Synthesis and Characterization of a Bio-Based Resin from Linseed Oil

Abstract

Summary: The use of renewable raw materials in the polymer industries is becoming increasingly popular because of environmental concerns and the need to substitute fossil resources. Plant oils with triglyceride backbones can be chemically modified and used to synthesize polymers from renewable resources (biopolymers). In the present study, linseed oil was epoxidized using a chemo-enzymatic method based on Candida Antarctica lipase B (CALB) as a biocatalyst and the modified linseed oil was cured using maleinated linseed oil and a commercial polyamide resin. The amount of epoxidation achieved depended on the amount of lipase used and was determined by infrared (IR) and nuclear magnetic resonance (NMR) spectroscopies. With 20% (weight per weight) catalyst concentration based on the wt % of oil a degree of epoxidation of > 90% was achieved. The cross-linking reaction of epoxidized linseed oil with the maleinated linseed oil and the polyamide resin was studied using differential scanning calorimetry (DSC). DSC traces showed that an increase in epoxidation degree lead to larger values for the exothermic enthalpy integrals of the curing reactions and hence to a higher reactivity of the linseed oil towards the cross-linking agents.

Introduction

During the last decades the attempts to develop bio-based products have increased considerably in number because of growing environmental consciousness and often lower prices of fast-growing plant based raw materials compared to products derived from petrochemical resources. The renewable materials can partially or in some rare cases1–5 even totally replace the petroleum based polymers. The most common class of renewable materials derived as a by-product from the food industry which is used for making bio-based polymers is the vegetable oils. Such plant oils are triglycerides with varying compositions in fatty acids depending on the individual plant, crop type, season and growing conditions. The fatty acid moieties contribute 94–96% of the total weight of one molecule triglyceride oil and these fatty acids may be saturated, unsaturated or polyunsaturated. The number of double bonds present in the triglycerides governs the level of reactivity of the oils and the physical properties of the natural oils depend very strongly on the degree of unsaturation and the relative amounts of different fatty acids. Vegetable oils do not meet the properties desired for polymer synthesis unless they are chemically modified either at the double bonds or carboxyl functionalities. Some of the modifications carried out at the double bonds of the fatty acid chain are acrylation, maleination, hydrogenation, halogenation, ozonolysis, dimerization, metathesis, epoxidation and hydroxylation.6–8 Of these modification methods, epoxidation and maleination of linseed oil were studied and are reported in the present article. Epoxidation is the conversion of the CC double bond of mono- and polyunsaturated fatty acids and their esters to the oxirane derivatives by chemical modification. The standard industrial production of these epoxidized oils is based on the in situ epoxidation9 where a peroxy acid is generated by reacting acetic or formic acid with hydrogen peroxide in the presence of strong mineral acids. Since strong acids are used, major drawbacks of the peracid based epoxidation are the increased risk of equipment corrosion and the potential hydrolysis of ester groups by undesired ring opening reactions with water. The oxirane ring opening will also reduce the yield of reactive groups. To prevent the undesirable ring opening, a new method of epoxidation using the enzyme lipase (Scheme 1) as a re-usable, regioselective and mild catalyst was proposed by Rusch gen Klass et al.10, 11 The lipase catalyzed chemo-enzymatic oxidation takes place in two reactions steps. In the first step, the unsaturated fatty acid is converted to an unsaturated peroxy fatty acid as an intermediate. This is followed by an autoxidation step (Prileshajev-epoxidation) which results in the formation of an epoxy acid.

Scheme 1. Modified structures of linseed oil after (1) chemoenzymatic epoxidation and (2) maleination process. *Two reaction mechanisms are possible with maleic anhydride, for example only one double bond is shown for each reaction mechanism in Scheme.16

Some authors have studied the influence of different reaction parameters on the level of epoxidation of soybean oil with immobilized Candida Antarctica Lipase B (CALB) as the catalyst;12 the parameters studied in these studies were hydrogen peroxide concentration, amount of fatty acid added to the mixture, temperature, enzyme activity, and solvent type.13, 14 The reported results already show that the catalyst concentration and temperature are the most important factors influencing the degree of epoxidation.

The objective of the present investigation was to find the optimum catalyst concentration for linseed oil epoxidation at otherwise constant reaction conditions. The progress of the epoxidation was characterized by infrared (IR) spectroscopy and nuclear magnetic resonance (NMR) spectroscopy. The cross-linking reaction of the oxirane groups of the epoxidized linseed oil with two types of curing agents, maleinated linseed oil and polyamide was studied by differential scanning calorimetry.

Epoxy resins can be cured with a wide variety of curing agents: amines, polyamides, imidazoles, polymercaptanes, anhydrides, and different types of latent curing agents. The properties of cured epoxy resins may have substantially different properties depending on the type of curing agent used. In the current investigation maleic anhydride modified linseed oil was used as a curing agent for the epoxidized linseed oil. Anhydride curing agents are preferred where a long pot life and severe curing conditions are required.15 Anhydrides with long chain backbones contribute good flexural strength to the end product.16 Maleic anhydride modified linseed oils (Scheme 1) can be used as flexible anhydride curing agents for epoxy resins that were also derived from linseed oil. When maleinated oil is used as a curing agent, the resulting polyester network contains maleinated groups, which are readily copolymerized with styrene.16 According to Warth et al.,16 maleic anhydride is attached to the unsaturated fatty acid either via Diels-Alder reaction or via ene-type reaction. Rheineckeu et al.,17 reported that in case of mono-unsaturated fatty acid esters the maleination reaction required at least 200 °C and followed an “ene”-type reaction, which results in the addition of anhydride groups in the allylic position of the fatty acid. The various isomers of most plant oils contain two unconjugated CC double bonds. Teeter et al.18 reported that “ene” type reactions take place in the case of such substrates and a conjugation of the double bonds is achieved leading to trans-trans isomers. The formed adduct undergoes a Diels-Alder reaction with another mole of maleic anhydride. In the present study, a maleinated linseed oil was synthesized and its cross-linking with epoxidized linseed oil was studied with DSC.

Polyamide resins are another category of curing agents that is generally used for coating applications. Polyamide resins may be derived from renewable resources and can then be regarded as bio-based resin cross-linkers. Their exothermal curing reaction with epoxy resins generates moderate heat during cure and introduces reactive amino groups into the polymer. Polyamides are generally based on step growth polymerization of triethylene tetramine with a mixture of dimeric (such as the C₃₆ diacid) and monomeric (for example, a C₁₈) fatty acids. In the present investigation, a polyamide derived from commercial tall oil fatty acid was cross-linked with the epoxidized linseed oil.

Experimental Part

Materials

Linseed oil was purchased from a local market. Candida antarctica lipase B immobilized on Immobead 150, recombinant from Aspergillus oryzae was purchased from Sigma-Aldrich. Hydrogen peroxide 35% v/v aqueous solutions, oleic acid, maleic anhydride, toluene (HPLC grade) were also purchased from Sigma-Aldrich. A solvent free viscous reactive polyamide resin derived from tall oil, which is used for curing epoxy resins in adhesives, coatings and sealants, was supplied from Arizona Chemicals (Viscosity 700–900 cps, amine number 335–360).

Methods

The epoxidation of linseed oil was carried out in a 250 mL three necked round bottom flask equipped with a temperature sensor. 25 g of the linseed oil were dissolved in 150 g toluene and different defined amounts of lipase (based on wt% of oil) were added in each experiment to study the effect of enzyme activity. Under mechanical stirring, 2 g of oleic acid were then added and the mixture was heated to 55 °C. Once the end temperature was reached, 25 g of hydrogen peroxide (aqueous solution, 35% v/v) were added drop-wise during 5 min using a dropping funnel as described by Vlecek et al.19 Stirring was continued for 24 h at 55 °C. After completion, the mixture was allowed to cool down to room temperature and the catalyst beads were removed by filtration, rinsed several times with toluene and subsequently air dried and stored for further use. The filtered liquid was transferred to a separator funnel and washed several times with distilled water and with 2% sodium bicarbonate aqueous solution. The separated organic layer was dried with anhydrous magnesium sulfate and filtered. The solvent was evaporated under vacuum in a rotary evaporator.

Maleination16 of linseed oil was carried out in a 250 mL three necked round bottom flask. 100 g of linseed oil were weighed in the flask and heated to 85 °C. Then 17 g of the finely ground maleic anhydride powder were added stepwise to dissolve completely in the oil and form a homogeneous solution. The mixture was then heated to 200 °C and this temperature was kept constant for 8 h under nitrogen atmosphere. After completion of the reaction, the reaction mixture was cooled to room temperature and high vacuum distillation was applied to remove the unreacted maleic anhydride.

The conversion of the double bonds to epoxy groups was monitored using a FTIR spectrometer (Brucker Equinox 55) and the spectra were recorded as an average of 32 scans in the spectral range between 4000 and 600 cm⁻¹ with a resolution of 4 cm⁻¹. The H¹ NMR spectra were recorded on a Varian 400 MHz high resolution FT-nuclear magnetic resonance spectrometer, dissolving the samples in CDCl₃. DSC measurements of the cross-linking reactions were performed on a METTLER TOLEDO 822e instrument (Greifensee, Switzerland) and the STAR^© software package was used for the quantitative evaluation of the DSC traces. The epoxidized linseed oil samples were mixed well with the two different curing agents (maleinated linseed oil and polyamide resin) at room temperature and approximately 5–6 mg of the mixture were weighed in an aluminum DSC crucible. The crucible was hermetically sealed and heated from 25 °C to 300 °C at a scanning rate of 10 °C min⁻¹ under a nitrogen atmosphere.

Results and Discussion

Characterization of Epoxidized Linseed Oil

In case of the linseed oil epoxidation, the rate of epoxide formation is strongly affected by the catalyst concentration. An amount of 2% of CALB based on wt% of oil did not show any notable changes in the oil structure after 24 h reaction time in the FTIR spectrum; however, when 5% of CALB were used, characteristic changes in the FTIR spectra indicated a decrease in double bond density and a corresponding increase in the oxirane peaks. Hence, an amount of 5% of catalyst beads is considered as the critical lower concentration necessary to epoxidize the oil using lipase catalyst. The spectral changes during the epoxidation are shown in Figure 1. The signals at 3030 cm⁻¹ and 720 cm⁻¹ correspond to the stretching vibration of the double bonds: CH, cis-CHCH, respectively. The most characteristic change during epoxidation is the occurrence of oxirane groups, which is being detected as the doublet at 822 and 833 cm⁻¹. In Figure 2, the decrease in the double bond peak at 3020 cm⁻¹ is shown as a complementary evidence of the epoxidation reaction. The formation of epoxy groups was supported by proton NMR results; the corresponding assignment of the signals for the oil has been reported in earlier work by other groups.20, 21 The chemical shifts and the corresponding peak assignments are listed in Table 1:

Figure 1. Changes in FTIR spectra of the linseed oil during epoxidation at various catalyst concentrations a) 20% b) 10% c) 5% d) 2% e) linseed oil.

Figure 2. FTIR spectra in the double bond region for the linseedoil during epoxidation 1)20% catalyst 2) 10% catalyst 3) 5% catalyst 4) 2% catalyst 5) linseed oil.

Table 1. Peak assignment for the linseed oil.

Signal (ppm)	Corresponding assignment
0.9–1.0	Hydrogens of the terminal methyl groups of linolenic acid (CH3CH2CHCH)
1.2–1.3	Aliphatic methylene hydrogen
1.6	Hydrogen in β-position to the carbonyl group (CH2CH2C(O)O)
2.0	Allyl hydrogens (CH2CHCH)
2.3	Methylene hydrogens in α-position to carbonyl groups (CH2C(O)O)
2.8	Hydrogen between two double bonds (CHCHCH2CHCH)
4–4.4	Methylene hydrogens from the glyceride moiety (CHCH2O)
5.3	Vinyl hydrogens (CHCH) and methine hydrogen from the glyceride group (CHOC(O))

The linseed oil has 6.6 double bonds per triglyceride molecule as was calculated from the ¹H NMR signals by the method described by Miyak. The reaction conversion was monitored by the area decrease of the double bond hydrogen signal in the 4.1–5.5 ppm region.22 The degree of epoxidation was calculated by integrating the signals in the 2.7–3.3 ppm range which correspond to the evolving cis-epoxy hydrogen signal.

The NMR results do not show any significant changes for the mixture treated with 2% of catalyst; at about 5% catalyst concentration the degree of epoxidation achieved was 58%. The degree of epoxidation for the 10% and 20% catalyst concentrations were 76% and 96% respectively. The 20% catalyst concentration showed drastic changes in the chemical structure especially in the double bond region and the observed shift in vinyl hydrogen was due to the hybridization change of the carbons (Figure 3). The sp² carbons were changed to sp³ carbon once after they were transformed into an oxirane ring. The other major change was observed with the allyl hydrogen signals: the original peak of the two double bonds is observed at 2 ppm which after epoxidation shifted to 1.45 ppm.23

Figure 3. ¹H NMR spectra of epoxidized linseed oil at various catalyst concentrations after 24 h reaction time a) 2% b) 5% c) 10% d) 20%.

Characterization of Maleinated Linseed Oil

The maleinated brown viscous oils showed specific absorption bands in the FTIR spectra that are related to saturated cyclic anhydride (1860 cm⁻¹) and unsaturated double bonds (910 cm⁻¹) (Figure 4).16 The absorption band at 1060 cm⁻¹, 1780 cm⁻¹ corresponds to the CO deformation and CO stretching band of maleic anhydride respectively (Figure 4).

Figure 4. FTIR spectra of a) maleinated linseed oil and b) linseed oil.

Curing of Epoxidized Linseed Oil

The epoxidized linseed oil was cured with maleinated linseed oil in the molar ratio of 1:2. Generally, higher concentrations of anhydride lead to higher glass transition temperatures but in the case of maleinated oil the final cured network was soft and flexible due to the flexible chain segments. DSC was applied to study the epoxidized linseed oil cure and the observed exothermic peak was attributed to the epoxy/anhydride copolymerization. The DSC trace shows that the peak maximum shifts from 130 °C to 150 °C with an increase in the degree of epoxidation (Figure 5). Simultaneously, the shoulder observed between 160 °C and 180 °C also became less pronounced with an increase in the level of epoxidation. However, in terms of the amount of energy released during cure, the exothermal enthalpy change remained almost the same for all three catalyst levels. A reason for this might be the reaction of anhydride groups with the double bonds of the epoxidized oil.

Figure 5. DSC traces of epoxidized linseed oil cured with maleinated linseed oil. The oil was epoxidized at various catalyst levels a) 20% b) 10% c) 5%.

The commercial polyamide used in the current investigation was a natural product derived from crude tall oil by modification with polyamines. An excessive amount of polyamide (ratio 1:1) was used for cross-linking the epoxidized linseed oils having different degrees of epoxidation. Two exothermic peaks were observed for the 5 and 10% lipase catalyzed epoxidized oil in the DSC trace (Figure 6). The first peak corresponds to the reaction of the double bonds of the fatty acid chains and the second peak corresponds to the reaction of epoxy groups with the amino end group. Triglycerides which are present in the oils have three functional groups, the reactivities of which are governed by the number of double bonds that can be measured by determining the iodine number, and their relative positions. When these trifunctional groups are incorporated, the polymer will be either branched or cross-linked. During thermal curing, the functional groups of triglycerides form new intramolecular and intermolecular chemical linkages via oxygenation and Diels-Alder reaction. An intramolecular linkage can occur not only inside a monomer (a triglyceride) but also inside a dimer (a molecule linked by two triglycerides), a trimer, and so on. It can occur either via Diels Alder reaction or the opening of double bonds inside a molecule. The observed first peak may be explained by these processes. The kinetics of the reaction between epoxy groups and the amine curing agent (polyamide having amine end-groups) are complex and the reaction is strongly catalyzed by hydroxyl groups that are formed during the reaction. The increase in exothermic cure energy for the 20% catalyst concentration shows that a larger number of epoxy groups are available for the reaction. The rate of consumption of epoxide is approximately proportional to the concentration of amine times the concentration of the epoxide.

Figure 6. DSC traces for the reaction of epoxidized linseed oil with polyamide resin at different degrees of epoxidation due to various catalyst concentration a) 20% catalyst concentration b) 10% catalyst concentration c) 5% catalyst concentration.

Conclusion

The epoxidation of linseed oil using a CALB-based chemo-enzymatic epoxidation method showed selective oxiran ring substitution in the double bonds of the fatty acid chains. The occurance of the oxirane group and the elimination of double bonds were detected by FTIR and ¹H NMR spectroscopies. The number of double bonds in the triglyceride backbone and the decrease in amount of double bond during epoxidation was quantitively evaluated using ¹H NMR. The ¹H NMR spectrum gave detailed information on the percentage of epoxidation based on monitoring the formation of the epoxy rings and the correspondingly decreasing level of unsaturation. Attachment of maleic anhydride to the double bonds of the triglycerides was also characterized using FTIR spectroscopy. The reactivity of the epoxy group with the maleinated oil and polyamide was deduced from the exothermic signals obtained from DSC experiments. The characterization of the epoxidized oil using other supportive methods like iodine value, epoxy oxygen group content, hydroxyl group content, acid value are currently under evaluation.