1. Introduction
Due to environmental concerns, new regulatory policies and a shift in consumer requirements, renewable resources for polymer materials have been widely studied in the last decade [
1]. In the past, as well as nowadays, fatty acids present in plants in the form of triglycerides are one of the most appropriate raw products for the manufacturing of bio-based materials [
2]. Various chemical modification methods, such as epoxidation and ring-opening, hydroformylation, transesterification, ozonolysis, amidation and thiol-ene coupling [
3], have been used to obtain various monomers and polymers with low toxicity, convenient availability and relatively low price [
4,
5,
6,
7]. Epoxides are one of the most versatile intermediates to be further used for the synthesis of different compounds and has a wide commercial use because of its high reactivity [
8].
Numerous epoxidation methods of different kind of vegetable oils, such as canola-rapeseed [
9,
10], palm [
11], cottonseed [
12], soybean [
13], castor [
14], linseed [
15], mahua [
16] and grape seed [
17] among others, have been reported. In addition, non-edible plant oils, such as tall oil [
18,
19,
20] and jatropha [
21,
22], have been successfully epoxidised previously. However, the use of vegetable oil for industrial uses is in direct competition with food and feed production. As mentioned, there are sources of plant-derived fatty acids that do not compete, such as tall oil [
23,
24].
Tall oil fatty acids (TOFA) are important renewable feedstock, which is obtained as a side stream from the softwood Kraft pulping process. After fractional distillation of crude tall oil, a more pure form of TOFA is obtained containing at least 97% free fatty acids (mainly a mixture of 48–52% of oleic and 43–48% of linoleic acid) and less than 3% other components, such as rosin acids and unsaponifiables [
25,
26,
27,
28]. The relatively high level of unsaturation makes TOFA a suitable raw material for the introduction of reactive functional group using the unsaturated hydrocarbon C=C double bonds, making them suitable for further processing into polymers [
29].
The most widely used epoxidation method is the well-known Prilezhaev reaction where peracid is reacted with olefins [
20,
30,
31,
32]. Peracids are conventionally formed in situ from hydrogen peroxide and short-chain carboxylic acid in the presence of highly acidic catalysts [
32,
33]. However, during epoxidation, acidic catalysts, such as sulphuric acid and acidic ion exchange resins, and carboxyl acids as oxygen carriers, such as formic or acetic acid, exacerbate undesirable side product formation through oxirane rings [
18,
20,
34]. Moreover, the higher the temperature of the epoxidation reaction, the greater the frequency of side reactions [
20]. Studies previously carried out by our group showed that if free fatty acids containing unprotected carboxyl groups, as they are in TOFA, are epoxidised, the use of acidic components has even more significance for side reactions occurrence [
18,
20]. To avoid the formation of side products, other epoxidation routes have to be explored.
Some studies indicate that chemo-enzymatic epoxidation, where acidic catalysts are replaced by lipase, overall is a milder route to free fatty acid conversion into epoxides [
19] than the well-known Prilezhaev rection. The advantages of chemo-enzymatic epoxidation are lower reaction temperature [
35], the absence of acidic catalysts [
36,
37] and even lipase reusability [
36], which if all taken in to account can result in substantially higher oxirane ring introduction into the substrate. In addition, lipases are highly selective to limit the frequency of side reactions [
38]. Moreover, it is possible to prevent the use of additional oxygen carriers if free fatty acids are chemo-enzymatically epoxidised. Lipases can turn free fatty acids into highly reactive peroxy fatty acids, which subsequently epoxidise unsaturated bonds [
39], thus improving the feasibility of reaction as there are no acidic catalysts and additional oxygen carriers needed. However, different factors, such as solvent, temperature, pH and presence of activators or deactivators etc., can influence the activity of lipases [
40].
After epoxidation, one of the most easily obtainable functional groups are hydroxyl groups, which are essential for polyurethane (PU) production. PUs are a class of polymers that are commonly used in a wide variety of applications to produce high-performance materials. The primary uses for PUs are flexible and rigid foams, sealants, elastomers, adhesives, and coatings [
30,
41,
42,
43,
44,
45,
46,
47]. Usually, PUs are obtained by polycondensation reaction between isocyanates and hydroxyl group containing compounds [
48,
49,
50]. Hydroxyl group compounds can be polyols with low, medium or high functionality with low, medium or high hydroxyl values (OH value), respectively. Polyols with high average hydroxyl group functionality are needed for the production of rigid PU foams to ensure high dimensional, mechanical and thermal stability of the material [
51]. A combination of epoxy ring-opening and transesterification or transamidation of fatty acids with polyfunctional alcohols can lead to such polyols, which would contain primary OH groups to ensure high cross-link density of obtained PU polymer matrix [
52,
53,
54].
The goal of this study was to compare two different TOFA epoxidation catalysts—ion exchange resin Amberlite IR-120 H and enzymatic catalyst Candida antarctica lipase B with a trade name Novozym® 435—and their influence on the properties of resulting polyols and rigid PU foams. In this study, a second-generation bio-based feedstock—TOFA—was epoxidised via two pathways resulting in two different epoxidised tall oil fatty acids (ETOFA). Afterwards, two different polyols were developed using the two different ETOFA and employing oxirane ring-opening and subsequent esterification reactions with two different polyfunctional alcohols (trimethylolpropane (TMP) and triethanolamine (TEOA)). The four developed polyols were used to obtain rigid PU foam thermal insulation material. Its common characteristics, such as thermal conductivity, apparent density and compression strength, were analysed and compared.
2. Materials and Methods
2.1. Materials
TOFA (trade name “FOR2”) with a high content of fatty acids (>96%), low content of rosin acids (1.9%) and unsaponifiables (1.8%) was ordered from Forchem Oyj (Rauma, Finland). Glacial acetic acid (AcOH), puriss, ≥99.8%; hydrogen peroxide (H2O2), purum p.a., ≥35%; acetanhydride, puriss, ≥99%; 4-(dimethylamino)pyridine (DMAP), reagent plus, ≥99%; N,N-dimethylformamide (DMF), ACS reagent, ≥99.8%, water content ≤150 ppm; potassium hydroxide, puriss, ≥85%; potassium iodide, ACS reagent, ≥99%; tetraethylammonium bromide, reagent grade, 98%; perchloric acid, ACS reagent, 70%; dichloromethane, puriss p.a., ACS reagent; anhydrous sodium sulphate, puriss; TMP, reagent grade, 97%, were ordered from Sigma-Aldrich (Schnelldorf, Germany). Amberlite IR-120 H, strongly acidic, hydrogen form and sodium thiosulphate fixanals 0.1 M and Hanus solution, volumetric 0.1 M IBr were ordered from Fluka (Steinheim, Germany). Lipase Novozym® 435 (immobilised on acrylic resin) was kindly supplied by Novozymes A/S (Bagsvaerd, Denmark). Tetrafluoroboric acid solution, 48 wt.% in H2O (HBF4), was ordered from Alfa Aesar (Kandel, Germany). TEOA, 99.2%, was ordered from Huntsman (Rotterdam, The Netherlands), and was used as purchased.
For the development of rigid PU foams, the following materials were used as purchased: two tertiary amine-based catalysts Polycat® 5, Polycat® NP10 as well as 30 wt.% of potassium acetate in diethylene glycol (PC CAT TKA 30) (Air Products and Chemicals Inc., Halfweg, The Netherlands); Niax Silicone L-6915 as a surfactant (Momentive Performance Materials Inc., Rotterdam, Germany); tris (1-chloro-2-propyl phosphate 99% (TCPP) as a flame retardant (Albermarle, Louvain-la-Neuve, Belgium)) and cyclopentane as a physical blowing agent (Sigma-Aldrich, Schnelldorf, Germany). Desmodur 44V20 L was purchased from (Covestro, Krefeld, Germany), and was used as the isocyanate component for all PU materials. It is a solvent-free product based on 4,4′-diphenylmethane diisocyanate (pMDI) and contains oligomers of high functionality. The average functionality is 2.8–2.9 and the isocyanate group (–NCO) content of 30.5–32.5 wt.%.
2.2. Epoxidation of TOFA with Ion Exchange Resin Amberlite IR-120 H
The epoxidation of TOFA was carried out in a four-necked round bottom flask. A thermocouple, mechanical stirrer, dropping funnel and a reflux condenser were attached to the necks of the flask. The epoxidation of TOFA was achieved by in-situ generated peroxyacetic acid, which forms from acetic acid and hydrogen peroxide in the presence of an acidic catalyst. During the epoxidation, the molar ratio of TOFA (double bonds-155 g I
2/100 g) to H
2O
2 and AcOH was 1.0:1.5:0.5. At first, the calculated amount of TOFA (700.0 g), acetic acid (128.5 g) and ion exchange resin Amberlite IR-120 H (140.0 g, 20 wt.% of TOFA weight), as the catalyst, was added to the flask. The flask was immersed in a thermostatic water bath (preheated to 40 °C). The speed of the mechanical stirrer was set to 600 rpm, and the mixture was started to stir. A hydrogen peroxide/water (35%/65%) solution (638.5 g) was poured into a dropping funnel. When the content of the flask reached 40 °C, hydrogen peroxide solution was added dropwise to the round bottom flask in a time interval of 30 min. Meanwhile, the temperature of the reaction medium was slowly increased to 60 °C. After the complete addition of hydrogen peroxide, the reaction medium was continued to stir for 6 h at 60 °C and 600 rpm [
18]. Afterwards, the reaction mixture was poured into a separating funnel and washed four times with warm (T = 60 °C) distilled water. The product was dried using a rotatory vacuum evaporator to remove water residues. As a result, ETOFA were obtained exhibiting the acid value of 144 mg KOH/g, oxirane content of 2.30 mmol/g and iodine value of 27.0 g I
2/100 g and characterised by pomegranate red colour. An acronym ETOFA_IR is used for the ETOFA synthesised via TOFA epoxidation with ion exchange resin Amberlite IR-120 H.
2.3. Epoxidation of TOFA with Novozym® 435
The epoxidation of TOFA was carried out in a four-necked round bottom flask. The flask was immersed in a water bath and equipped with a mechanical stirrer, a reflux condenser, a thermocouple and a dropping funnel. Using the data that was obtained from the previous study [
55], the optimal epoxidation parameters were determined and used for TOFA epoxidation.
During the epoxidation, the molar ratio of double bond present in TOFA and H2O2 was 1.0:2.0. The required amount of TOFA (700.0 g) was poured in the flask, and necessary amount of Candida antarctica lipase B (Novozym® 435) (22.4 g, 3.2 wt.% of TOFA weight) was added. The mixture of TOFA and catalyst was heated to 44 ± 0.1 °C. Afterwards, the necessary amount of 32% H2O2 water solution (1038.2 g) was added to the reactants dropwise through the dropping funnel. The rate of addition speed was adjusted so that the whole peroxide was added within 30 min. After the complete addition of hydrogen peroxide, the reaction medium was continued to stir for 5.5 h at 44 ± 0.1 °C and 500 rpm. Afterwards, the reaction mixture was poured into a separating funnel and washed four times with warm (T = 60 °C) distilled water. The product was dried using a rotatory vacuum evaporator to remove water residues. As a result, ETOFA were obtained exhibiting the acid value of 146 mg KOH/g, oxirane content of 3.28 mmol/g and iodine value of 17.0 g I2/100 g and characterised by pomegranate red colour. An acronym ETOFA_E is used for the ETOFA obtained from TOFA epoxidation with lipase catalyst Novozym® 435.
2.4. Synthesis of Polyols Using Two Different ETOFA
High functionality polyols were synthesised by functionalising ETOFA_IR or ETOFA_E, which were obtained by epoxidising TOFA using two epoxidation catalysts either Amberlite IR-120 H or Novozym
® 435. Bio-based polyols were synthesised by opening the oxirane ring of ETOFA_IR or ETOFA_E and subsequent esterification with various polyfunctional alcohols, such as TMP and TEOA. For bio-polyols synthesised from intermediate ETOFA_IR, the following acronyms were used ETOFA_TMP_IR and ETOFA_TEOA_IR depending on the used polyfunctional alcohol for epoxide ring-opening. For polyols obtained from TOFA epoxidation with lipase catalyst Novozym
® 435, the following acronyms were used ETOFA_TMP_E and ETOFA_TEOA_E. The general scheme of bio-polyol development is depicted in
Figure 1.
Due to the two different chemical processes that are carried out simultaneously, namely, oxirane ring-opening reaction and esterification reaction, the molar amount of the polyfunctional alcohol needed for polyol synthesis is calculated using the following equation:
where n
MP is the molar amount of the polyfunctional alcohol used for oxirane ring-opening and esterification reaction (TMP or TEOA), in mol; n
KG is the molar amount of the ETOFA carboxylic groups, in mol and n
EG is the molar amount of the ETOFA oxirane groups, in mol.
To obtain TOFA-based bio-polyols, the oxirane ring-opening was first carried out in the four-necked round bottom flask. The multifunctional alcohol/amine and tetrafluoroboric acid solution, 48 wt.% in H
2O and as a catalyst, 0.4 wt.% of ETOFA mass, was added, see
Table 1 for corresponding mass for each type of polyols. The flask was immersed into an oil thermobath, and a mechanical stirrer was inserted into the central neck. A purge gas tube, Liebig condenser, and a dropping funnel were attached to the vacant necks. The mixer was set to 400 rpm, and the flow of purge gas (argon) through the flask was provided, while the content of the flask was heated up to 120 °C. When the required temperature of 120 °C for oxirane ring-opening was reached, 200 g of ETOFA were added dropwise to the flask in a time interval of 20 min. After the complete addition of ETOFA, the reaction medium was continued to stir for 30 min at the temperature of 120 °C to open the oxirane rings completely. Afterwards, the synthesis temperature was increased to carry out the esterification reactions. The ETOFA_TMP_IR and ETOFA_TMP_E polyol synthesis were carried out at 200 °C, whereas ETOFA_TEOA_IR and ETOFA_TEOA_E synthesis were carried out at 180 °C (
Figure 2). The stirring of the reaction medium and the argon gas flow was retained until the acid value of the product decreased below 5 mg KOH/g [
54]. After which, the synthesis was considered to be finished, and the bio-polyol was obtained for further rigid PU foam development.
2.5. Characterisation of Products and Precursors
The obtained bio-polyols were characterised by hydroxyl and acid values calculated according to ISO 4629-2:2016 and ISO 2114:2000 testing standards using titrimetric methods. The epoxy content was calculated in accordance with ASTM D1652-04:2004. The viscosity of polyols was measured at 25 °C using the Thermo Science HAAKE (Medium-High Range Rotational Viscometer, Thermo Fisher Scientific, Waltham, MA, USA). Polyol density was determined using a series of hydrometers. In a thermostatic bath at 20 °C, a graduated cylinder filled with polyol was immersed for 20 min, and afterwards, the density was measured. The moisture content was measured using the Denver Instrument Model 275KF automatic titrator (Denver Instrument, Bohemia, NY, USA) using Karl Fisher titration.
Polyol structure was analysed using Fourier-transform infrared spectrometry data (FTIR), which were obtained with a Thermo Scientific Nicolet iS50 spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) at a resolution of 4 cm
−1 (32 scans). The FTIR data were collected using attenuated total reflectance technique with ZnSe and diamond crystals. Moreover,
1H NMR spectra for the samples were recorded on a Bruker spectrometer (Bruker BioSpin AG, Fällanden, Switzerland) at 500 MHz. The chemical shifts (δ) are reported in ppm. The residual chloroform peak was used as an internal reference (δ = 7.26 ppm). Size exclusion chromatography from Knauer equipped with refractive index detector (Detector RI) and polystyrene/divinylbenzene matrix gel column with a measurement range up to 30,000 Da at tetrahydrofuran (THF) eluent flow of 1.0 mL/min was used to analyse the number-average molecular weight (M
n) and number-average functionality (f
n) of the synthesised bio-polyols. The polyols f
n was calculated based on hydroxyl values and M
n as seen from Equation (2) [
52].
where f
n is the number-average functionality, OH groups/mol; M
n is the number-average molecular weight, g/mol; OH
valuel is the hydroxyl value of the polyol, mg KOH/g and 56,100 is equivalent weight of KOH, in milligrams.
2.6. Rigid PU foam Preparation and Characterisation
For rigid PU foam development, ETOFA-based bio-polyols were used. For the PU foams obtained from bio-based polyols synthesised from ETOFA using Amberlite IR-120 H and polyfunctional alcohols like TMP and TEOA, the following acronyms PU_TMP_IR and PU_TEOA_IR were used. Similarly, for the PU foams obtained from bio-based polyols synthesised from ETOFA using Novozym
® 435 and polyfunctional alcohols like TMP and TEOA, the following acronyms PU_TMP_E and PU_TEOA_E were used. In
Table 2, the developed rigid PU foam formulations are depicted. In order to obtain rigid PU foams using newly synthesised bio-polyols a previously synthesised bio-polyol with lower functionality based on tall oil (TO) esterification with TEOA was also used (TO_TEOA with OH value of 334 mg KOH/g, the water content of 0.45 wt.%, the viscosity of 280 mPa·s at 25 °C, f
n = 2.4 and M
n = 391 g/mol). The polyol component was obtained by weighing all the required components presented in
Table 2 (polyols, blowing agent, flame retardant, catalysts and surfactant) and stirring them with a mechanical stirrer at 2000 rpm for 1 min. The polyol system was then conditioned in a sealed container at room temperature for at least 2 h to de-gas the mixed air, and afterwards, PU foams were prepared. The isocyanate index was chosen to be 150 for all PU foams. To produce rigid PU foams, isocyanate (pMDI) and a polyol portion were weighed and mixed at 2000 rpm for 15 s with a mechanical stirrer. After that, the reactive mixture was poured into an open-top mould [
54].
The content of renewable materials was determined based on the mass of renewable materials used in the formulation of the PU foams. The rigid PU foam formulations were designed to obtain foams with an apparent density of ~40 kg/m3. The physical and mechanical properties of the foams were measured in accordance with the following standards: foam density—ISO 845:2009, closed cell content—ISO 4590:2003, compression strength—ISO 844:2009 and thermal conductivity—ISO 8301:1991. The compression strength of the PU foams was tested parallel and perpendicular to foam rise with one offset from ISO 844:2009 standard—sample size; cylinders with a diameter of 20 mm and a height of 22 mm were tested. The mechanical testing of PU foams was done using Zwick/Roell 1000 N testing machines (Zwick Roell Group, Ulm, Germany)