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Mohan Lal Sukhadia University, Udaipur 313 001,. Rajasthan, India. Received 4 June 2014; re-revised and accepted 28 January 2015. A new terpolymer resin ...

Indian Journal of Chemistry Vol. 54A, February 2015, pp. 193-198

Model-free thermal degradation kinetics of bio-based phenolic resin derived from vanillin oxime Narendra Pal Singh Chauhana, *, Nirmala Kumari Jangidb, Jyoti Chaudharyc, Ritu Tomara & Paridhi Katariac a

Department of Chemistry, Bhupal Nobles Post Graduate College, Udaipur 313 001, Rajasthan, India Email: [email protected]/ [email protected]

b

Department of Chemistry, University College of Science, Mohan Lal Sukhadia University, Udaipur 313 001, Rajasthan, India

c

Department of Polymer Science, University College of Science, Mohan Lal Sukhadia University, Udaipur 313 001, Rajasthan, India

Received 4 June 2014; re-revised and accepted 28 January 2015 A new terpolymer resin is synthesized from vanillin oxime and p-bromoacetophenone with formaldehyde by solution condensation technique in the presence of an acid. The structure of the terpolymer resin is confirmed by IR and NMR spectroscopy. The average molecular weight and the polydispersity index for the terpolymer are found to be 12,852 g/mol and 2.65 as calculated by gel permeation chromatography. The phase transition temperature (102-214 °C) has been determined by differential scanning calorimetry (DSC). Thermal degradation characterstics and kinetic parameters such as activation energy are investigated by TG-DTG. The apparent activation energy of terpolymer resin using Kissinger-Akahira-Sunose, and Friedman methods are 10.2 and 10.8 kJ mol−1, respectively. The most likely solid state decomposition process is D3 deceleration type as shown by Craido method and master plots. Keywords: Resins, Terpolymer resins, Phenolic resins, Vanillin, Oximes, Decomposition, Degradation, Model-free kinetics, Solid state mechanism

One of the major environmental challenges is the proper disposal of solid waste, which is mainly generated by the industries with the aim to conserve natural resources and reduce environmental pollution. Removal of metal from wastewater by ion exchange resins and removal of trace metals from contaminated soils using EDTA incorporating resin trapping techniques are being widely used for the last few decades1,2. Recently, the development of novel eco-friendly materials, such as resins partially or fully based on biomass, with high mechanical and thermal properties has become the focus of attention in this field3-5. One of the most widely employed biomaterials is lignin and its derivatives.

Lignins, such as Kraft lignin and lignosulfonates, have been the subject of studies as a raw material for partial substitution of phenol in resol and novolac resins6-8. As a result of these studies, several phenolic bio-resins whose cost is partially decoupled from the oil market and with a lower environmental impact have been developed. Vanillin can be produced from phenolic compounds such as phenolic stilbenes, lignin, isoeugenol, eugenol, ferulic acid, vanillic acid, aromatic amino acid, sugar beet pulp, wheat straw and biomass substances9. Apart from its flavoring properties, vanillin also exhibits several bioactive properties10. Moreover, vanillin is also used for other purposes such as constituent in cosmetic and drug preparations. Very recently, a highly bio-based thermoset for use in polymer composites, the ligninderived chemical, vanillin, was methacrylated in a twostep, one-pot synthesis to produce a vinyl ester resin (87 cP at 25 °C) with a 1:1 mole ratio of a monofunctional monomer, methacrylated vanillin-tocrosslinking agent, glycerol dimethacrylate11. Polyvanillin was prepared by electrochemical reductive polymerization of divanillin in aqueous sodium hydroxide12. The polar and hydrophilic natures of the lignocellulosic fibers and the non-polar characteristics of many thermoplastic matrices could alter composite properties, owing to the lack of adhesion and non-uniform dispersion of the fibers in the matrix13. When phenolic resins are used as matrices and lignocellulosic fibers are used as reinforcement materials, the problem of lack of adhesion can be considerably minimized by favorable interactions between polar hydroxyl groups of the phenolic matrix and the hydrophilic fibers. This represents an important advantage of this matrix as compared to the hydrophobic thermoplastic matrices14, since the intensity of the intermolecular interactions through the interface controls various properties of the composites15. Another advantage of using phenolic matrices is their low processing temperature (C=O) group having a C═O double bond, which is in conjugation with the aromatic nucleus. The bathochromic shift from the basic value, i.e., 255-325 nm, (Π-Π*) and 265-300 nm (>C=N), may be due to the combined effect of conjugation (due to chromophore) and oxime hydroxy group (auxochrome). The FT-IR spectrum of VOFBA is shown in Fig. S1 (Supplementary Data). The characteristic peak at 3424 cm−1 is due to O–H stretching. The peaks observed at 2929 cm−1 are due to C–H stretching vibration of aldoxime group. The characteristic peak observed at 1678 cm−1 is due to C=N stretching of vanillin oxime moiety. The band at 1538 cm−1 is due to C-C–C stretching of the aromatic ring carbons. The in-plane O-H deformation vibration appears as a

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strong band at 1489 cm−1. The O-H out-of-plane bending vibration gives rise to a broad band in the region 840 cm−1. Appearance of the band at 1401 cm−1 is due to C-H stretching of methylene bridge between aromatic moieties. The medium band obtained at 983 cm−1 suggests C–N stretching. The 1,3,4,5-tetra-substitution of aromatic benzene ring can be recognized from strong to medium/weak absorption bands at 1013 and 840 cm−1, respectively. The proton NMR spectrum of VOBAF terpolymer resin was scanned in DMSO-d6 solvent (Fig. 1). The chemical shift (δ) ppm observed is assigned on the basis of data available in literature22. The terpolymer resin VOBAF shows an intense weakly multiplet signal at 2.4 (δ) ppm, which may be attributed to methyl proton of Ar–COCH3 group, whereas the signal at 2.5 may be due to DMSO-d6. The medium signal at 3.35−3.37 (δ) ppm may be due to methoxy proton of vanillin oxime moiety. The signal at 3.40−3.78 (δ) ppm may be due to the methylene proton of Ar–CH2–Ar moiety. The signal in the region 6.82−6.85 (δ) ppm is attributed to phenolic protons. The weak multiplet signal (unsymmetrical pattern) in the region of 7.2−7.8 (δ) ppm may be attributed to aromatic proton (Ar–H). The signal appearing at 8.2 (δ) ppm may be due to the hydroxyl protons of aldoxime group. The position of the signal of protons of aldoxime group is slightly shifted downfield, indicating clearly the intramolecular hydrogen bonding of –OH group. The DSC curves of the terpolymer resin at different heating rates showed a single endothermic peak at

Fig. 1 − 1H NMR spectrum for terpolymer VOBAF.

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INDIAN J CHEM, SEC A, FEBRUARY 2015

Fig. 2 − DSC scans at different scanning rates.

102−214 oC (Fig. 2). With an increase in heating rate, both the onset temperature and peak temperature shifted to higher temperatures23,24. There were no other noticeable exothermic peaks in different runs indicating that the curing reactions were completed. The DSC curves also seemed to suggest that the curing process of the resins was mainly dominated by condensation reactions. The addition reactions for the formation of methylol compounds may have been completed by and large during the resin synthesis stage. The endothermic peak was probably due to expulsion of water molecules formed during polycondensation process. This result is consistent with TG-DTG data discussed vide infra. To characterize the thermal stability of phenolic resin, TG-DTG analysis was carried out. Figure 3 shows the TG-DTG curves at different scanning rates. It is known that phenolic resin degraded in three steps: post-curing, thermal reforming and ring stripping25. In the initial stage, the mass loss was due to the evaporation of water, which was formed by condensation reaction of methylol groups. The second mass loss was due to the loss of water formed by condensation reaction of methylene and phenolic OH

Fig. 3 − TG-DTG thermograms at different scanning rates.

NOTES

as well as between two hydroxyl groups and elimination of side groups like acetyl, bromine, methoxy, etc. (Fig. 3b and 3d). In the third event, the mass loss was due to the loss of carbon monoxide and methane formed by degradation of the methylene bridge26. It was observed that VOBAF showed lower thermal stability in the initial stage which may be due to the fact that methylol content in VOBAF is less. This indicates that the terpolymer resin is more or less linear with little substitution. The Kissinger-Akahira-Sunose method27,28 was utilized to determine the values of apparent activation energies of thermal decomposition from plots of ln(β/T2) against 1000/T (Supplementary data, Fig. S2). The average apparent activation energies was found to be 10.19 kJ mol-1. The Friedman method29 is a widely used method of kinetic analysis, to study the thermal properties of the polymer. From the plots of lnβ(dα/dT) against 1/T, the mean activation energy was found to be 10.76 kJ mol-1 over the range of a given conversion. The activation energy values calculated for a particular model with the differential (Friedman) and the integral (K-A-S) methods are almost the same. The apparent activation energy calculated for α = 0.10-0.70, by both the methods is given in Table S1 (Supplementary data). To obtain the master curves as a function of the reaction degree, the fourth rational expression of Senum & Young, Z(α) = 0.01−1.5 was used. This gives errors lower than 10−15 % for x = 20. This equation was used to obtain the master curves as a function of the reaction degree (corresponding to different models listed in Supplementary data, Table S2). Activation energy was calculated by fitting the K-A-S equation to temperature and heating rate values at 10 oC/min-1 for a given conversion value, α. This Ea is called ‘apparent activation energy’ as it is the sum of chemical reaction and physical process in thermal degradation. The temperature range of 215−315±10 oC with main thermal decomposition fraction of 50% is more meaningful for polymers, since decomposition mechanism may change when conversion rate is higher than 0.6. Hence, a conversion range from 0.1−0.7 may offer a simplified and more meaningful modeling of thermal decomposition behavior. The decomposition conversion thereafter is less meaningful for polymer due to high temperature and sample mass losses. Therefore, α value between 0.10 and 0.70 was plotted (Fig. 4). It can be seen that activation energy depends on the degree of conversion, indicating that the

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Fig. 4 − Experimental master plots obtained from TG-DTG in the selected range of conversion versus the theoretical values.

degradation of the system follows a complex mechanism. Figure 4 shows the z(α) – α master and experimental curve of the terpolymer. In earlier studies22, the thermodegradation solid state mechanism was found to be D2 (two dimensional diffusion) mechanism whereas the present work shows D3 (three dimensional diffusion) solid state mechanism, indicating that it is a relatively better biocidal material that the material reported earlier. In the present study, a terpolymer resin VOBAF based on the condensation reaction of vanillin oxime and p-bromoacetophenone with formaldehyde in the presence of an acid as catalyst has been prepared. Structure of terpolymer is elucidated by FT-IR and 1 H NMR spectra. The molecular weight (12,852 g/mol) and polydispersity index (2.65) are determined by size exclusion chromatography. From TG-DTG, the energy of activation evaluated from the KAS method (10.2 kJ mol−1) and Freidman method (10.8 kJ mol−1) are found to be nearly equal. The thermal degradation mechanism for terpolymer is a decelerated three dimensional diffusion (D3) type, which is a solid-state process based on a phase boundary controlled reaction. This bio-based terpolymer is thermally stable at elevated temperatures and therefore may be used in industry where thermally stable polymers are required. Supplementary data Supplementary data associated with this article, i. e., Figs S1 and S2 and Tables S1 and S2, are available in the electronic form at http://www.niscair.res.in/jinfo/ijca/IJCA_54A(02)193 -198_SupplData.pdf.

INDIAN J CHEM, SEC A, FEBRUARY 2015

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