and C18:3) that accounted for ~77% of HOFA were first conjugated under the catalysis of alkali followed by DielsAlder addition with maleic anhydride and subsequent hydrolysis, resulting in a mixture of carboxylated fatty acids. Methyl ester of linolenic acid that accounts for ~58% of HOFA was used as a model compound to study the conjugation and maleation reactions and optimize the reaction conditions. The formation of HO-polyacid derived from HOFA was confirmed by NMR and titration methods. Curing of a commercial epoxy (DER 332) using HO-polyacid indicates the terminal and mid-chain carboxylic acid groups exhibited similar curing reactivity, while the resulting material showed relatively low modulus, strength and Tg owing to presence of some dangling fatty chains in the network structure. These dangling chains were resulted from the monocarboxylic acids in HO-polyacid which could react with epoxy but could not form the crosslinks. Addition of the rigid MPA as a co-curing agent could greatly regulate the performance of the cured resins. The modulus of the cured resin increased continuously with MPA content in the co-curing agents, ranging from 0.84 to 1.49 GPa as the HO-polyacid/MPA molar ratio varied 26


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from 6/0 to of 3/3, meanwhile Tg increased greatly from 60.5 to 135.0 °C. On the other hand, flexural strength increased rapidly with MPA content and reached a maximum at the HOpolyacid/MPA molar ratio of 4/2, while yield strain showed little change with MPA addition. However, both strength and strain exhibited great drops as the MPA content increased further. In summary, the hempseed oil-based polycarboxylic acid in combination with a rosin-derived anhydride offers a good solution to regulate the mechanical and thermal properties of epoxy resins and may find potential applications in coatings and adhesives.

Supporting Information MS spectrum of conjugated linoleic acid (CLA), MS spectrum of triacid, FTIR Spectra of Methyl linolenate (ML), Conjugated linoleic acid (CLA), Maleated linoleic acid (LA-MA) and Triacid, 1H

NMR Spectrum of Triacid in DMSO-d6, SEM of DER 332 cured with HO-polyacid and MPA

in molar ratios of 3:3. (Magnification= 500× and 1000×). (PDF)

Acknowledgement The authors are grateful for the financial support from the Federal Highway Administration (FHWA, project # DTFH61-14-C-00013).

References 27



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For Table of Contents Use Only

31



A new type of polycarboxylic acid (HO-polyacid) prepared from the hempseed oil was utilized as a curing agent for epoxy resin and the rosin-derived anhydride acid (MPA) as a cocuring agent was adopted for adjusting the stiffness/flexibility of the cured resins.

32


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Graphic Abstract

A new type of polycarboxylic acid (HO-polyacid) prepared from the hempseed oil was utilized as a curing agent for epoxy resin and the rosin-derived anhydride acid (MPA) as a co-curing agent was adopted for adjusting the stiffness/flexibility of the cured resins.


Use of Hempseed-Oil-Derived Polyacid and Rosin ... - ACS Publications

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