doi.org/10.26434/chemrxiv.7295654.v1

Synthesis and Characterization of Crosslinked Polymers fromCottonseed OilRangana Wijayapala, Satish Mishra, Bill Elmore, Charles Freeman Jr, Santanu Kundu

Submitted date: 04/11/2018 • Posted date: 05/11/2018Licence: CC BY-NC-ND 4.0Citation information: Wijayapala, Rangana; Mishra, Satish; Elmore, Bill; Freeman Jr, Charles; Kundu, Santanu(2018): Synthesis and Characterization of Crosslinked Polymers from Cottonseed Oil. ChemRxiv. Preprint.

In this study, crosslinked polymers were synthesized from cottonseed oil. Unsaturated fatty acids, the majorcomponents of cottonseed oils, were initially epoxidized. A network polymer was then formed by crosslinkingthe epoxidized oil with maleic anhydrate. Mechanical properties of these polymers were altered by varying theamount of maleic anhydrate. These polymers have a tensile modulus of the order 1 MPa and are stable in theacidic and alkaline environment.

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Synthesis and Characterization of Crosslinked Polymers from Cottonseed Oil Rangana Wijayapala1*, Satish Mishra1, Bill Elmore1, Charles Freeman2, Santanu Kundu1*

1 Dave C Swalm School of Chemical Engineering, Mississippi State University, Mississippi State, MS

39762, USA

2 School of Human Sciences, Mississippi State University, Mississippi State, MS 39762, USA

Correspondence to: Rangana Wijayapala, Santanu Kundu (E-mail: rangana@che.msstate.edu, santanukundu@che.msstate.edu )

((Additional Supporting Information may be found in the online version of this article.))

ABSTRACT

Traditional plastics are usually obtained from petroleum feedstocks. Petroleum-based plastics are not

readily degradable, residing in landfills for a long time without significant deterioration. In this study,

crosslinked polymers were synthesized from cottonseed oil. Unsaturated fatty acids, the major components

of cottonseed oils, were initially epoxidized. A network polymer was then formed by crosslinking the

epoxidized oil with maleic anhydrate. Mechanical properties of these polymers were altered by varying the

amount of maleic anhydrate. These polymers have a tensile modulus of the order 1 MPa and are stable in

the acidic and alkaline environment. Our results provide a new synthetic strategy to obtain a network

polymer from cottonseed oil.

INTRODUCTION

Polymers are traditionally being synthesized from petroleum-based chemicals. As polymers with various

chemical structures suitable for particular applications are continually being developed, the demand for

polymeric materials is increasing. When accompanied by the increasing societal pressure to move away

from fossil fuel resources, the need for finding alternative sources for polymer synthesis is important.1,2 The

renewable and carbon-neutral sources are highly desired due to the environmental aspects, thereby making

plant-based materials an excellent choice for the development of polymeric materials from sustainable

products with potentially “tuneable” biodegradability. A variety of complex biomolecules including

cellulose, starch, proteins and natural oils have been studied extensively.3,4 Among those, plant-based oils

consist of chemical structures uniquely suited for the synthesis of thermoset and thermoplastic polymers

through a relatively straightforward process. Unsaturated fatty acids present in plant oils provide an

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excellent starting molecular “backbone” upon which to build and propagate a polymerization reaction.5,6

Plant oils are non-toxic, biodegradable, and abundantly available. These economic and environmental

advantages of the plant oils make them attractive alternatives to petroleum-based materials.1,7

When considering all the plant-based oil, cottonseed oil is in the group which contains a high amount of

unsaturated fatty acid.8 Cottonseed oil consists of 65-70 wt% of unsaturated fatty acids, and among them,

18-24% is monounsaturated (oleic) and 42-52% is polyunsaturated (linoleic)—providing a 2:1 linoleic to

oleic ratio. Approximately, 26-35% of the fatty acids are saturated (palmitic and stearic).9-12 The unsaturated

alkenes present in the fatty acid open the possibility of polymerizing the fatty acid molecules via an

esterification reaction. However, to achieve this polymerization, alkenes need to be converted into more

active functional groups.

The epoxidation reaction can convert alkenes into oxirane rings, which are favorable for performing

nucleophilic substitution reactions.13-15 An oxirane ring can be converted into a number of different

functional groups, such as alcohols, glycols, alkanolamines, carbonyl compounds, olefinic compounds, and

may subsequently be incorporated into polymer syntheses of polyesters, polyurethanes, or epoxy resins

depending upon the desired applications.14-17 Different reactants and catalysts have been studied for the

epoxidation reaction step. In one instance, percarboxylic acids were used with an acid catalyst for the

epoxidation oleic acid methyl ester.18 Also, chemo-enzymatic epoxidation of fatty acids was reported with

immobilized Candida antarctica lipase B enzyme and H2O2.19 Epoxidation of methyl esters, obtained from

high-oleic sunflower oil was studied using titanium-grafted silica catalyst.20 Therefore, it is clear that

epoxidation of vegetable oils has received considerable attention and efficient epoxidation reaction

pathways are readily available.

Previous publications have been reported the feasibility of synthesis of the plant oil based polymer primarily

using soybean oil. Different polymerization routes have been attempted. Cationic copolymerization of

soybean oil with styrene has been conducted resulting in a rigid thermoset polymer.21 Another study

reported the synthesis of hybrid latexes from a soybean oil-based waterborne polyurethane and acrylics via

emulsion polymerization.22 In a few instances, polymerization of cottonseed oils has been reported. For

example, styrene was copolymerized with the cottonseed oil molecules.23,24.

The purpose of this study was to synthesize a rigid and stable polymer using cottonseed oil. To achieve this

goal, polymers were synthesized by crosslinking epoxidized cottonseed oil with maleic anhydride. In the

initial step, epoxidation of cottonseed oil was conducted using hydrogen peroxide. Unsaturated sites in the

fatty acids were converted to epoxides during this reaction step. Following the epoxidation reaction, maleic

anhydride was introduced to the reaction mixture. Maleic anhydride can bind to the epoxidized position in

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the fatty acid chain to produce rigid crosslinked polymeric materials. Fourier-transform infrared

spectroscopy (FTIR), thermogravimetric analysis (TGA), and nuclear magnetic resonance spectroscopy

(NMR) analysis were used to analyze the products after each reaction steps. The chemical synthesis

framework established in this study demonstrated the production of value-added products from cottonseed

oil. This reaction scheme can also be efficiently adopted with all other plant oils with unsaturated fatty acid.

EXPERIMENTAL

Materials

The purified cottonseed oil (CSO) with an iodine value between 109 and 120 and with a density of

0.92 g/cm3 was purchased from Fisher Scientific. Hydrogen peroxide (50% v/v Sigma-Aldrich) was used

for the epoxidation reaction along with glacial acetic acid (Sigma-Aldrich) and sulfuric acid (Sigma-

Aldrich).

Epoxidation of Cottonseed oil

A three-neck round-bottom flask (250 mL) with a reflux condenser was used to perform the epoxidation

reaction. Cottonseed oil (94 g) was heated to 55 °C using a heating mantle. After the cottonseed oil reached

to the desired temperature, glacial acetic acid (9.8 mL) and sulfuric acid (0.8 mL) were introduced to the

flask—maintaining the temperature (55 °C) for another 10 min. Hydrogen peroxide (58 mL) was added to

the reaction mixture drop by drop using a syringe to initiate the epoxidation reaction. After adding all H2O2,

the reaction temperature was increased to 70 °C and was maintained for 8 h. Epoxidized CSO was separated

by using a solvent (ethyl ether) extraction process. Collected product was characterized by using FTIR

(Thermo Nicolet Nexus 6700 instrument), and the NMR (Bruker AVANCE III 500 MHz). Also, iodine

analysis was conducted using ASTM D5554-15 method to determine the number of unsaturated carbon

bonds in the epoxidized cottonseed oil.

Crosslinking of epoxidized cottonseed oil (ECSO)

Epoxidized cottonseed oil was used for the crosslinking reaction with maleic anhydride. The first stage of

the reaction was conducted under a nitrogen blanket by maintaining a continuous flow of nitrogen into the

three-neck round-bottom flask (250 mL). A condenser was fitted to the round-bottom flask assembly to

collect vapors produced during the first reaction phase (i.e. epoxidation step).

Initially, an appropriate amount of maleic anhydride was dissolved in the ECSO and reaction was conducted

at 80 °C with stirring for 3 h. The amount of maleic anhydride used was determined by maintaining ECSO:

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maleic anhydride (mal) mole ratios as 2:3, 2:4, 2:5, 2:6 and 2:7. At the end of the 3 hours, a viscous liquid

from this reaction was obtained and transferred into a glass dish.

The second step of the reaction was conducted under the vacuum. The reaction temperature was increased

up to 150 °C inside a vacuum oven and maintained for 4 h. At this reaction step, the formation of a rigid

polymeric specimen was noted. The sample was removed from the glass dish for further analysis. FTIR

analysis was used to characterize the chemical structure of the produced polymer film. Thermal stability of

the polymers was investigated using the TGA (TA instruments Q600) and DCS (TA instruments Q2000)

analysis.

Investigation of Mechanical Properties

For tensile testing, dogbone coupons with dimensions prescribed in the ASTM D638 standard were

obtained by using a punch. The tensile testings were conducted using a custom built tensile test set-up

(Figure S1).25 Here, the dogbone sample was stretched using a moving stage (M414, Physik Instrument).

Force was measured by recording the deflection of a cantilever using a micro-epsilon capacitance based

displacement sensor (DT 6220 ). Using a monochrome camera (Grasshopper3, Point Grey Research Inc.)

the distances between three drawn lines on the sample were tracked. These values were then used to estimate

the stretch value and stretch rate. For all tests, the stage was moved with the velocity of 1 mm/s and the

corresponding stretch rate was in the range of 0.0364 ± 0.0019 1/s. For each ECSO: mal ratios, a tensile

test was repeated 3 times and the obtained data was filtered through the Savitzky-Golay filter. Further

details about the experimental set-up can be found in the Supporting Information (Figure S1).

Results and Discussion

Synthesis of epoxidized cottonseed oil

Cottonseed oil (CSO) consists of 65-70 % unsaturated fatty acid (linoleic and oleic). The linoleic fatty acid

molecule contains two unsaturated places (C=C), whereas, oleic acid has one unsaturation. Also, in CSO

linoleic: oleic molar ratio is 2:1. That results in five C=C bonds (four from linoleic and one from oleic) in

one triglyceride molecule in the CSO. The C=C double-bonds in fatty acids can undergo epoxidation

reaction resultant in oxirane groups. As shown in Figure 1, during the first reaction step, peroxy acid

(H3CCOOOH) was produced by adding hydrogen peroxide (H2O2) and acetic acid (CH3COOH). Then,

peroxy acid reacted with the CSO and epoxidized cottonseed oil (ECSO) was obtained. The percentage of

conversion of C=C double-bonds to oxirane groups was determined by using the iodine value test (ASTM

method D5554). As shown in Figure 2, during the first two hours of the reaction, 60% conversion of C=C

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bonds were observed reaching a maximum of 95% conversion in 7 h. This was further investigated by using

FTIR (Figure 3). In the FTIR spectrum, the absorption peak at 3015 cm-1 represents the =C-H (stretching).

This peak intensity started to decrease with the epoxidation reaction time and faded after 7 h. The very

weak bands displayed by the FTIR at 1651.0 cm−1 and 3052.0 cm−1 were due to the CH=CH stretching.

These bands also started to diminish over time as the epoxidation reaction progressed. However, a new

band appeared in the FTIR spectra over time at 823.0 cm−1. This band can be attributed to the formation of

epoxy groups.27 The intensity of this band increased with the progress of epoxidation reaction. Therefore,

FTIR data further supports the conversion of C=C double-bonds in oxirane groups.

Figure 1. Reaction scheme for the polymerization of cottonseed oil.

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Figure 2. Conversion of iodine percentage with epoxidation reaction time. Average of three runs are shown

and the error bars represent one standard deviation.

Figure 3. (a) Comparison of FTIR spectra investigating the epoxidation reaction products after 1h, 2h, 4h, and 8h of

reaction time. Comparison of FTIR peaks: =C-H stretching at 3015 cm-1 (b) and oxirane group at 823.0 cm−1 (c)

before and after the epoxidation reaction.

0 1 2 3 4 5 6 7 8 9 1020

30

40

50

60

70

80

90

100C

onve

rsio

n of

iodi

ne (%

)

Time (h)

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When comparing the 1H NMR spectra of the untreated CSO and reacted ECSO, resonances appeared in

both spectra between 3.8 and 4.4 ppm. This can be identified as the glycerol center in the triglycerides.

Three carbons in the triglycerides center have two different chemical environments, resulting in two

different resonances. Also, the NMR spectrum of the untreated CSO indicates the presence of C=C, due to

the resonances appearance at the δ = 5.2–5.5 ppm. However, C=C resonance peaks disappeared in the

reacted ECSO NMR spectrum and new resonance appeared at a range of 2.8 – 3.2 ppm. These new

resonance peaks can be attributed to the presence of H atoms in the newly formed oxirane rings. Therefore,

in comparison of CSO and ECSO NMR spectra, it explains the conversion of C=C to oxirane during the

epoxidation reaction.

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Figure 4. NMR spectrum comparison of CSO and ECSO.

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Crosslinked cottonseed oil polymer synthesis

As explained previously, maleic anhydride was used as to crosslink ECSO. Maleic anhydride undergoes a

nucleophilic substitution reaction with the oxirane groups of cottonseed oil molecule. When considering

the amount of unsaturated fatty acid in the cottonseed oil, each cottonseed oil triglyceride molecule

contained five C=C bonds. Furthermore, one maleic anhydride molecule was able to crosslink with two

ECSO molecules by forming two ester bonds. Therefore, five maleic anhydride molecules are enough to

reach the maximum possible crosslinking in between two ECSO molecule, if the reaction reaches 100%

completion. By changing the molar ratio of ECSO: maleic anhydride (mal) it is possible to tailor the

crosslinked density. Therefore, ECSO: mal molar ratios of 2:3, 2:4, 2:5, 2:6 and 2:7 were selected.

In Figure 5, FTIR spectra comparing CSO, ECSO, and crosslinked polymers with different ratios of ECSO:

mal are presented. A sharp, intense peak was observed around 1740 cm-1 in both CSO and ECSO spectrums.

This peak represents the C=O group present in the cottonseed oil molecule. However, a broad peak

appeared in the wavenumber range 1700 – 1765 cm-1, after the ECSO reaction with maleic anhydride as

shown in Figure 5b. During the crosslinking reaction, new C=O groups (from maleic anhydride molecules)

were attached into the main ECSO molecule. However, those newly formed C=O groups are in a different

chemical environment than the original C=O groups in the CSO molecule and due to that occurrence new

C=O peak, emerges and overlaps with the initial CSO carbonyl (C=O) peak at 1740 cm-1. This new ester

C=O bond formation can be identified by this FTIR spectrum comparison (Figure 5b).

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Figure 5. FTIR spectrums of CSO, ECSO, and maleic anhydride crosslinked CSO (a), and a comparison of

carbonyl (C=O) FTIR peak broadening with maleic anhydride crosslinking (b).

The crosslink density was tailored by changing the mole ratio of ECSO to maleic anhydride. Evidence of

change in crosslink density is evident from the physical appearance of the crosslinked polymers (Figure

S2). A sticky semi-solid polymer was obtained for the sample with 2:3 mole ratio of ECSO: mal. However,

the sample became solid (rigid) as the ECSO: mal ratio was increased to 2:5. However, it was noted that,

by increasing the ECSO: mal ratio to 2:7, we observed a non-homogeneous sample caused by unreacted

maleic anhydride trapped inside the polymer sample (figure S2).

Thermal gravimetric analysis (TGA) was conducted to determine the thermal properties of resulting

polymers. All the cross-linked polymers followed a similar thermogram trend as shown in figure 6. The

decomposition of crosslinked polymer initiated around 400 °C and reached less than 10 % weight % at 450

°C. However, ECSO: mal 2:3 polymer nearly 20% weight drop before reaching to 450 °C temperature. This

is because of unreacted epoxide groups, which decompose around 200 °C.

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Figure 6. TGA thermograms of CSO, ECSO, and ECSO: mal crosslinked polymer.

Tensile test results for three different ECSO: mal ratios are shown in Figure 7, in which nominal stress (𝜎𝜎)

is plotted against the stretch ratio, λ. The mechanical responses have been found to depend on then ECSO:

mal ratio. However, all three samples fail at λ≈1.25, despite different amounts of maleic anhydride used.

Since the sample is elastomeric in nature, the stress vs stretch ratio data were fitted with the neo-Hoookean

model to estimate the elastic modulus, E (Young’s modulus). The neo-Hookean model can be expressed as 28 𝜎𝜎 = 𝐸𝐸

3�𝜆𝜆 − 1

𝜆𝜆2�

Fitting this equation with the experimental data (Figure 7), we obtain E = 1.12, 1.95, and 1.34 MPa for the

ECSO: mal mole ratios of 2:4, 2:5, and 2:6, respectively. With increasing maleic anhydride content, the

elastic modulus increases initially (from 2.4 to 2.5) but then decreased (for 2.6). This is further evident in

Figure 7, as for a given stretch ratio the stress value increased with increasing maleic anhydride content

(viz. 2.4 and 2.5) and the stress decreased for 2.6. The increase of maleic anhydride content initially resulted

in higher crosslink density and an increase in tensile modulus. However, a further increase in maleic

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anhydride content may possibly saturate the oxirane group in the ECSO molecule rather than crosslinking

two ECSO molecules resulted in slightly lower modulus.

Figure 7. Nominal stress (𝜎𝜎0) versus stretch ratio (𝜆𝜆) obtained from tensile tests for ECSO:mal 2:4,

ECSO:mal 2:5, ECSO:mal 2:6. The markers are experimental data and lines are neo-Hookean

model fit.

The mesh size (ξ) of the network polymer obtained here can be estimated using the relationship29

𝜉𝜉 = �𝑘𝑘𝐵𝐵𝑇𝑇𝜇𝜇�1/3

, where, 𝜇𝜇 is the shear modulus, kB is the Boltzmann constant, and T is temperature (≈ 295

K). Here, considering an incompressible materials we used 𝜇𝜇 ≈ 𝐸𝐸/3. The estimated ξ values are 3.6, 2,

and 3 nm for the ECSO: mal ratios of 2:4, 2:5 and 2:6, respectively. These values match reasonably well

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bond lengths present in the crosslinked structure as shown in Figure 1. Because of a tight network structure,

the network cannot be stretched significantly.

The stability of synthesized materials was tested using aqueous acid (HCl (1.0 M), H2SO4 (1.0 M), H3PO4

(1.0 M)), aqueous base (KOH (1.0 M), NaOH (1.0 M)). A known weight of the crosslinked polymer

(ECSO:mal 2:5) samples were submerged 24 h inside the desired solvent, and no degradation of the samples

was observed.

CONCLUSIONS

During this study we have shown the feasibility of using cottonseed oil to synthesize rigid polymer. Here,

95% of C=C bonds in the cottonseed oil were converted into epoxide groups. Furthermore, we have

demonstrated the physical and mechanical properties of the resulting polymer can alter by changing the

crosslink density. Lower mole ratio of ECSO: maleic anhydride 2:3 was produced sticky viscous semi-solid

materials, whereas, rigid network material was obtained for the higher mole ratios of ECSO: maleic

anhydride (2:4, 2:5, and 2:6). The reaction scheme applied here can also be used for other natural oils

consisting of unsaturated fatty acids.

ACKNOWLEDGEMENT

This work was partially supported by Cotton Inc. We also would like to acknowledge financial support

from Swalm School of Chemical Engineering.

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104, 293. 16. Sharpless, K. B.; Woodard, S. S., Pure Appl. Chem. 1983, 55, 1823. 17. Chua, S.C.; Xu, X.; Guo, Z., Process Biochem. 2012, 47, 1439. 18. Rios, L. A.; Weckes, P.; Schuster, H.; Hoelderich, W. F., J. Catal. 2005, 232, 19. 19. Törnvall, U.; Orellana-Coca, C.; Hatti-Kaul, R.; Adlercreutz, D., Enzyme Microb. Technol. 2007,

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Supporting Information

Synthesis and Characterization of Crosslinked Polymers from

Cottonseed Oil

Rangana Wijayapala1*, Satish Mishra1, Bill Elmore1, Charles Freeman2, Santanu Kundu1*

1Mississippi State University, Dave C Swalm School of Chemical Engineering, Mississippi State, MS

39762, USA

2Mississippi State University, School of Human Sciences, Mississippi State, MS 39762, USA

Correspondence to: Rangana Wijayapala, Santanu Kundu (E-mail: rangana@che.msstate.edu, santanukundu@che.msstate.edu )

2

Figure S1. (A) A schematic of dogbone shape sample marked with three lines in the gauge region (not scaled with original sample dimensions). Here, lT0 is the length of the gauge region, bT0 is the gauge region breadth, and tT0 is sample thickness. (B) Sample at different strain (ε=λ-1) values, (B1) ε=0, (B2) ε=0.1, (B3) ε=0.3.

Tensile tests were performed at an ambient temperature (22 °C) using a custom-built tensile test machine.

To obtain the samples in dogbone shape, a sheet of the sample was prepared with a thickness in the range

of 0.3 mm- 0.4 mm. The dogbone shape of samples was cut out from the sheet using an ASTM D638

punch. A schematic of the dogbone shape sample is indicated in Figure S1A. As shown in the Figure S1B1,

the sample was supported in this set-up by using 4 supporting pins attached to two supporting blocks. The

bottom supporting block was kept fixed while the top supporting block was attached to a moving stage

through a brass plate cantilever. The sample was stretched by moving the stage at a speed of 1 mm/s (B1-

B3). To estimate the stretch and strain values, the samples were marked with three lines in the gauge

3

region. A monochrome camera (Grasshopper3, Point Grey Research Inc.) was used to capture images (10

fps) during the experiments. An image processing program was implemented in MATLAB to measure the

relative distance between 3 lines (1-2, 2-3, and 1-3) with an accuracy of 0.08 mm. Distances between the

lines were divided by initial distances (unstretched length) to calculate three individual stretch values. An

average of these three stretch values (𝜆𝜆) is considerd. For all experiments, we have used a steetch rate of

0.00364 ± 0.0019 1/s. The application of stretch led to a bent in the cantilever. A micro-epsilon capacitance

based displacement sensor (DT 6220) was used to measure the cantilever deflection. The resultant force

was estimated from the known cantilever stiffness. Further, dividing the obtained force by sample gauge

(neck) initial area (bT0×tT0) estimates the nominal stress. A framework in NI Instruments LabVIEW was

developed to control the setup and to capture the time-lapsed. A framework in MATLAB was developed

for filtering the data and post-processing the results.

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Figure S2. The physical appearance of CSO:Mal polymers. (CSO:Mal 2:3 has shown sticky semi-solid nature and CSO:Mal 2:7 consists with unreacted maleic anhydride)

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