2.1. Materials
The following reactants were used for the synthesis of the bio-based monomers: rapeseed oil (RO, PREOL, Lovosice, Czech Republic), technical-grade oleic acid (OA, 70%, Penta Chemicals, Prague, Czech Republic, CAS:112-80-1), hydrogen peroxide (30%, technical grade, Lach-Ner, Brno, Czech Republic, CAS: 7722-84-1), methanol (Penta Chemicals, Prague, Czech Republic, CAS: 67-56-1), potassium hydroxide (Lach-Ner, Brno, Czech Republic, CAS: 1310-58-3), diethyl ether (Lach-Ner, Brno, Czech Republic, CAS: 60-29-7), sulfuric acid (Lach-Ner, Brno, Czech Republic, CAS: 7664-93-9), potassium carbonate (Lach-Ner, Brno, Czech Republic, CAS: 584-08-7), formic acid (Lach-Ner, Brno, Czech Republic, CAS: 64-18-6), acrylic acid (AA, Sigma-Aldrich, Schnelldorf, Germany, CAS: 79-10-7), hydroquinone (Lach-Ner, Brno, Czech Republic, CAS: 123-31-9), chromium (III) 2-ethyl hexanoate (ThermoFisher, Kandel, Germany, CAS: 3444-17-5), ethyl acetate (Lach-Ner. Brno, Czech Republic, CAS: 141-78-6), and sodium carbonate (Lach-Ner, Brno, Czech Republic, CAS: 497-19-8). All the chemicals were used as received.
In all emulsion polymerization reactions methyl methacrylate (MMA, CAS: 80-62-6), butyl acrylate (BA, CAS:141-32-2), and methacrylic acid (MAA, CAS: 79-41-4) were used as monomers (Sigma-Aldrich, Schnelldorf, Germany), Disponil FES 993 (sodium salt of fatty alcohol polyglycol ether sulfate, BASF, Prague, Czech Republic) as surfactant, ammonium persulfate (Lach-Ner, Brno, Czech Republic, CAS: 7727-54-0) as initiator, and 2-amino-2-methyl-1-propanol (AMP 95, Sigma-Aldrich, Schnelldorf, Germany, CAS: 124-68-5) as neutralizing agent. All the chemicals were used as received without further purification.
2.2. Synthesis and Characterization of Bio-Based Monomers
In the first step, the methyl ester of RO (ME_RO) and methyl ester of OA (ME_OA) were obtained by transesterification of RO and esterification of OA. The transesterification was carried out with 420 g of RO, 92 g of methanol, and 3.7 g of potassium hydroxide as a catalyst. The reaction temperature was 60 °C and the reaction time was 90 min. The esterification was carried out with 220 g of OA, 152 g of methanol, and 287 g of diethyl ether as cosolvent. The mixture was heated to 42 °C and 28 g of sulfuric acid as a catalyst was added. After 4 h, the esterification was stopped by neutralization of the catalyst with potassium carbonate. Then, diethyl ether and methanol were removed by distillation. Formed ME_RO and ME_OA were separated from glycerol or water in a separatory funnel.
The second step involved the epoxidation of the double bonds in ME_RO and ME_OA resulting in an epoxidized methyl ester of RO (EME_RO) and epoxidized methyl ester of OA (EME_OA), respectively. The epoxidation reaction was carried out in a batch reactor with 420 g of the respective methyl ester (ME_RO, ME_OA), 345 g of hydrogen peroxide, 60.3 g of formic acid as a catalyst, and 3.2 g of sulfuric acid as an additional catalyst. The mixture of the given methyl ester (ME_RO, ME_OA), formic acid, and sulfuric acid was cooled to 8–10 °C and then hydrogen peroxide was gradually added over 30 min at a stirring speed of 300 rpm. The reaction mixture was heated to 60 °C for 3 h. The reaction was stopped by the addition of potassium carbonate until the pH of the epoxide phase was neutral. After that, the aqueous phase was removed by decantation in a separatory funnel.
The last step involved the acrylation of EME_RO and EME_OA by AA, resulting in acrylated methyl ester of RO (AME_RO) and acrylated methyl ester of OA (AME_OA), respectively. The laboratory procedure involved mixing 50 g of EME_RO and EME_OE, respectively, 17 g of AA, 0.15 g of hydroquinone, and 0.5 g of chromium (III) 2-ethyl hexanoate in a three-necked round-bottom flask, equipped with a reflux condenser, magnetic stirrer, and a thermometer. The solution was vigorously stirred and kept at 100 °C using an oil bath for 6 h. After that, the excess amount of AA was neutralized using a saturated solution of sodium bicarbonate. An extraction was then performed, using a mixture of ethyl acetate and water (1:1 w/w). The separated organic layer was desiccated with sodium carbonate, and ethyl acetate was evaporated from the product with constant stirring at room temperature (RT).
The prepared methyl esters, epoxidized methyl esters, and acrylated methyl esters of RO and OA were further characterized in terms of their chemical structure, the content of non-epoxidized and non-acrylated methyl esters of different higher fatty acids, and the iodine value. The chemical structure of the compounds was investigated using infrared (IR) vibration spectroscopy on a Nicolet iS50 FTIR spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) equipped with a build-in diamond ATR (attenuated total reflection) crystal in the region of 4000–400 cm
–1 (data spacing = 0.5 cm
–1). Concurrently, Raman spectra were acquired by a Nicolet iS50 spectrometer with an FT-Raman module (Nd:YAG excitation laser λ = 1064 nm, power = 0.5 W, data spacing = 1 cm
–1) in the region of 4000–200 cm
–1. The methyl esters of different higher fatty acids was analyzed by gas chromatography coupled with mass spectroscopy (GC-MS). The Agilent 7890B/5977A Series GC/MSD (Agilent Technologies, Waldbronn, Germany) equipped with an autosampler (Agilent 7693) and operating in the electron ionization (EI) mode was used for monitoring and identification of compounds. The electron energy of EI was 70 eV, the source temperature was 300 °C, the quadrupole temperature was 150 °C and the transfer line temperature was 300 °C. MS data were acquired over a mass range of 50–500 at a rate of 6 scan/s. A TRACE™ TR-FAME capillary column, 60 m × 0.25 mm I.D., film thickness 0.25 µm (Thermo Fisher Scientific, Waltham, MA, USA) under gradient conditions was used for separation of all compounds in the sample. Helium at a flow rate of 1 mL/min was used as the carrier gas with the following oven temperature program: 70 °C held for 3.5 min, the gradient of 90 °C/min to 160 °C held for 2 min, the gradient of 5 °C/min to 200 °C held to 1 min, the gradient of 2 °C/min to 240 °C held to 50 min. The amount of 0.5 µL sample was injected to GC under split mode injection with a 1:50 split ratio at 250 °C. The measurements were carried out two times. Samples were prepared as follows: 0.25 g of a tested material was weighed and dissolved in 3 g of acetonitrile. Then the solution was diluted five times with acetonitrile. C17:0 methyl ester (≥99.0%, Merck KGaA, Darmstadt, Germany) was used as the internal standard. The area of the peaks (A) and thus the representation of the individually detected compounds were related to the internal standard (AIS). The iodine value, which is a measure of the unsaturation of an organic compound, was determined using the Hanuš method [
44]. The method consists of adding the excess of iodine monobromide (that reacts with double bonds in the unsaturated compound), followed by the treatment with potassium iodide and the determination of liberated iodine by titration with sodium thiosulfate.
2.3. Synthesis and Characterization of Latexes
Two series of latexes were prepared using the technique of semi-continuous non-seeded emulsion polymerization. The latex copolymers were synthesized starting from an initial composition of standard acrylic monomers (denoted as REF) on one hand and different mixtures of the bio-based monomers (AME_RO, AME_OA) and acrylic monomers, on the other hand, as can be seen in
Table 1. The latex samples were labeled as X_y, where X represents the bio-based raw material type (RO or OA), and y is the percentage content of the respective bio-based monomer in the monomer mixture. The MMA/BA ratio of 21/28 (
w/w) was maintained in all monomer compositions.
The latexes were produced in a 700 mL glass reactor under an inert atmosphere (nitrogen) at 85 °C. The reactor charge consisting of 16.25 g of demineralized water, 0.125 g of Disponil FES 993, and 0.175 g of ammonium persulfate was heated to the polymerization temperature. Then the monomer emulsion consisting of 50 g of the monomer mixture (
Table 1), 57.5 g of demineralized water, 3.7 g of Disponil FES 993, and 0.35 g of ammonium persulfate was dosed into the reactor at the feed rate of about 2 mL/min. After that, the reaction mixture was allowed to polymerize for 2 h. Each latex sample obtained from the corresponding initial monomer composition was synthesized 3 times.
After the synthesis, the latexes were filtered, and the coagulum was collected. The coagulum content and monomer conversion were determined by the gravimetric method and were calculated according to Equations (1) and (2) [
45].
where
m1 is the weight of a liquid latex portion; m
2 is the weight of the latex portion dried to a constant weight at 110 °C;
mC is the weight of the dried coagulum;
mL is the weight of the total filtered latex;
mT is the total weight of all the materials put in the reaction flask;
mI is the weight of the initiator;
mS is the weight of the surfactant (active matter);
mM is the weight of the total monomers.
The pH of latexes was then adjusted to 8.5 with AMP 95 (50% aqueous solution). The storage stability of latexes was evaluated according to changes in the average particle size and the zeta potential after storing the latexes at 40 °C for 60 days. The average particle sizes and the zeta potential of the latex particles dispersed in the water phase were detected by dynamic light scattering (DLS) using a Litesizer 500 instrument (Anton Paar GmbH, Graz, Austria). The concentration of a solid polymer in the water phase was 0.01 wt.% and the measurements were conducted at 25 °C.
The molar mass distribution of latex copolymers was determined by asymmetric flow field flow fractionation (AF4) coupled with a multi-angle light scattering (MALS) detector. The employed AF4-MALS instrumental set-up consisted of an Agilent 1260 Infinity II chromatograph (Agilent, Santa Clara, CA, USA) with a quaternary pump and an autosampler with a Wyatt Technology AF4 system Eclipse coupled with a MALS photometer DAWN and an Optilab refractive index (RI) detector. The system was completed with an online viscometer ViscoStar for a more detailed characterization of the molecular structure of some of the analyzed samples. All detectors and software were acquired from Wyatt Technology (Santa Barbara, CA, USA). The separation was achieved with a long channel of 350 μm thickness and regenerated cellulose membrane of 10 kDa cut-off. The carrier was tetrahydrofuran (THF). A linear cross flow gradient from 2.5 mL/min to 0.1 mL/min over 15 min followed by 0.1 mL/min for 20 min and zero cross flow for 10 min was used for the separation with the channel flow and detector flow of 1 mL/min and 0.3 mL/min, respectively. Samples were prepared in THF at the concentration of ≈2.5 mg/mL, left to dissolve for at least 48 h, filtered with 0.45 μm filter, and injected in the amount of 100 μL. ASTRA 8 was used for data collection and processing, VISION was used for the operation of the Eclipse (both Wyatt Technology).
2.4. Preparation and Characterization of Coatings
Liquid latexes were applied onto glass panels using a blade applicator. The thickness of the wet coatings was 120 μm. There were no coalescing agents used. After curing at RT (23 ± 1 °C) and 40 ± 5% of relative humidity for 1 week, the resulting films were evaluated in terms of their gloss, water contact angle (WCA), adhesion, glass transition temperature (Tg), and chemical composition. The gloss of coatings, cast on glass panels coated with black matte paint (RAL 9005), was measured by means of a micro-TRI-gloss μ instrument (BYK-Gardner, Geretsried, Germany) using a gloss-measuring geometry at 60°. WCAs were examined using an optical tensiometer Attension Theta (Biolion Scientific, Espoo, Finland). A water drop of 1 µL was always applied and the steady-state WCA value was subtracted at the time of 10 s. Ten measurements were performed for each coating sample at RT and 40 ± 5% RH. The adhesion (expressed as pull strength) of coatings to a glass substrate was evaluated in accordance with ISO 4624 using an Elcometer 510 Automatic Adhesion Tester (Elcometer Instruments, UK). Tg was determined by differential scanning calorimetry (DSC) on a Pyris 1 DSC instrument (Perkin-Elmer, Waltham, MA, USA). The measurements were performed under an inert atmosphere (nitrogen) atmosphere at a heating rate of 10 °C/min from –50 °C to 120 °C (Tg was determined from the second heating curve). Fourier transform infrared (FT-IR) spectroscopy was used to detect the incorporation of vegetable oil-based monomers into latex copolymers. Infrared spectra of the samples were recorded on a Nicolet iS50 FTIR spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) using a build-in diamond ATR (attenuated total reflection) crystal in the region of 4000–400 cm–1 (data spacing = 0.5 cm–1).