The need for the production of synthetic polymers from renewable and sustainable resources also affects the area of emulsion polymerization. The bio-based monomer (BM) was synthesized from camelina oil (CO) and itaconic acid through transesterification and epoxidation of CO, followed by itaconation, resulting in a blend of methyl esters of CO-originated fatty acids functionalized with reactive methyl itaconate groups. Various amounts of BM were copolymerized with standard acrylic monomers (0−30 wt. % of BM in the monomer mixture) using the emulsion polymerization technique to obtain film-forming latexes. Infrared and Raman spectroscopies evidenced the successful incorporation of BM into the structure of latex polymers. The ultra-high molar mass nanogel fraction was detected by asymmetric flow-field flow fractionation coupled with a multiangle light scattering (AF4-MALS) for BM comprising copolymers; the higher the BM content, the more extensive the nanogel fraction. Crosslinking of latex polymers induced by BM testified to the reactivity of itaconated functions in emulsion polymerization and provided additional evidence of the copolymerization ability of the BM. The incorporation of BM also resulted in hardness and glass transition temperature enhancement (about 11% and 9°C, respectively, in the case of 30 wt. % of the BM content in contrast to 0 wt. % of the BM content in the copolymer). Coatings with excellent transparency and gloss were obtained from all latexes regardless of the BM content used. Slightly increased water repellency (about 7 ° increased water contact angle value) and significantly improved the water whitening resistance of the coatings (about 80% decreased water whitening after 1-day long water exposure) were found for coatings comprising 30 wt. % of BM in the copolymer, where the water whitening phenomenon was highly dependent on the BM content.
Research Article
Utilization of bio-based monomer derived from camelina oil and itaconic acid for the synthesis of film-forming latexes
https://doi.org/10.21203/rs.3.rs-5030135/v1
This work is licensed under a CC BY 4.0 License
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Latex coating
Emulsion polymerization
Bio-based monomer
Itaconic acid
Camelina oil
Water resistance
Synthetic polymer latexes are advanced polymeric materials, manufactured using emulsion polymerization, a process that involves the free radical polymerization of emulsified monomers, resulting in the formation of polymer latex particles dispersed in an aqueous medium [1–4]. Emulsion polymerization products find extensive applications as protective and decorative coatings, printing inks, adhesives, and paper coatings [5, 6]. Different monomers can be utilized in emulsion polymerization; among the most ordinary are various acrylic and methacrylic acid esters [7]. However, these monomers are commercially obtained from petroleum feedstock. The negative environmental impact leads to increased attention to renewable and eco-friendly alternatives [8–15], where plant oils represent the promising feedstock [16–18]. Various plant oils and their derivatives have been used as bio-based monomers (BM) in polymeric reactions, including rapeseed [19], linseed [20], soybean [21], sunflower [22], castor [23], and camelina [24] oil. Camelina oil (CO) is produced by cold pressing the oilseed of the cruciferous plant Camelina sativa [25]. Camelina does not compete with other crops in the food industry, since it is frequently grown as a non-food oilseed crop. It contains mainly unsaturated fatty acids (80–90%) with a relatively high content of polyunsaturated acids. In short, it can be stated that the most represented are fatty acids with an 18-carbon chain and the ratio between unsaturated acids with one, two, or three double bonds is almost equal [26–30].
The application of plant oil-based monomers in emulsion polymerization remains quite a challenge due to their hydrophobic nature [31]. Moreover, the double bonds present in unsaturated fatty acids are not reactive enough to participate in polymerization; instead, they are preferred for functionalization which results in the attachment of groups, reactive in free radical polymerization [32]. For example, plant oil derivatives with acryloyl groups, prepared by epoxidation of double bonds [33–35] and subsequent acrylation of epoxy groups, have emerged as valuable BMs for the emulsion polymerization [35, 36]. Unfortunately, their manufacturing process requires a non-negligible amount of petroleum-based acrylic acid that is not only environmentally unfriendly but also can possess offensive odors, irritation, and allergenic potential. Furthermore, eye contact is extremely harmful; high exposure can even trigger pulmonary edema [38–41].
Replacement of acrylic acid with itaconic acid in plant oil functionalization is a possible solution to these problems. Itaconic acid is manufactured by fermentation of carbohydrates using various fungi, such as Aspergillus terreus or Ustilago maydis [42–48]. The solid state of itaconic acid also solves the problems arising from the volatility of acrylic acid [49]. Itaconic acid has been utilized in the synthesis of polyesters [50–52], UV-curable coatings [53], and polymeric latexes, although a negative influence on the overall monomer conversion and the copolymerization rate was observed in the emulsion polymerization of acrylic latexes prepared using itaconic acid [53, 54]. Since itaconic acid possesses two carboxylic groups, it cannot be used in the same way as acrylic acid in the case of plant oil functionalization, because the reaction would result in gelation [55]. Utilizing monoesters of itaconic acid is one way to overcome this inconvenience. Several methods of the synthesis of monoesters of itaconic acid have been studied. Báez et al. [56] obtained monomethyl itaconate by esterification of itaconic acid with a large excess of methanol with acetyl chloride used as the initiator. The same synthesis route was also studied by López-Carrasquero et al. [57], who performed the purification step using recrystallization in hot n-heptane or n-hexane. Richard et al. [58] prepared monoesters of itaconic acid with various alcohols (from C1 to C22) from itaconic anhydride. Li et al. [59] successfully used monomethyl itaconate for synthesizing itaconated soybean oil which was further used as a replacement for acrylated soybean oil in the synthesis of crosslinked networks. However, the successful utilization of itaconated derivatives of plant oils in emulsion polymerization has not been reported to the best of our knowledge.
The synthesis of acrylated BMs derived from various plant oils (rapeseed oil, linseed oil, and CO) and acrylic acid and their ability to copolymerize with conventional acrylic monomers in standard emulsion polymerization was demonstrated in our previous research [30, 60]. In this work, we used itaconic acid, specifically its monomethyl ester (MMI), for the functionalization of CO to provide itaconated BM that can be utilized effectively in the emulsion polymerization for the synthesis of film-forming latexes. The effect of the BM content on the properties of final latex materials was studied, including storage stability, chemical structure, glass transition temperature, molar mass, gloss, hardness, and water resistance.
Chemicals
Camelina oil (CO, The National Agricultural and Food Center, Pstruša, Slovakia) was modified through transesterification and epoxidation procedures that involved the utilization of hydrogen peroxide (30%, technical grade), methanol, potassium hydroxide, potassium carbonate, and formic acid. All chemicals were obtained from Lach-Ner (Czech Republic). Itaconic acid (Acros Organics, Belgium) was modified through esterification using methanol (Lach-Ner, Czech Republic), benzoyl chloride (Acros Organics, Belgium), toluene (Penta Chemicals, Czech Republic), and petroleum ether (Penta Chemicals, Czech Republic). The synthesis of BM also involved the use of triphenylphosphine (Sigma-Aldrich, Germany), hydroquinone (Lach-Ner, Czech Republic), ethyl acetate (Lach-Ner, Czech Republic), and sodium bicarbonate (Lach-Ner, Czech Republic).
Methyl methacrylate (MMA), butyl acrylate (BA), and methacrylic acid (MAA) were used as conventional monomers in all emulsion polymerization reactions (Sigma-Aldrich, Germany). Disponil FES 993 (sodium salt of fatty alcohol polyglycerol ether sulphate, BASF, Germany) was used as the surfactant. Ammonium persulphate (Lach-Ner, Czech Republic) served as the initiator, and 2-amino-2-methylpropan-1-ol (AMP 95, Sigma-Aldrich, Germany) was used as the neutralizing agent. All chemicals were used as received without additional purification.
Synthesis and Characterization of Bio-Based Monomer
A multi-step process was employed to obtain the final BM. Initially, CO transesterification was performed according to references [60, 61] to yield a blend of methyl esters of fatty acids (ME). The detailed proportion of methyl esters of CO-derived fatty acids in the ME intermediate product is given in the reference [30]. In the next step, the double bonds in ME were epoxidized to obtain a blend of epoxidized methyl esters of fatty acids (EME). Furthermore, monomethyl itaconate (MMI) was prepared according to the procedure described in reference [59]. Finally, a blend of methyl esters of fatty acids functionalized with reactive methyl itaconate groups (BM) was synthesized from EME using a reaction of the epoxy group with MMI following reference [59].
The synthesized BM and its intermediate products were characterized by the iodine value, epoxy index (EI), proton nuclear magnetic resonance (1H NMR) spectroscopy, infrared (IR) vibration spectroscopy, and Raman spectroscopy. The iodine value was measured according to a Hanus method (EN ISO 3961) [62] using a Mettler Toledo T50 Excellence Titrator (Mettler Toledo, USA). The epoxy value was determined according to the CSN EN ISO 3001 standard. 1H NMR spectra were obtained from a Bruker 500 Avance spectrometer (Bruker, Billerica, MA, USA) at 300 K. The samples (50 µl) were dissolved in CDCl3 (1 ml). Spectra were referenced internally to residual CDCl3 and reported relative to Me4Si (δ = 0 ppm). The intensity of the bands is referenced to the methyl ester group (3H). IR spectra were recorded 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). Raman spectra were obtained from the same spectrometer using the FT-Raman module (Nd:YAG excitation laser, λ = 1064 nm, power = 0.5 W, data spacing = 2 cm–1) in the region of 4000–200 cm–1.
Thermodynamic Calculations of Itaconation Reaction
Theoretical calculations of the itaconation reaction were performed using the Gaussian 16 program package [63]. The optimal geometries of the reactants (epoxidized methyl esters of fatty acids and MMI) and the products (itaconated methyl esters of fatty acids) were optimized using a hybrid M06–2X functional without any constraints (energy cutoff: 10 − 5 kJ/mol: final RMS energy gradient < 0.01 kJ/mol/Å). The M06–2X functional is a reliable approach to determining the thermochemistry of organic compounds [64]. Applying a sufficiently large 6–311 + + G (d,p) basis set [65, 66] including diffuse and polarization functions, allows a balanced description of all investigated species.
Synthesis and Characterization of Latexes
Various amounts of BM were copolymerized with standard acrylic monomers (0−30 wt. % of BM in the monomer mixture) using semicontinuous non-seeded emulsion polymerization. The monomer composition of the latex samples is described in Table 1. The monomer composition of all synthesized latexes maintained a constant MMA/BA ratio of 21/28 (w/w). A detailed procedure for the latex synthesis is given in the reference [60]. Each latex sample was prepared three times to determine reproducibility. The coagulum content and overall monomer conversion were determined using standard methods described in reference [67]. The latexes were alkalized to pH 8.5 by 50% AMP 95 aqueous solution.
The apparent viscosity of the latexes was assessed at 25°C using a Brookfield LVDV-E Viscometer (Brookfield Engineering Laboratories, USA) at 100 rpm according to CSN ISO 2555. The storage stability of the final latex copolymers was evaluated according to changes in their particle size and zeta potential after storage for 3 months at elevated temperature (40°C). The average particle size (hydrodynamic diameter) and zeta potential of the latex particles dispersed in water were detected at 25 ° C by dynamic light scattering (DLS) using the Litesizer 500 instrument (Anton Paar, Austria). The latexes were diluted to approximately 0.01 wt. % before the DLS measurement.
Table 1 Monomer composition of the reference latex (REF) and BM-based latexes (L)
Characterization of Copolymer Structure
The molar mass distribution, molar mass average, and the root mean square (RMS) radius (also called the radius of gyration) were obtained by asymmetric flow field flow fractionation combined with a multiangle light scattering detector (AF4-MALS). The experimental setup consisted of an Agilent 1260 Infinity II chromatograph (Agilent, Santa Clara, CA, USA) coupled with an A4F system Eclipse, a MALS photometer DAWN, and an Optilab refractive index (RI) detector. A channel of 350 µm thickness with a regenerated cellulose membrane (10 kDa cutoff) was utilized for the separation using tetrahydrofuran (THF) as the carrier. The cross-flow profile was as follows: 2.5 ml⋅min–1 for 5 min, linear gradient to 0.1 ml /min over 15 min, 20 min at 0.1 ml/min, and 10 min with zero cross flow. The channel flow and detector flow were 1 and 0.5 ml /min, respectively. Samples were prepared as THF solutions with an estimated concentration of 2.5 mg/ml. The solutions were filtered with a 0.45 µm filter, and injected in a volume of 100 µl. Data collection and processing were performed using ASTRA 8, while VISION 3 was used to operate the Eclipse instrument. Eclipse, detectors, and software were obtained from Waters Wyatt Technology (Santa Barbara, CA, USA).
The cross-link density was determined through swelling experiments on dry gel polymer samples submerged in toluene at 40°C for 14 days. The method using a set of equations [68] based on the Flory and Rehner theory [69] was utilized to calculate the cross-link density, expressed as moles of cross-links per cm3 of a polymer network. Calculations were performed for the MMA/BA/MAA (42/56/2 by weight) copolymer using density and solubility parameter literature data for poly(MMA), poly(BA), and poly(MAA) homopolymers [67, 68]. Data from the literature for the poly(BM) homopolymer were absent, therefore, the copolymerized BM was not considered in the calculations. The chemical composition of latex copolymers was studied by IR vibration spectroscopy and Raman spectroscopy under the same experimental conditions as previously mentioned.
Preparation and Characterization of Latex Coatings
Latexes were cast onto glass panels using a wet film applicator, creating coatings of 120 µm wet thickness. No coalescing agents or latex paint additives were used. The coatings were left to dry under ambient conditions (22 ± 1°C, 40 ± 5% of relative humidity) for 10 days. Subsequently, the coating materials were evaluated for their chemical composition, gloss, water contact angle (WCA), water whitening, hardness, glass transition temperature (Tg), gel content, and crosslink density.
For the gloss measurement, the coatings were applied using a blade applicator onto a glass panel coated with black matte paint (RAL 9005). Gloss was measured with the micro-TRI-gloss µ instrument (BYK-Gardner, Germany) using a gloss measurement geometry at 60 °. The WCAs were measured using an optical tensiometer (Dataphysics Instruments, Germany) applying a 10 µl water drop. Ten measurements under ambient conditions were taken for each sample.
To objectively assess the transparency and water whitening of coating films applied to glass panels, light transmission was measured at a wavelength of 500 nm (corresponding to cyan light which is sensitive for the human eye) employing a ColorQuest XE spectrometer (Hunterlab, USA). The coatings were submerged in distilled water for 4 and 24 h and the transmittance of the exposed coating film area was recorded. The extent of water whitening W (%) was then calculated using Eq. (1):
$$\:W=100\times\:\:\frac{{T}_{0}-{T}_{t}}{{T}_{0}}$$
1
where T0 is the coating transmittance before exposure to water and Tt is the coating transmittance measured immediately after exposure to water.
The hardness of the coatings was evaluated according to CSN EN ISO 1552 using a Persoz-type pendulum TQC SP0500 (TQC Sheen, The Netherlands). The Tg of the coating polymers was measured using the differential scanning calorimetry (DSC) technique on a DSC Q2000 instrument (TA Instruments, USA) at a heating rate of 10°C/min from −40 to 100°C.
Characterization of Bio-Based Monomer
The number of double bonds, calculated from the iodine value, and the number of epoxy groups, expressed as the epoxy value, were determined in ME, EME, and BM (Table 2). The epoxy groups in EME were evidenced by an epoxy value, and their formation was also confirmed by a drop in the level of unsaturation, although some double bonds were found to be non-epoxidized. After the itaconation of EME, an increased iodine value was determined for BM, which can be attributed to the presence of methyl itaconate functions in BM molecules. A theoretical amount of MMI that could be bonded via epoxy groups in 100 g of EME is approximately 46.8 g (calculated based on the determined epoxy value for EME). However, the results of the iodine values for EME and BM indicate that the real amount of reacted MMI was much lower; approximately 11.1 g of MMI was bonded in 100 g of BM, which means that only approximately 25% of epoxy groups had been successfully itaconated.
| Sample | ME | EME | BM |
|---|---|---|---|
| Iodine value (g I2/100 g) | 180.2 ± 5.2 | 32.3 ± 1.2 | 51.7 ± 2.1 |
| Moles of double bonds/100 g | 0.701 ± 0.020 | 0.127 ± 0.005 | 0.204 ± 0.008 |
| Epoxy value (moles of epoxy groups/100 g) | 0 | 0.325 ± 0.010 | 0 |
The itaconation of EME was followed by NMR spectroscopy. 1H NMR spectrum of EME (Fig. 1) shows characteristic signals of residual vinylene function at 5.25–5.60 ppm (denoted a), the methyl ester group at 3.62 ppm (denoted b), and the CH group of epoxy cycles in the region 2.8–3.2 ppm (denoted c and d). Upon synthesis of BM, the high conversion of the epoxy function is evident from the disappearance of the signals c and d. The introduction of methyl itaconate into the BM structure is proven by the appearance of new signals at 6.34, 5.68, and 3.32 ppm (denoted e, f, and i), assigned to fully esterified itaconic acid. The signal of the methyl itaconate group (signal h) is shifted to a lower field compared to that of the fatty acid methyl esters (signal b) because of the electronic effects of the methylidene function. The signal at 4.88 ppm (signal g) was assigned to the CH group in the fatty acid tail in the α-position relative to the itaconate function. The integral intensities of the signals show that each fatty acid tail in EME contains ~ 0.78 itaconate functions on average. Note that the residues of the starting MMI were detected at 6.41, 5.79, and 3.35 ppm (signals denoted by *), but their content is only ~ 20 mol. % (~ 8 wt. %).
Successful itaconation was further supported by IR and Raman spectra of BM focusing on the appearance of the characteristic C = C stretching band at 1644 cm–1 and the C = O stretching band at 1719 cm–1 (Fig. 2). Note that the bands are significantly shifted in comparison to the fatty acid functions as a result of conjugation. The itaconate function also gives the characteristic C–H stretching band of the methylidene group at 3109 cm–1. The formation of hydroxy function after itaconation is evidenced by the increase in the O–H stretching band at ~ 3500 cm–1 in the IR spectrum.
Composition of Bio-Based Monomer According to Thermodynamic Calculations
The real composition of the synthesized BM cannot be easily determined due to the formation of many structurally similar compounds (theoretically above 30). The compounds differ in the number of methyl itaconate groups and their position in the fatty acid chain. (In the itaconation reaction step, the epoxy group is transformed into one itaconate and one hydroxyl group). These compounds are almost impossible to separate and identify by common chromatographic methods (standards are not commercially available). Therefore, the thermodynamic calculations of Gibbs energy for the individual itaconation reactions of EME intermediate product with MMI were carried out to estimate the compounds preferred to occur in the BM final product. The calculations were based on theoretical quantum chemistry (thermodynamic values of the individual reactants and products are not tabulated) implementing the actual reaction conditions (temperature of 293 K and relative permittivity of EME of 6.02). It should be mentioned that the thermodynamic calculations for the particular epoxidation reactions of ME intermediate product were not the subject of this work but were presented in the recent study [72]. Low Gibbs energies ranging from − 242 to − 264 kJ⋅mol–1 were calculated for the individual epoxidation reactions, suggesting their strongly spontaneous character under specified reaction conditions.
The illustrations of the compounds that may be formed in BM as a result of itaconation of the most represented epoxidized methyl esters of unsaturated fatty acids, namely oleic (C18:1), linoleic (C18:2), and linolenic (C18:3), with calculated Gibbs energy values are presented in Figs. 3−5. As for the double-epoxidized methyl esters of linolenic acid, all the possible reactions were not included because a high number of structurally similar compounds can be formed, requiring time-consuming calculations of Gibbs energies that are not expected to differ significantly. Compounds based on a triple-epoxidized methyl ester of linolenic acid were also not addressed as this intermediate product had not been proven to be formed [73].
In the case of the presented itaconation reactions, Gibbs energy values ranging from − 16 to − 58 kJ⋅mol–1 were calculated, suggesting that itaconation is less spontaneous compared to epoxidation. The probable reason is the strong exothermic character of epoxidation due to the decomposition of hydrogen peroxide. Focusing on the mono-itaconation reactions, the more spontaneous (i.e., advantageous) position of the methyl itaconate group appears the more distant bonding from the fatty acid head (i.e., the methyl ester group of fatty acid), probably for steric reasons. The calculated Gibbs energies of the mono-itaconation reactions were also found to decrease with the increasing number of double bonds present in the fatty acid tail. This result lets us assume that the presence of double bonds increases the spontaneity of the mono-itaconation reaction. Regarding the double-itaconation reactions, the trends are ambiguous. As for the number of methyl itaconate groups formed, Gibbs energies ranging from − 16 to − 35 kJ⋅mol–1 were calculated for the mono-itaconation reactions, while Gibbs energies in the range of − 37 to − 58 kJ⋅mol–1 were obtained for the double-itaconation reactions. These results indicate that the formation of compounds containing two itaconate functions in the final BM is more spontaneous, i.e., favorable according to the thermodynamic calculations.
Characterization of Latexes
The properties of prepared latexes are presented in Table 3. All synthesized latexes exhibited a low coagulum content which was slightly affected by the amount of copolymerized BM; the higher the BM concentration in the monomer mixture, the higher the coagulum content formed during latex synthesis. The amount of BM was also shown to have a considerable impact on monomer conversion; decreasing conversion was found with increasing BM content in the case of latexes synthesized using 20, 25, and 30 wt. % of BM in the monomer mixture. This phenomenon could be related to the lower water solubility of BM compared to standard acrylic monomers, which probably limited its transport in water and incorporation into the existing latex particles [74–76]. The decreased conversion and colloidal stability during latex synthesis were also the cause of why no further attempts were made to incorporate a higher BM content into the latex copolymer.
Further tests revealed that neither the viscosity nor the particle diameter of freshly prepared latexes were influenced pronouncedly by the amount of introduced BM. On the contrary, the zeta potential of the freshly prepared latexes (being inherently negative because of the presence of carboxyl and sulphate functions originating from copolymerized MAA, persulphate initiator, and adsorbed anionic surfactant) was found (meant in absolute value) to be proportional to the amount of introduced BM. This phenomenon can be attributed to partial hydrolysis of the methyl ester functions [77] in the BM molecules under acidic environment (pH ∼ 2) of emulsion polymerization, which caused the formation of additional carboxyl groups. The zeta potential values (in absolute terms) above −40 mV indicate that all the freshly prepared latexes exhibited colloidal stability [78]. After storage at 40°C for 3 months, neither visible coagulation nor a significant increase in the average hydrodynamic diameter of the particles were observed. However, a mild reduction in the zeta potential (the absolute value) was found, which can predicate the desorption of some emulsifiers from latex particles [79]. Despite this fact, all latexes can still be considered long-term stable.
| Sample | Coagulum content (%) | Conversion (%) | Viscosity | Hydrodynamic diameter (nm) | Zeta potential (mV) | ||
|---|---|---|---|---|---|---|---|
| (mPa⋅s) | Initial state | After storage | Initial state | After storage | |||
| REF | 0.7 ± 0.6 | 94.3 ± 1.0 | 11.1 ± 0.4 | 92.1 ± 1.9 | 94.0 ± 1.6 | −42.2 ± 0.2 | −38.5 ± 1.1 |
| L_5 | 1.1 ± 0.2 | 93.5 ± 0.2 | 18.9 ± 0.3 | 87.4 ± 2.2 | 87.0 ± 1.3 | −40.1 ± 3.3 | −37.3 ± 2.1 |
| L_10 | 0.7 ± 0.1 | 93.7 ± 0.6 | 19.5 ± 0.2 | 85.0 ± 1.1 | 85.0 ± 1.8 | −42.8 ± 0.9 | −36.7 ± 1.0 |
| L_15 | 1.2 ± 0.1 | 93.1 ± 0.5 | 18.6 ± 0.2 | 84.8 ± 1.5 | 85.8 ± 1.4 | −48.1 ± 0.3 | −41.7 ± 0.6 |
| L_20 | 1.3 ± 0.1 | 91.6 ± 0.8 | 19.2 ± 0.3 | 83.8 ± 0.8 | 84.3 ± 1.4 | −49.4 ± 1.1 | −42.9 ± 0.8 |
| L_25 | 1.7 ± 0.2 | 88.8 ± 1.1 | 19.8 ± 0.1 | 80.7 ± 1.6 | 80.0 ± 1.6 | −48.9 ± 2.4 | −43.6 ± 0.1 |
| L_30 | 2.6 ± 0.1 | 84.5 ± 0.6 | 19.3 ± 0.2 | 87.0 ± 0.9 | 87.5 ± 0.9 | −51.5 ± 2.3 | −43.8 ± 0.5 |
Characterization of Copolymer Structure
The weight-average molar mass (Mw), dispersity (Ð), the z-average RMS radius, and the polymer and nanogel fractions are summarized in Table 4. Similarly to our previous research on latex copolymers synthesized from acrylated BMs [30], the RI fractograms show two peaks (an example is shown in Fig. 6). The peak at lower retention times can be assigned to dissolved individual macromolecules, while that at higher retention times corresponds to swollen cross-linked latex particles (nanogels). As expected, with increasing BM content, nanogels become more compact, which is evident from the decreasing retention time and RMS radius, as shown in Fig. 7. Figure 7 also reveals a slightly decreasing RMS radius with increasing retention time. This tendency was found for all the tested samples. Since the retention time in AF4 increases with increasing hydrodynamic volume, decreasing RMS radius may suggest that the central parts of the nanogels become more compact despite their increasing hydrodynamic volumes.
The level of cross-linking incorporated into latex polymers was assessed, considering the nanogel content obtained from A4F and the cross-link density obtained through swelling experiments (Table 4). As expected, the nanogel content and cross-link density were related to the amount of introduced BM. This effect is a consequence of the copolymerization of multi-itaconated BM fractions represented mainly by linoleic and linolenic fatty acids, which has already been documented in our previous work on latex copolymers with acrylated BM counterparts [30].
| Sample | Soluble polymer | Nanogel | RMS radius (nm) | Cross-link density × 10− 6 (moles of crosslinks⋅cm–3) | |||
|---|---|---|---|---|---|---|---|
| Mw (103 g⋅mol–1) | Ð | Fraction (%) | Mw (106 g⋅mol–1) | Fraction (%) | |||
| REF | 6000 | 36.0 | ≅ 100 | -b | -b | 120 | -c |
| L_5 | 4300 | 56.1 | 41.7 | 82 | 58.3 | 109 | 3.51 ± 0.2 |
| L_10 | 6190 | 12.2 | 20.7 | 89 | 79.3 | 79 | 7.59 ± 0.1 |
| L_15 | 339 | 20.4 | 19.1 | 88 | 80.9 | 71 | 12.6 ± 0.4 |
| L_20 | 349 | 15.2 | 19.3 | 114 | 80.7 | 68 | 18.8 ± 0.6 |
| L_25 | -a | -a | 22.0 | 81 | 78.0 | 60 | 34.5 ± 1.1 |
| L_30 | -a | -a | 14.4 | 133 | 85.6 | 59 | 61.2 ± 2.9 |
a Value was impossible to determine due to a low signal of the MALS detector.
b The nanogel fraction was not present.
c Impossible to determine.
IR spectroscopy was employed to follow the incorporation of BM into latex polymer chains (Fig. 8). For this purpose, a protocol previously described for acrylated fatty acid methyl esters was used [60]. The increased content of BM in the starting monomer mixture resulted in polymers with increased intensity of the absorption bands at 2931 and 2855 cm–1 assigned to the antisymmetric and symmetric C–H stretching modes of the methylene groups, νa(C–H, CH2) and νs(C–H, CH2), respectively. Thus, the occurrence of methylene groups forming the fatty acid tails could be evidence of successful BM copolymerization. A similar effect was observed in the Raman spectra (Fig. 8), where the increased amount of incorporated BM well correlated with the increased intensity of the symmetric CH stretching band of the methylene groups, νs(C–H, CH2), at 2854 cm–1.
Evaluation of Coatings
The coating properties of the latex films are presented in Table 5. All coating samples appeared smooth, glossy, and transparent, with gloss and transmittance values comparable to the REF latex coating. It was found that copolymerized BM decreased coating wettability; almost all BM-comprising coating samples exhibited about 7 ° higher WCA than REF coating. This phenomenon could be attributed to the hydrophobic nature of the long fatty acid chains in the BM structure. However, the presence of hydrophilic hydroxyl groups, formed as a result of the ring-opening reaction of the epoxy group (see e.g. Figure 3), probably diminished this effect.
In our previous research on acrylated BMs derived from various vegetable oils, including CO [30, 60], a decrease in Tg and hardness of BM-comprising coatings was found revealing the plasticizing effect of the fatty acid tails [80, 81] attached to the acrylic polymer backbone. The present study found the opposite effect of copolymerized itaconated BM on the Tg and coating hardness; the higher the BM concentration, the higher the Tg and hardness. This phenomenon can be explained by the significant contribution of rigid methyl itaconate groups [59].
| Sample | Gloss at 60° (GU) | Transmittance at 500 nm (%) | WCA (°) | Hardness (%) | Tg (°C) |
|---|---|---|---|---|---|
| REF | 83.5 ± 0.1 | 91.4 ± 0.3 | 64.9 ± 1.5 | 5.8 ± 0.2 | 1.3 ± 0.3 |
| L_5 | 84.6 ± 0.1 | 91.2 ± 0.5 | 70.6 ± 3.5 | 8.4 ± 0.3 | 8.4 ± 0.4 |
| L_10 | 84.5 ± 0.2 | 90.5 ± 0.4 | 72.7 ± 2.7 | 8.8 ± 0.6 | 9.3 ± 0.2 |
| L_15 | 83.8 ± 0.1 | 90.0 ± 0.2 | 72.5 ± 1.9 | 10.3 ± 0.5 | 10.2 ± 0.3 |
| L_20 | 84.6 ± 0.1 | 90.3 ± 0.7 | 72.6 ± 3.7 | 12.3 ± 0.9 | 11.9 ± 0.3 |
| L_25 | 84.2 ± 0.5 | 90.2 ± 0.5 | 72.3 ± 0.7 | 10.6 ± 0.6 | 11.1 ± 0.6 |
| L_30 | 84.1 ± 0.1 | 90.3 ± 0.3 | 72.6 ± 0.9 | 14.8 ± 1.1 | 12.8 ± 0.2 |
a Impossible to determine.
One of the most pressing shortcomings of common latex protective coating films is their poor water resistance, which manifests itself in the deterioration of protective properties, as well as the appearance of water-whitening effect. The latter phenomenon is caused by light scattering induced by water clusters formed in the film interior due to water penetration [82]. The results of the water-whitening measurements are presented in Fig. 9. It was found that the higher the content of the introduced BM, the lower the water-whitening of the coating films. Moreover, employing high amounts of BM (25 and 30 wt.%) can dramatically improve the resistance of coatings against water whitening, which extends the possibility of their use in various outdoor applications. This phenomenon can be related to the introduced cross-linking that does not allow the water clusters to grow large enough to scatter light visible to the human eye [83].
The bio-based monomer derived from camelina oil and itaconic acid was prepared via a multi-step synthesis consisting of transesterification, epoxidation, and itaconation using monomethyl itaconate. The itaconated derivative of camelina oil was utilized as a reactive comonomer with standard petroleum-based acrylic monomers (0−30 wt. % of the bio-based monomer was included in the total monomer mixture) to produce film-forming polymer latexes using a semi-continuous emulsion polymerization technique. Successful polymerizations with conversions exceeding 90% and a coagulum content below 1.5 wt. % were performed in the case of compositions containing up to 20 wt. % the bio-based monomer in the monomer mixture. A drop in monomer conversion and an increase in coagulum content were observed for the compositions with a higher bio-based monomer content (conversion of 88.8 and 84.5% and coagulum content of 1.7 and 2.6% for the compositions comprising 25 and 30 wt. % of the bio-based monomer, respectively). The latex storage stability was not affected by the incorporation and content of the itaconated derivative. Infrared and Raman spectroscopies evidenced the successful incorporation of BM into the structure of latex polymers. AF4-MALS showed the presence of cross-linked polymers (nanogels) with a molar mass exceeding 108 g⋅mol–1, which is a consequence of the copolymerization of double-itaconated fractions originated mainly from linoleic and linolenic fatty acids. Regarding the coating properties, incorporation of the bio-based monomer did not deteriorate the gloss and transparency of the coating films. The copolymerization of the bio-based monomer caused an increase in Tg and hardness (about 11°C and 9%, respectively, comparing the reference and the highest bio-based monomer content coating compositions). This phenomenon indicates the suppression of the long fatty acid tails plasticization by the rigidity of methyl itaconate groups. Utilization of the bio-based monomer significantly enhanced the water-whitening resistance of coating films. These aspects make the itaconated derivative of camelina oil an attractive candidate for replacing standard petroleum-based monomers in the production of protective latex coatings with enhanced water resistance.
Acknowledgments
We are grateful to the Ministry of Education, Youth and Sports of the Czech Republic, for support through project no. UPA/SG321005. This work was also supported by the Slovak Grant Agency (VEGA 1/0461/21).
Author contributions Conceptualization: J.M.; Methodology: J.H., Š.P., V.L, E.K., and J.M.; Investigation: M.K., J.H., Š.P., and D.K.; Resources: M.K., and J.M.; Data curation: M.K.; Writing—original draft preparation: M.K., J.H., M.H., and J.M.; Writing—review and editing: J.M.; Visualization M.K., J.H., and M.H; Supervision: J.M. All authors have read and agreed to the published version of the manuscript.
Data availability
Data will be made available on request.
Conflict of interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to infuence the work reported in this paper.
Ethical approval
There are no ethical issues involved in this study.
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