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From Fossil to Bio–Based AESO–TiO2 Microcomposite for Engineering Applications

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05 November 2024

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07 November 2024

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Abstract
Environmental issues and reduction of fossil resources led to the partial or total substitution of petroleum based materials with natural raw renewable ones. The continuous decrease in world oil reserves and unpredictable price fluctuations compelled both academia and the industry to invest massively in the production of new materials. One expanding domain is the obtaining of engineering materials from raw renewable natural resources as sustainable eco–friendly polymers for different applications. This is the first study on the obtaining, thermal, morphological, dielectric and wettability characterization of a composite from thermally cured solvent–free bio–based epoxidized and acrylated soybean oil containing low quantities of reactive diluent and TiO2 microparticles as filler. The microcomposite also possessed reduced initial matrix formulation viscosity and without the toxic curing agents. The material was characterized by Fourier–transform infrared spectroscopy, differential scanning calorimetry, thermogravimetry, scanning electron microscopy, broadband dielectric spectrometry and contact angle measurements. Here we demonstrate that the composite exhibits superior thermal stability, glass transition temperature, dielectric constant and hydrophobicity to the pristine matrix. All investigations recommend the microcomposite for optoelectronic devices and as layer in thin–film transistors. This study brings new contributions to green chemistry and sustainable materials.
Keywords: 
Subject: Chemistry and Materials Science  -   Polymers and Plastics

1. Introduction

Environmental and economic issues together with reduction of petroleum oil reserves have determined both academia and industry to seek replacements derived from natural and sustainable resources (lignin, rosin acids, vegetable oils, chitosan, fats) [1,2,3,4,5]. Epoxides are one of the most important classes of resins used in obtaining different thermosets for various application fields. Epoxy resins account for 70% of the commercial markets and have a wide range of applications, from adhesive and coatings to aerospace [6,7]. Cured epoxy matrices exhibit excellent abrasive, chemical resistance and thermal, mechanical and electrical behavior, being well known as excellent insulators [8,9].
However, most epoxy resins are synthesized from the petrochemical environmentally hazardous epoxy monomer diglycidyl ether of bisphenol A (BPA). BPA is an endocrine–disrupting compound which also affects the central and immune system [10,11,12]. Furthermore, a thermosetting formulation may be comprised of the epoxy resin, curing agent, reactive diluent, filler, activator, catalyst etc. Hence, the BPA based epoxies are cured with low molecular weight petroleum based hardeners, such as amines, anhydrides, polyacids, Lewis acids, known to be toxic agents to both human health and the environment [13,14,15,16,17,18]. Moreover, most of them increase system viscosity. Epoxy resins are known for their high viscosity at room temperature [15,16], and poor toughening [17], these aspects leading to difficulties in processing and applications overcome by solvent addition of which most are toxic. Reactive diluents may surpass these impediments. Jagtap and More [19] have recently reviewed reactive diluents for epoxy resins. Of these, the acrylate–based multifunctional monomer trimethylolpropane triacrylate (TMPTMA) significantly enhanced the crosslinking density of epoxy acrylates without adding toxicity.
It is therefore that all the mentioned risk factors compelled the need to develop sustainable epoxy systems from renewable resources [20], following one of the green chemistry’s principles stated by Anastas and Eghbali [21]. Among these resources, during the last two decades vegetable oils, especially soy–bean and linseed, have received increasing attention due to their availability, low cost, chemical functionality and facile processing [22,23]. Due to their very low reactivity during polymerization, the double bonds in vegetable oils need to be converted into reactive functional groups. Vegetable oils can undergo epoxidation and, cured with an adequate hardener, produce a bio–based sustainable epoxy resin [24,25]. Due to its versatility, the epoxide ring can be further converted into a wide palette of functional groups through different reactions, such as with acids (e.g. acrylic), anhydrides or hydroxyl (alcohols, thiols, diols) [26]. Therefore, the development of soy–based epoxy resins for engineering applications remains a challenge for the polymer composite industries. Hence, both academic and industrial research is greatly expanding in exploring the feasibility of producing polymer composites based on epoxidized soybean oil. Such bio–based compounds may constitute raw materials to obtain polymeric formulations for micro– and nanocomposites (e.g. with metal oxides) for protection of different substrates (wood, metal, plastic) [27,28,29,30,31]. One example of metal oxide micro– and nanoparticles is TiO2 which are very good semiconductors, have high chemical resistance, low toxicity and excellent UV photoactivity [32]. Other examples include Ag nanoparticles or their combined systems (Ag–TiO2) which are biocompatible and have excellent antibacterial properties. ZnO nanoparticles offer excellent UV and antibacterial protection. CeO nanoparticles are excellent insulators due to their high chemical resistance. TMPTMA may replace traditional curing agents in different materials: (i) macroporous poly (glycidyl methacrylate–co–trimethylolpropane trimethacrylate)s with fine controlled porosity; (ii) poly(hydroxyethyl methacrylate) based nanocomposites applied in dentistry; (iii) organic column obtained through controlled or free–radical polymerization used as stationary phase in the technique of capillary liquid chromatography, and (iv) silicone rubber [19].
This is the first report the obtaining and characterization of a thermo–curable formulation based on epoxidized and acrylated soybean oil (AESO) containing TMPTMA as reactive diluent. This formulation was used as matrix in obtaining a bio–based composite with TiO2 microparticles filler (AESO–TiO2). The scope of this research is: (i) to reduce system viscosity; (ii) to simplify the system and its synthesis towards a green approach by eliminating the use of environmental and health hazardous solvent and curing agent in the matrix formulation for engineering sustainable microcomposites.

2. Experimental Section

2.1. Materials

The epoxidized soybean oil is a product under the commercial name LankroflexTM E2307 from Valtris Speciality Chemicals (Manchester, UK) (molar mass 975.399 g mol–1; density 0.997 g mL–1; acid value 0.7 mg KOH g–1; viscosity 325 mPa·s at 25oC; boiling point 150oC at 0.5 kPa; flash point 310oC). According to the manufacturer, Lankroflex™ E2307 is a low odor epoxy resin, certified safe for food contact and with medical approval. It is used in all types of PVC formulations in combination with metal soap stabilizers. It is also used as plasticizer for coatings applications, being an excellent water, oils and solvents repellent. Acrylic acid (98 % purity) was purchased from Acros Organics, Germany, and used as received. Methylhydroquinone was a product of Aldrich Chemistry, China. Triethylamine was a product of Sigma–Aldrich, Germany. Trimethylolpropane trimethacrylate (TMPTMA) was purchased from Sigma–Aldrich, USA and used as received. The TiO2 (IV) microparticles (99%) (d = 1–1.3 µm) were purchased from abcr GmbH, Germany.

2.2. Synthesis of AESO

100 g (0.102 mol) epoxidized soybean oil, 34.3 g acrylic acid (0.465 mol) and 0.04 g (3.2·10–4 mol) methylhydroquinone, as thermal polymerization inhibitor, were mixed in a glass flask with a round bottom (500 mL capacity) provided with three necks, water cooler and mechanical stirrer. The temperature required for the reaction was ensured with a silicon oil bath equipped with a thermostat. The reaction mixture was preheated to 40oC for 30 minutes followed by the adding of 0.15 mL triethylamine. To complete the chemical reaction, the mechanical stirring was continued for another 6 hours at 70–80oC. After the completion of the reaction the AESO was washed a few times in a funnel with saline solution (28 g 100 mL–1) to remove the unreacted acrylic acid and dried on anhydrous sodium sulfate. After filtration, 267 g of AESO were obtained with a reaction yield of 98.8%. The final product was of yellow–orange coloring and oily texture. The characterization of AESO by 1H–NMR technique is detailed in the supplementary materials (Text S1, Figure S1).

2.3. Obtaining of the AESO–TiO2 Microcomposite

44.75 g AESO and 2.521 g (5.63%) TMPTMA were stirred until the homogeneous acrylated epoxy formulation for the matrix (AESO–m) was obtained. Afterwards the AESO–m formulation was mixed with 0.89 g (2%) TiO2 microparticles in an ultrasonication bath (S 15 Elmasonic from Elma Schimdbauer GmbH, Singen, Germany) for 15 minutes and the AESO–TiO2 formulation was obtained. The thermally curable AESO–TiO2 liquid formulation was of a white pasty texture. In order to obtain the AESO–TiO2 composite the experimental thermal conditions needed to be established. In this sense, 10 mg of composite formulation was first placed in a pierced and sealed shut aluminum crucible and a differential scanning calorimetry (DSC) heating curve was recorded from room temperature to 300oC with a 10oC min–1. A wide exothermal peak was observed at 268oC, describing the thermal curing process, shown in the supplementary materials (Figure S2). Then the rest of the composite formulation was heated in an oven (Vulcan 3–130 from Dentsply Ceramco, York, PA, USA) in the same conditions and held 10 minutes at 268oC to obtain the solid white and hard AESO–TiO2 microcomposite (Figure 1b).

2.4. Methods

2.4.1. Proton Nuclear Magnetic Resonance (1H–NMR)

The 1H–NMR spectrum of AESO was recorded on an Advance DRX 400 (Brüker, Germany) in CDCl3 as solvent. The device was equipped with a 5 mm direct detection z–gradient probe.

2.4.2. Fourier–Transform Infrared Spectroscopy (FT–IR)

The FT–IR spectra were registered on a Vertex 70 apparatus (Brüker, Germany) equipped with a MIRacleTM ATR accessory provided with diamond crystal plate with a 1.8 mm diameter. The liquid formulations were measured within KBr pellets and the solid cured matrix and composite using the ATR module.

2.4.3. Thermogravimetric Analysis (TGA)

The thermogravimetric curves were recorded on a STA 449 F1 Jupiter apparatus (Netzsch–Gerätebau GmbH, Selb, Germany). Approximately 14 mg of sample was placed and heated in alumina crucibles. A heating rate of 10 oC min–1 was applied in nitrogen atmosphere (flow rate 50 mL min–1) up to 700 oC.

2.4.4. Differential Scanning Calorimetry (DSC)

The differential scanning calorimetry curves were recorded on a DSC 200 F3 Maia apparatus (Netzsch–Gerätebau GmbH, Germany). Approximately 10 mg of sample was placed heated in aluminum crucibles which were sealed shut with pierced lids. Experiments were conducted in nitrogen atmosphere (flow rate of 50 mL min–1) and with a heating rate of 10 oC min–1. Calibrations were made with standard indium.

2.4.5. Scanning Electron Microscopy (SEM)

The scanning electron microscopy micrographs were recorded on a Verios G4 UC scanning electron microscope (Thermo Fisher Scientific, Brno–Cernovice, Czech Republic). The samples were fixed onto aluminum stubs using double adhesive carbon tape and afterwards coated with 6 nm platinum with a Leica EM ACE200 sputter coater (Leica, Vienna, Austria) for providing electrical conductivity and preventing charge accumulation during the electron beam exposure. There was used a secondary electron detector (ETD detector – Everhart–Thornley detector) in order to highlight the size and shape of the particles at an acceleration voltage of 5 kV and a spot size of 0.4 nA.

2.4.6. Broadband Dielectric Spectroscopy

The broadband dielectric spectroscopy measurements were undertaken on a Broadband Dielectric Spectrometer (Novocontrol Technologies, Germany). The dielectric parameters were recorded isothermally, in a frequency window between 1 Hz and 1 kHz and at temperatures between –50 oC and 250 oC.

2.4.7. Contact Angle Measurements

Static contact angle measurements were performed on a CAM–200 instrument (KSV Instruments, Helsinki, Finland) with sessile drop profile analysis. The measurements were undertaken at room temperature by placing a 1 μL drop of liquid on the sample surface. Each contact angle value was a mean of five measurements taken on different surface sites.

3. Results and Discussion

3.1. Structural Characterization by FT–IR

Figure 2a shows the comparative FT–IR spectra of LankroflexTM E2307 and the AESO formulation. The signals at 822 cm–1 and 1244 cm–1 in the FT–IR spectrum of LankroflexTM E2307 demonstrate the presence of the epoxy rings [33]. The FTIR spectrum in AESO confirms the synthesis of its structure via the formation of the peaks at 3460 cm–1 corresponding to the –OH moiety from epoxy ring opening and associated with intermolecular polymeric hydrogen bonding [34]. The wider peak at 1738 cm–1 is due to the appearance of supplementary carbonyl (C=O) entities. The vibration of the –CH=CH2 in acrylate is depicted by the signals at 1409 cm–1, 985 cm–1 and 812 cm–1. The –CH=CH– stretching vibration is represented by the peak at 1630 cm–1 [34].
Figure 2b shows the FTIR spectrum of the microcomposite AESO–TiO2. Compared to the FT–IR spectra in Figure 2a, the complex FT–IR spectrum of the composite AESO–TiO2 contains more signals originating also from the bulky TMPTMA structure and its curing with and within the AESO matrix [35]. According to the literature, the peaks in the range 3000–3750 cm–1 and the one at 3258 cm–1 may correspond to the stretching vibrations of different hydrogen bonds between –OH entities. The peak at 1640 cm–1 corresponds to the terminal C=C bonds of TMPTMA and its very low intensity indicates their implication in curing reactions. Due to their bulky nature, the TMPTMA molecules become sterically hindered, restricting mobility in the three–dimensional composite network and leaving some unreacted double bonds in the system [36]. The peak at 1774 cm–1 could be attributed to some new carbonyl structures, such as conjugated anhydrides with 5 members and/or cyclic aldehydes [37].

3.2. Thermal and Morphological Characterization

Figure 3a, b show the thermogravimetric curves, with their corresponding first derivatives (DTG) curves, respectively. Table S1 in the supplementary material lists the data extracted from the thermal analyses: the static heat resistant index (Ts), calculated with Equation (1) [38], as criterion to assess thermal stability; the temperature at 5 % (T5%) mass loss; the temperature at 30 % (T30%) mass loss; the temperature at maximum rate of degradation (Tpeak); mass loss (%) of each thermal degradation stage (M); the residual mass (%) at the end of the thermal degradation process (700 oC) (R) and the glass transition temperature (Tg).
Ts = 0.49·[T5% + 0.6·(T30% − T5%)]
The thermogravimetric curves (Figure 3a) show a significant increase in the thermal stability of the materials after curing, from Ts = 150 oC for AESO to Ts = 180 oC and 183 oC for the cured matrix (AESO–m) and composite (AESO–TiO2), respectively. The DTG curves indicate a thermal degradation profile in three overlapping stages, almost indiscernible in the TGA curves. The Tpeak in the DTG curves shifts toward higher values with curing and TiO2 addition. Also, the mass loss (M) decreases for thermal degradation stages I and II and increases for stage III of AESO–m and AESO–TiO2 compared to AESO (Table S1). The three overlapping thermal degradation stages are due to initial random chain scission followed by almost simultaneous branching and crosslinking phenomena in AESO [39]. Stage III of thermal degradation (440oC–490oC), where the mass loss increases (Table S1), is better evidenced for AESO–m and composite AESO–TiO2 where the crosslinks in the matrix decompose (DTG curve peak Tmax in Table S1 and Figure 3a).
The DSC second heating curves of the studied materials (Figure 3b) show the presence of a single neat Tg, demonstrating the formation of a fully thermally cured composite, a confirmation of a good miscibility of the components and a uniform distribution of the TiO2 microparticles within the composite matrix. These aspects also led to the increasing the Tg of the composite from –20 oC (AESO–m) to –10 oC (AESO–TiO2), as confirmed by the SEM measurements in Figure 3c, d.

3.3. Wettability

For materials to be used in applications domains such as anti–weathering protective coatings and in microelectronics it is also crucial for them to possess hydrophobicity [40]. The water contact angle of the matrix and composite was measured. It was observed that compared to the matrix, with a contact angle value below 90o, the microcomposite possesseed a contact angle value above 90o. This aspect implies that the incorporation of TiO2 microparticles renders the composite hydrophobic. The contact value of the matrix was 88o, indicating a hydrophilic behavior. The micro composite manifested hydrophobic character, displaying a contact angle value of 96o. An explanation could reside in the microparticles mobility, as demonstrated by the increased permittivity in the dielectric studies discussed in the next section, allowing them to migrate in the polymer matrix, leading to a higher packing degree and forming a microstructured compact texture. This phenomenon not only increases surface hydrophobicity, but at the same time generates a good compatibility with the polymer matrix, as previously shown by DSC and SEM results. Moreover, the anchoring of TiO2 microparticles may expose alkyl moieties at the solid−air interface, which, together with the suitable morphology, also increases the contact angle value of the microcomposite [41].
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Figure 4. (a) Water contact angle of the cured epoxy matrix and (b) of the microcomposite.
Figure 4. (a) Water contact angle of the cured epoxy matrix and (b) of the microcomposite.
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3.4. Broadband Dielectric Spectroscopy

The dielectric properties play a crucial role in assessing the use of epoxy resins as electrical insulators, and are greatly influenced by their molecular structure [42]. The behaviors of dielectric constant, ε’, and dielectric loss, ε”, parameters with frequency are presented in Figure 5. The isothermal plots are selected at room temperature. The ε’ parameter is related with the dipolar activity of a material. Following the Figure 5a, ε’ decreases gradually towards increasing frequency, as generally observed for polymer materials [43]. The magnitude of ε’ of the composite is higher than that of the cured matrix. The enhanced magnitude of ε’ for the composite may be due the incorporation of TiO2 filler in the matrix that induces supplementary polarizable units [44]. Increased permittivity values of the matrix and the composite at lower frequencies may be associated either with the Maxwell–Wagner effect, when the alternating current and applied potential are in phase, or direct current conductivity, defined as the result of increased ion mobility, or both, detailed in the literature [45,46]. The dielectric loss encompasses the dissipation energy required to align the polarizable units in the direction of an external electrical field. Similar with ε’(f) dependences, the magnitude of ε” (Figure 5b) decreases considerably, due to the vegetable oil entities.
The isochronal plots of ε’ and ε” vs. temperature of the studied materials are comparatively displayed in Figure 5c, d. The spectra are selected at 1 Hz. At temperatures lower than 0oC, a clear relaxation process is retrieved as a step–increase in the dielectric spectra of ε’, higher for the composite than for the matrix (Figure 5c), and as a well–defined dielectric peak in the spectra of ε” (Figure 5d). According with the DSC data, the relaxation process is connected with the glass transition temperature of the materials, found in both ε’(T) and ε”(T) profiles of the two materials. At temperatures between 100oC and 200oC, a secondary step increase is detected in the ε’(T) profiles of both samples. This step–increase may suggest hindering or cleavage of physical bonds [47]. In the ε”(T) profiles of the structures, no relaxation process is detected between 100oC and 200oC. Due to its enhanced dielectric properties compared to the cured matrix, the microcomposite may also be recommended in applications such as optoelectronic devices and as dielectric layer in thin–film transistors [48].

4. Conclusions

This is the first report on a composite from solvent and hardener–free bio–based acrylated and epoxidized soybean oil matrix with 2% TiO2 microparticle filler. The material possessed reduced initial matrix formulation viscosity and was obtained through thermal curing. The components showed excellent miscibility within the cured matrix. Here we demonstrated that the microcomposite exhibited superior thermal stability, glass transition temperature, hydrophobicity and dielectric behavior to those of the pristine cured matrix. All results recommend the microcomposite for optoelectronic devices and as layer in thin–film transistors.

Supplementary Materials

The following supporting information can be downloaded at the website of this paper posted on Preprints.org., Text S1: Characterization of The Acrylated and Epoxidized Soybean Oil (AESO); Figure S1: The 1H–NMR spectrum of AESO; Figure S2: The first DSC heating curve of AESO–TiO2 formulation; Table S1: Thermal analyses data.

Author Contributions

Conceptualization, C.-D.V. and D.R.; methodology, C.-D.V., L.R. and D.R., investigation C.-D.V., L.R., D.R. and M.A.; writing-original draft preparation, C.-D.V. and M.A.; writing–review and editing, C.-D.V., D.R.; project administration, C.-D.V and D.R. All authors have read and agreed to the published version of the manuscript.

Acknowledgments

This article is dedicated to the memory of Acad. Bogdan C. Simionescu (1948–2024).

Conflicts of Interest

The authors claimed no conflicts of interest.

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Figure 1. (a) Synthesis of AESO and (b) obtaining of the cured epoxy matrix and microcomposite.
Figure 1. (a) Synthesis of AESO and (b) obtaining of the cured epoxy matrix and microcomposite.
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Figure 2. (a) FT-IR spectra of the epoxidized soybean oil and AESO and (b) FT-IR spectrum of the microcomposite.
Figure 2. (a) FT-IR spectra of the epoxidized soybean oil and AESO and (b) FT-IR spectrum of the microcomposite.
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Figure 3. (a) Thermogravimetric curves, (b) DSC curves, (c) surface SEM micrographs of the cured matrix and (d) microcomposite. Scale marks at 5 µm and 1 µm.
Figure 3. (a) Thermogravimetric curves, (b) DSC curves, (c) surface SEM micrographs of the cured matrix and (d) microcomposite. Scale marks at 5 µm and 1 µm.
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Figure 5. (a), (b) The evolution of dielectric constant and dielectric loss with frequency at room temperature. (c), (d) Comparative evolution of dielectric constant and dielectric loss with temperature at a frequency of 1 Hz.
Figure 5. (a), (b) The evolution of dielectric constant and dielectric loss with frequency at room temperature. (c), (d) Comparative evolution of dielectric constant and dielectric loss with temperature at a frequency of 1 Hz.
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