Abstract
Methacrylated lignin model compounds (LMCs, i.e., eugenol and guaiacol) monomers are ideal candidates as styrene replacements because they have low volatilities and can free-radically polymerize with vinyl ester resins. This article reports the synthesis of methacrylated eugenol (ME), methacrylated guaiacol (MG) using LMCs and methacrylic anhydride in the presence of 4-dimethylaminopyridine (DMAP) as a catalyst. ME and MG were characterized using FT-IR, 1H-NMR and 13C-NMR. The thermal and mechanical properties of the samples prepared at 30°C from o-cresol epoxy based vinyl ester resin (VEOCN) using MG and ME, respectively, as reactive monomers, in the presence of benzoyl peroxide (2 phr) as initiator were further investigated using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA/DTG/DTA) and universal testing machine (UTM). Chemical and corrosion resistance of cured VEOCN samples coated on steel panels were also evaluated as a function of percentage weight loss and with the help of scanning electron microscopy (SEM), upon immersing the VEOCN samples in 1 m HCl, 1 m NaOH and 1 m NaCl solutions for 90 days. Thermal, mechanical and chemical performance of VEOCN using ME and MG was also compared with VEOCN samples containing styrene and methyl methacrylate (MMA) as reactive monomers.
1 Introduction
In the polymer industry, concerns over the environmental issues due to the volatile organic content (VOC) emissions coupled with increasingly strict legislations have forced us to move away from the use of petroleum based monomers towards the use of monomers from the renewable and forestry feedstock. These bio-based and green monomers are considered to be the wave of the future. Styrene and different acrylates are the commonly used petroleum based reactive monomers, employed to lower down the viscosity of vinyl ester (VE) resin and to facilitate the processing of VE resin in liquid molding techniques for the fabrication of large scale composite parts. These VE resins encompass exceptional mechanical and chemical properties coupled with outstanding corrosion resistance and heat performance, which makes them a good selection for many end uses, such as polymer matrix composites and especially fiber reinforced polymer (FRP) for construction applications [1–3], coatings for solvent storage tanks, sewer pipes [4–6] and adhesives [7], etc. Petroleum based reactive monomers, i.e., styrene and different acrylates, also act as a linear chain extender and improve the polymer performance such as crosslink density, mechanical strength, percent elongation, hardness, chemical resistance, scratch resistance and surface finish, etc., as they delay the onset of gelation during curing and also address the diffusion limitation issue [8–14]. However, as these petroleum based reactive monomers have been designated as a hazardous pollutant due to their high VOC, the attempts are being made to replace these monomers either with bimodal blends of VE monomers [15, 16] or with non-volatile fatty acid monomers obtained from renewable resources [17] in order to reduce emission.
There are a number of reasons to study the effect of methacrylated lignin model compounds (LMC) compounds, i.e., methacrylated eugenol (ME) and methacrylated guaiacol (MG), as reactive monomers on the mechanical and physico-chemical performance of VE resin. These methacrylated LMC compounds, i.e., ME and MG, are an excellent alternative to these petroleum based reactive monomers because of their low cost and low volatility [18]. In addition, these reactive monomers are derived from renewable resources, i.e., lignin. Therefore, not only would the use of these methacrylated LMC compounds as reactive monomer for VE resin in liquid molding applications reduce the VOC emissions, thereby reducing the health and environmental risks, they would also promote tglobal sustainability.
In this paper, ME and MG were explored to study the structure-property relationship of these bio-based reactive monomers with previously synthesized o-cresol epoxy novolac based VE resin (VEOCN). The thermal, mechanical and chemical performance of VEOCN samples cured with ME and MG were also compared with the VEOCN samples cured with petroleum based reactive monomers, i.e., styrene and methyl methacrylate (MMA). The main objective of this study is to provide important information for the feasibility of replacing hazardous petroleum based reactive monomers with monomers from the renewable and forestry feedstock.
2 Materials and methods
2.1 Materials
The o-cresol based novolac resin (OCN) and the epoxy resin based on OCN resin (EOCN) were synthesized according to the method reported in our previous research article [19]. Eugenol (99% pure, Lobachemie, Mumbai, India), guaiacol (99% pure, Lobachemie, Mumbai, India), methacrylic anhydride (94% pure, Alfa Aesar, Heysham, England), and 4-dimethylaminopyridine (DMAP) (99% pure, Alfa Aesar, Heysham, England), were used for the synthesis of ME and MG. Styrene (99.5% pure, ACROS ORGANICS, NJ, USA), MMA (98% pure, ACROS ORGANICS, NJ, USA), MG and ME were used as reactive monomers in the present study.
2.2 Synthesis of vinyl ester resin from o-cresol epoxy novolac (VEOCN)
VE resin (Figure 1) was prepared using 1:0.9 molar ratios of OCN resin (epoxide equivalent weight=164 g/eq. by pyridinium chloride method [20]) and methacrylic acid in the presence of triphenyl phosphine (1 phr by weight of epoxy resin) and hydroquinone (200 ppm) at 85±1°C for 3.5 h to obtain a product with an acid value of ∼7 mg of KOH/gm solids determined according to ASTM D 1636 as reported earlier [19].
Reaction Scheme for the synthesis of VEOCN.
2.3 Synthesis of bio-based reactive monomers, i.e., MG and ME
A mixture of DMAP (2 mole% of methacrylic anhydride) and eugenol/guaiacol was placed into 100 ml round bottom flask and contents were stirred on a magnetic stirrer. N2 gas was subsequently purged for 1 h into the reaction vessel to remove moisture and oxygen. Methacrylic anhydride (1.2 moles) was further added and the reaction mixture was stirred at room temperature for 3 h. The temperature was then increased further to 45°C for 24 h. The reaction mixture was further cooled to room temperature and diluted with 150 ml ethyl acetate. Unreacted methacrylic acid and methacrylic anhydride was then removed by washing the organic phase with saturated aqueous NaHCO3 solution until the carbon dioxide no longer reacted. Organic phase thus obtained was then washed with 1.0 m aqueous NaOH solution. The phase was then neutralized by 1.0 m aqueous HCl solution, and was further dried over sodium sulphate. Distillation under reduced pressure was carried out in order to remove ethyl acetate and pure ME/MG was obtained [18].
2.4 Characterization
2.4.1 Structural characterization:
The structural characterization of methacrylated LMCs, i.e., ME and MG was done using FT-IR, 1H-NMR and 13C-NMR spectroscopy. The FTIR spectra of the samples were recorded by putting a small drop of sample between two KBr discs. Perkin Elmer FTIR Spectrometer was used for this purpose. The 1H-NMR and 13C-NMR spectra of the samples were recorded using Bruker Avance II 400 NMR spectrometer, taking CDCl3 as solvent and tetramethylsilane as an internal standard.
2.4.2 Determination of physical properties:
Viscosities of the VEOCN samples containing 40% w/w styrene, ME, MG and MMA were measured by using a Brookfield Viscometer (LVDV II + Pro, Brookfield, USA) using spindle no. L62 at 120 rpm. Density measurements for the above samples were performed at 25°C using pycnometer.
2.5 Curing and decomposition behavior
The samples for assessing the curing and decomposition behavior were prepared by mixing 10/4/0.2 (w/w) of VEOCN, reactive monomers – styrene, MMA, MG and ME- in the presence of free radical initiator-benzoyl peroxide, respectively, in a small glass vial and stirred vigorously with a glass rod at 30°C until the mixtures became homogeneous in nature. Differential scanning calorimetry (DSC) scans under dynamic conditions were obtained with a program rate of 10°C/min from 35°C to the temperature at which the exothermic reactions were completed.
From the dynamic DSC scans, a temperature was selected at which an appreciable rate of curing could be observed. DSC runs under isothermal conditions at the selected temperatures were then carried out to determine the apparent time required for the completion of each of the curing reactions. The curing conditions (temperature and time) thus determined were used to cure the resin samples for subsequent studies on thermal stability. Thermal stability of the samples cured isothermally at 90±2°C in hot air oven was evaluated by thermogravimetry. An EXSTAR TG/DTG 6300 was used to record TG/DTG traces in nitrogen atmosphere (flow rate=200 ml/min) at a heating rate of 10°C/min witha sample size of 11±5 mg. The relative thermal stability of the resin was quantitatively estimated by comparing the temperatures for a particular degree of weight loss.
2.6 Mechanical behavior
In mechanical characterization, the mechanical properties such as compressive strength (as per ASTM D3410), flexural strength (as per ASTM D790) and tensile strength (as per ASTM D3039) of VEOCN cured using styrene, MMA, MG and ME as reactive monomers were determined by Hounsefield-25KN universal testing machines at a deformation rate of 2 mm/min.
2.7 Chemical resistance
Mild steel panels (15 cm×10 cm), prepared according to British standard specification 1449, were used to evaluate the chemical and corrosion resistance of VEOCN samples containing styrene, MMA, MG and ME as reactive monomers. One side of the panel was coated with coaltar epoxy primer to protect against chemical environment used to evaluate the chemical and corrosion resistance. While the tested side of the panel was coated with VEOCN resin containing styrene, MMA, MG and ME as reactive monomers and cured at 90±5°C for 24 h. Chemical and corrosion resistance behavior in terms of percent weight loss of the cured resin samples coated on mild steel panels was studied by plunging the coated panels of identified weights in 1 m HCl, 1 m NaOH and 1 m NaCl solutions for 90 days at room temperature.
Percentage weight loss was then calculated using the following formula:
where, Wi=Dry weight of the sample before immersion
Wf=Dry weight of the sample after immersion
2.8 Scanning electron microscopy (SEM)
SEM was also used to analyze morphological change on the surface of coated VEOCN resin samples (containing styrene, MMA, MG and ME as reactive monomers) due to chemical exposure given to study the chemical and corrosion resistance of the cured VEOCN resin samples. The SEM analysis was performed by JSM-6610 machine and to enhance the conductivity of the samples, a thin film of gold was mounted on the samples before the SEM photographs were taken.
3 Results and discussion
Lignin is an abundantly present amorphous natural polymer which is derived from wood and is an integral part of the secondary cell walls of plants. Lignin is a complex polymer of aromatic alcohols known as monolignols and is rich in aromaticity with the capacity to produce lower molecular weight aromatic compounds when broken down. Because of its effortless availability and structure, lignin is now becoming an interesting material for the use as a component to produce viable and green monomers for the polymer industry. In this article, we have used eugenol and guaiacol, the pyrolytic products of lignin, as reactive monomers to investigate the thermal, mechanical and chemical behavior of the VEOCN. The chemical structure and synthesis of ME and MG have been shown in Figure 2. The synthesis of ME and MG is accomplished by direct esterification of eugenol and guaiacol using highly reactive methacrylic anhydride in the presence of DMAP as catalyst. The by-product formed during the synthesis of ME and MG, i.e., methacrylic acid and unreacted methacrylic anhydride are removed by successive washings of the organic phase with a saturated aqueous NaHCO3 solution and a 1.0 m aqueous NaOH solution followed by a 1.0 m aqueous HCl solution. ME and MG are obtained as pale yellow to colorless liquids. The purity of the synthesized reactive monomers, i.e., ME and MG, is estimated to be ≥98% based on the spectroscopic data. The physio-chemical properties of all the reactive monomers, i.e., styrene, ME, MG and MMA and VEOCN resin samples containing these reactive monomers have been summarized in Table 1.
Reaction scheme for the synthesis of ME and MG.
The physico-chemical properties of the reactive monomers and the VEOCN resin samples containing these reactive monomers.
| Samples | Viscosity (in cps at 25°C) | Density (in g/cm3 at 25°C) |
|---|---|---|
| Styrene | 0.75 | 0.91 |
| MMA | 0.84 | 0.94 |
| ME | 29.62 | 1.20 |
| MG | 19.37 | 1.24 |
| VEOCN+Styrene | 3114 | 0.99 |
| VEOCN+ME | 4370 | 1.39 |
| VEOCN+MG | 4827 | 1.34 |
| VEOCN+MMA | 2906 | 1.03 |
FTIR spectra shown in Figure 3(A) and (B) illustrate the notable absorption peaks due to carbonyl wagging vibration at 1739 cm-1, terminal C=C wagging vibration at 1638–1637 cm-1 and terminal C=CH2 bending vibration at 947–946 cm-1 due to the methacrylate group in ME and MG, respectively. Moreover, C=CH2 bending vibration associated with the allyl group present in ME is observed at 916 cm-1. In the 1H-NMR spectra in Figure 4(A) and (B) of ME and MG, characteristic resonance signals due to aromatic protons are observed at 6.8–7.1 ppm, =CH2 protons of the methacrylate group at 5.9, 6.3 ppm (carbon 2), -O-CH3 protons at 3.7 ppm (carbon 3) and -CH3 protons of the methacrylate group (carbon 1) at 1.9 ppm. The additional proton resonance signals due to -CH2 (carbon 4), =CH2 (carbon 6) and =CH- (carbon 5) protons of the allyl group present in ME are also observed at 3.3, 5.1 and 5.8 ppm, respectively. Figure 5(A) and (B), shows the 13C-NMR spectra of MG and ME, respectively, in which the appearance of peaks at 123.7 ppm and 135.5 ppm due to the =CH2 (carbon 2) and CH2=C-CH3 (carbon 3) carbons confirm the absence of homopolymerization of the reactive monomers, i.e., MG and ME during their synthesis. Moreover, in Figure 5(A) and (B), the peaks observed at 18.5 ppm, 56 ppm, 138–139.5 ppm, 151 ppm and 165.5 ppm are due to the -CH3 (carbon 1), -OCH3 (carbon 10), the aromatic carbon bearing methacrylate group (carbon 5), the aromatic carbon bearing -OCH3 group (carbon 11) and the carbonyl carbon (carbon 4), respectively. The peaks of the allylic group present in ME are also observed at 40 ppm (carbon 12), 116 ppm (carbon 14) and 137.5 ppm (carbon 13) in Figure 5(B).
FT-IR of (A) ME and (B) MG.
1H-NMR of (A) ME and (B) MG.
13C-NMR of (A) ME and (B) MG.
3.1 Curing behavior
The average molecular weight of the VE resin was determined as 636 g/mol by GPC. Reactive monomers, styrene, MMA, MG and ME, was added to the resin by calculating the ratio of the vinyl group of the reactive monomer to that of the VE group, taking into account the molecular weight of each monomer and the number of vinyl groups per monomer. Typical DSC scans for the curing of the samples prepared by mixing 10/4/0.2 (w/w) [21] of VEOCN, reactive monomers, styrene, MMA, MG and ME, in the presence of free radical initiator-benzoyl peroxide, respectively, at a program rate of 10°C/min are shown in Figure 6. An exothermic heat flow during the curing of the VEOCN samples containing styrene, MMA, MG and ME as reactive monomers was observed in the range of 85–150°C. The curing behavior of VE resins varies with the chemical structure of the VE resin and the different reactive monomers. The curing process is initiated by generating free radicals by decomposing the benzoyl peroxide under the influence of temperature within the resin mass. These free radicals thus formed react to open the double bond of the vinyl groups of the VEOCN and the reactive monomers, styrene or MMA or MG or ME. Once opened, the resin vinyl group is highly reactive and rapidly undergoes an exothermic reaction by combining with several vinyl groups available from the unreacted resin as well as the reactive monomers and thus forms a thermosetting network [22]. Under the same curing conditions, low onset temperature (Ti) and peak temperature (Tp) can be taken as indicators of the high reactivity of the reactive monomers with VE resin during curing reactions. The onset temperatures (105.21°C, 93.20°C, 94.05°C and 97.73°C) and peak temperatures (118.16°C, 100.37°C, 110.24°C and 115.58°C) for VEOCN samples containing styrene, MMA, MG and ME, respectively, illustrated in Table 2, depict that the reactivity of MG and ME towards VEOCN is comparable with MMA and higher than that of styrene as reactive monomer, during curing in the presence of benzoyl peroxide (BP) as free radical initiator.
Typical DSC scans for curing at 10°C/min of VE resin samples (A) VEOCN + MG and (B) VEOCN + ME.
The DSC results of the VEOCN samples cured using styrene, MMA, MG and ME as reactive monomers, respectively, during curing.
| Sample | Onset temperature (°C) | Peak temperature (°C) | Endset temperature (°C) |
|---|---|---|---|
| VEOCN+styrene | 105.21 | 118.16 | 127.21 |
| VEOCN+MMA | 93.20 | 100.37 | 106.32 |
| VEOCN+MG | 94.05 | 110.24 | 121.66 |
| VEOCN+ME | 97.73 | 115.58 | 126.77 |
The reason for lower reactivity of styrene towards VEOCN than ME and MG may be due to the greater resonance stability of styrene free radical which might hinder the curing reaction between VEOCN and styrene. This fact results in the rise in onset and peak temperatures of VEOCN samples containing styrene as compared to the VEOCN samples containing MMA, ME and MG. However, the allylic C-H bonds present at the α-carbon of the ester group in the MMA, ME and MG do not undergo extensive degradation chain transfer (DCT). This may be due to the stabilization of these allylic free radicals by the ester group, which lowers the reactivity of these radicals towards DCT but simultaneously the reactivity of these radicals towards chain propagation reaction is enhanced [23]. Furthermore, methacrylate radicals present in MG and ME are more polar compared to those present in styrene and consequently more reactive towards the vinylic C=C of the methacrylate group present in the VEOCN. The enthalpy of the reaction (ΔH) illustrated in Table 2, is also observed to be higher in the VEOCN sample containing styrene than in the samples containing MG and ME which indicates a lower degree of curing or crosslinking in VEOCN sample containing styrene than in the samples containing MG and ME [24]. When comparing the reactivity of ME and MG from Figure 6 and Table 2, it can be seen that ME is found to be less reactive than MG, as the allyl substituent present at the para position of the methacrylate group in ME forms a stable free radical ion due to the resonance stabilization and this stable free radical ion of the allyl substituent present in ME competes with the normal propagation reaction step during the curing reaction between ME and the VEOCN resin system and inevitably disrupts and decelerates the chain growth by DCT reaction [25]. The resulting allyl free radical does not readily add to the VE monomer but once all the methacrylic C=C groups of the ME molecule undergo copolymerization with vinylic C=C groups of the VEOCN, the allylic C=C groups would further take part in the copolymerization [26]. The resonance stabilization of the allylic free radical in the case of ME is shown in Figure 7.
The resonance stabilization of the allylic free radical in ME molecule taken for study.
3.2 Decomposition behavior
The thermal stability of thermosetting resins have been assessed by thermogravimetric analysis (TG/DTG/DTA), where the sample mass loss due to the volatilization of the degraded by-products is monitored as a function of a temperature ramp. The thermal stability of thermosetting resins is one of the significant aspects in determining its end use and is greatly influenced by polar moieties present on the VE resins and reactive monomers which can be further altered by the physical and chemical structure and the secondary interactions between the cured network systems. Figure 8 shows the noticeable single step mass loss on the comparative thermogravimetric (TG) scans of the VEOCN samples containing styrene, MMA, MG and ME as reactive monomers and benzoyl peroxide as free radical initiator. The rate of decomposition at a specific temperature can be determined as the tangential slope of the TGA trace. The temperature at the maximum rate of decomposition, i.e., Tmax, can be taken as an indicator of the extent of the crosslinking in cured VEOCN samples containing different reactive monomers. The Tmax are 416°C, 415°C, 420°C and 423°C for cured VEOCN samples containing styrene, MMA, MG and ME, respectively. TA has also proved to be a useful analytical technique in evaluating kinetic parameters such as the energy of activation, which provides valuable quantitative information regarding the stability of the thermosetting resins. A higher activation energy corresponds to better thermal stabilities of the cured samples of the resins. Various methods have been reported in the literature by different investigators to estimate the kinetic parameters of thermal degradation reaction. In this article, the activation parameter, i.e., the activation energy (Ea), for the cured VEOCN samples containing styrene, MMA, MG and ME as reactive monomers, has been worked out by using the well known non-isothermal integral method by Dharwadkar and Kharkhanawala [27] as it is independent of the sample size and the heating rate used:
Comparative TGA scans for cured VE resin samples containing styrene, MMA, MG and ME as reactive monomers.
where α is the fraction degraded, Ea the activation energy, R the gas constant, Ti the temperature of inception of the reaction, Tf the temperature where the reaction get completed, θ = (T-Ts) (the difference between the maximum decomposition temperature (Ts) and temperature under consideration (T), C a constant. The values of ln[ln(1–α)–1] are plotted against the corresponding θ values and Ea is calculated from the slope (m) of the straight line. The activation energies of decomposition (E) are calculated by using equations 1 and 2 and the values of Ea have been summarized in Table 3.
Temperature of 1–20% weight loss in TGA and activation energy (Ea) of cured VEOCN samples containing styrene, MMA, MG and ME as reactive monomers, respectively.
| Sample | Temperature (°C) at% weight loss | Ea (KJ/mol) | ||
|---|---|---|---|---|
| 5% | 10% | 20% | ||
| VEOCN+styrene | 307 | 345 | 369 | 183.57 |
| VEOCN+MMA | 305 | 330 | 360 | 179.09 |
| VEOCN+MG | 309 | 351 | 374 | 200.27 |
| VEOCN+ME | 325 | 360 | 387 | 209.59 |
When comparing the Tmax values from Figure 8, the temperatures at which 5, 10 and 20% weight loss incurred and the Eas data for cured VEOCN sample summarized in Table 3, it is observed that the VEOCN samples containing MG and ME show better thermal stability than the samples containing styrene and MMA. The reason for this behavior can be attributed to the higher reactivity of the methacrylate group present in ME and MG which increases the degree of crosslinking in cured VEOCN samples and also the presence of aromatic moieties in MG and ME monomers which provides rigidity in cured VEOCN network. Also, it is noted that the VEOCN samples containing ME are found to be even greater than that of MG since the presence of the allyl group also increases the reactive sites and lead to higher degree of crosslinking.
3.3 Mechanical performance
The mechanical behavior of the VE resins shows great dependence on the physical structure and chemical composition of the VE resin and reactive monomers. Secondary interactions between the polymeric chains and the degree of crosslinking of the cured VE resin system also affect the mechanical properties of the VE resin samples. From Figure 9, it can be observed that the VEOCN samples prepared from MG as bio- based reactive monomers show excellent mechanical properties, i.e., compression, flexural and tensile strength as compared to the VEOCN samples containing ME, styrene and MMA as reactive monomers. This might be due to higher reactivity of vinylic C=C of the methacrylate group present in MG which leads to the formation of a superior crosslink network and also to the presence of aromatic moiety which provides rigidity to the cured VEOCN network system. Moreover, the increased polymeric chain interactions, i.e., the intermolecular hydrogen bonding and polar attraction between the secondary hydroxyl groups present in VEOCN and the carbonyl group of MG, also tend to decrease the chain mobility, providing resistance to the matrix deformation. Due to the tendency of ME to undergo copolymerization using its two crosslinkable funtionalities, i.e., the allylic group as well as the methacrylic group, cured VEOCN network formed using ME should show higher mechanical performance than MG. But due to the presence of these two crosslinkable moieties in ME, the degree of crosslinking exceeds its maximum limit which leads to the brittleness in the VEOCN samples containing ME and hence, a deterioration of the mechanical performance of these samples is observed compared to the VEOCN samples containing MG. The mechanical properties of the cured VEOCN samples containing ME are still observed to be higher than those of the VEOCN samples containing styrene and are comparable to those of the VEOCN samples containing MMA, because of the presence of more reactive methacrylate free radical and the aromatic ring present in ME during curing which resulted into a better and morer rigid crosslinked network.
The values of tensile, flexural and compression strengths of VEOCN samples containing styrene, MMA, MG and ME as reactive monomers.
3.4 Performance of VEOCN samples against environmental exposure
The degree of crosslinking and secondary interactions between the cured network systems are the most important structural aspects to prevent the polymeric chains from dissolving in any solvent. The increased degree of crosslinking and secondary interactions, i.e., the hygrogen bonding as a consequence of polar moieties, stop the segmental mobility of the polymeric chains which further prevent the permeation of the solvent molecules to any crosslinked structure and provide a better environmental resistance. Chemical and corrosion resistance studies were performed as a function of % weight loss upon immersing the VEOCN coated panels into a chemical environment, i.e., 1 m HCl, 1 m NaOH and 1 m NaCl solutions, for 90 days and the results are shown in Table 4. Highest acid, base and salt resistance were observed for the cured VEOCN samples containing MMA compared to that of VEOCN samples containing styrene, ME and MG. The reason for this can be attributed to the highest reactivity of the group present in the least sterically hindered MMA which may have a better alignment of the polymeric chains and provides minimal free space in the cured VEOCN network system and further prevents the penetration of solvent molecules into the crosslinked network system. On the contrary, the randomness would increase due to the presence of the aromatic moieties of ME and MG which in turn would provide afree space and facilitate the attack of the solvent molecules on the cured VEOCN network systems containing ME and MG, as reactive monomers. The chemical and corrosion resistance of the VEOCN sample containing ME and MG was still observed to be higher compared to that of the VEOCN samples containing styrene which is due to the maximum degree of crosslinking of the cured network formed by VEOCN containing ME and MG, during curing. Also, the chemical and corrosion resistance of the VEOCN samples containing ME was found to be even greater than that of VEOCN samples containing MG due to its potential ability to undergo copolymerization using two crosslinkable functionalities, i.e., the allylic group as well as the methacrylic group, which led to a higher degree of crosslinking.
Chemical and corrosion resistance of VEOCN samples containing styrene, MMA, MG and ME.
| Solvent used | No. of days | Samples | |||
|---|---|---|---|---|---|
| Chemical resistance of VEOCN+ME (in terms of % weight loss) | Chemical resistance of VEOCN+MG (in terms of % weight loss) | Chemical resistance of VEOCN+styrene (in terms of % weight loss) | Chemical resistance of VEOCN+MMA (in terms of % weight loss) | ||
| HCl | 30 | 0.39 | 0.41 | 0.50 | 0.02 |
| 60 | 0.25 | 0.45 | 0.60 | 0.04 | |
| 90 | 0.63 | 0.67 | 0.69 | 0.04 | |
| NaOH | 30 | 0.40 | 0.52 | 0.60 | 0.18 |
| 60 | 0.58 | 0.63 | 0.74 | 0.30 | |
| 90 | 0.69 | 0.75 | 0.86 | 0.65 | |
| NaCl | 30 | 0.02 | 0.03 | 0.03 | 0.01 |
| 60 | 0.04 | 0.05 | 0.05 | 0.01 | |
| 90 | 0.31 | 0.33 | 0.33 | 0.25 | |
The effect of the chemical environment given to the cured VEOCN samples coated on the mild steel panel was also evaluated using sSEM. It is evident from the Figure 10 that the maximum surface deterioration or cracks were observed on the surface of the cured VEOCN samples containing styrene as reactive monomer, whereas only roughness or small cracks on the surface of VE resin samples containing as MMA, MG and ME reactive monomers were observed after chemical exposure which shows the better chemical resistance of VE resin samples containing MMA, ME and MG. These results also confirmed the data obtained during the study of the chemical and corrosion resistance behavior as a function of % weight loss of the cured VEOCN samples.
SEM images of cured VEOCN samples containing MMA, styrene, ME and MG as reactive monomers, respectively. Exposed to 1 m HCl: (A) MMA, (D) styrene, (G) ME and (J) MG; Exposed to 1 m NaOH: (B) MMA, (E) styrene, (H) ME and (K) MG; Exposed to 1 m NaCl: (C) MMA, (F) styrene, (I) ME and (L) MG.
4 Conclusion
The synthesis of bio-based and non-volatile monomers from the renewable feedstock rather than from the petroleum reserves has been the center of major research efforts due to the increasing awareness for the sustainable development of the environment and to avoid perturbing the ecosystem. Two potential bio-based and non-volatile reactive monomers for the curing of VE resins, i.e., MG and ME, were synthesized from the methacrylation of LMCs, and the chemical structures of these reactive monomers based on methacrylated LMCs were confirmed by FT-IRand 1H NMR, 13C-NMR. From the DSC results, it was discovered that MG and ME show comparable reactivity with MMA towards VEOCN, whereas the reactivity of MG and ME was found to be stronger than that of styrene. The thermal stability and mechanical pertinence with regard to their tensile, flexural and compression strengths were superior in VEOCN samples containing environmentally friendly MG and ME as reactive monomers compared to those samples containing styrene. Compared to petrochemical reactive monomer, i.e., styrene, VEOCN samples cured using MG and ME as reactive monomers showed higher chemical and corrosion resistance against the chemical exposure given during the work. These results indicate that the bio-based and non-volatile LMCs, i.e., MG and ME, have a great potential to replace commonly used petroleum based reactive monomers.
About the authors
Shipra Jaswal obtained her Bachelor’s degree in 2006 and her Master’s degree in 2008 from Himachal Pardesh University, Shimla, Himachal Pardesh, India. She is currently pursuing her PhD on the synthesis of hyperbranched vinyl ester resins based on natural products, from the National Institute of Technology, Hamirpur, Himachal Pardesh, India. She is also working in a project as principal investigator under Women Scientist Scheme-A approved by the Department of Science and Technology, New Delhi, India. She has six publications in national and international journals and presented at a number of conferences.
Bharti Gaur is an Associate Professor in the Department of Chemistry, National Institute of Technology-Hamirpur, Himachal Pradesh, India. She received her BSc degree in 1986 from St. Bedes College Shimla. She did her postgraduate studies in Organic Chemistry at Sardar Patel University, Anand, Gujarat, India, in 1988. She obtained her PhD degree from HBTI Kanpur, India, in 1993. She has taught in a number of undergraduate and postgraduate institutes before joining NIT Hamirpur as Assistant Professor in 2010. She has more than 28 research papers in national and international journals and presented in a number of conferences and has filed two patents in the field of energetic binders for solid rocket propellants. Her current research interests include microbial fuel cell, the synthesis of proton exchange membrane for fuel cells, nanocomposites, the synthesis of adhesives, and coatings.
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