Study of Non-Isothermal Decomposition and Kinetic Analysis of 2 , 4-Dihydroxybenzoic Acid-Melamine-Formaldehyde Copolymer

A copolymer (2,4-DHBAMF) synthesized by the condensation of 2,4-dihydroxybenzoic acid and melamine with formaldehyde in the presence of acid catalyst using varied molar proportions of the reactants. A composition of the copolymer has been determined by elemental analysis. The number average molecular weight has been determined by conductometric titration in nonaqueous medium. The copolymer has been characterized by UV-Visible, IR and H NMR spectral analysis. Thermogravimetric analysis was carried out to study the decomposition and various kinetic parameters. Freeman Carroll and Sharp Wentworth methods have been applied for the calculation of kinetic parameters while the data from Freeman Carroll method have been used to determine various thermodynamic parameters such as order of reaction, energy of activation, frequency factor, entropy change, free energy change and apparent entropy change. The results indicate that given copolymer have potential as matrix resin for long term applications at temperature up to 350 C.


Introduction
The thermal degradation study of polymers has become a subject of recent interest.It is very important property of polymer, which decides the thermal stability and processability of the polymer.The study of thermal behavior of polymers in air at different temperature provides important information about its practical applicability.
Shah et al 1 synthesized the terpolymer from salicylic acid -formaldehyde -resorcinol.The terpolymer was characterized by FTIR and elemental analysis.The thermal analysis (TGA) was performed at the heating rate of 10 0 C/min. in nitrogen atmosphere.Bonde and coworkers 2 synthesized and characterized polymeric chelates of azelaoyl bis-p-chlorophenyl urea with Mn +2 , Co +2 , Ni +2 , Cu +2 and Zn +2 ion.Thermal data have been analyzed by Freeman-Carroll and Sharp-Wentworth methods.
Thermal analysis (TA) is a typical analytical technique to describe the relationship between physicochemical changes and temperature [3][4][5][6] .In order to synthesize polymers having numerous practical applications, there is a need to investigate the effect of heat on the polymers in order to establish their thermal stability.Singru et al 7 synthesized copolymers by the condensation of p-cresol and melamine with formaldehyde in the presence of an acid catalyst and using varied molar proportion of the reacting monomers.Thermal studies of the resins were carried out to determine their mode of decomposition, the activation energy (Ea), order of reaction (n), frequency factor (Z), entropy change (S), free energy change (F) and apparent entropy change (S*).Thermal decomposition curves were discussed with careful attention of minute details.The Freeman-Carroll and Sharp-Wentworth methods have been used to calculate thermal activation energy and thermal stability.In an earlier communication a large number of copolymers were synthesized from substituted phenols and acids with formaldehyde [8][9][10] .However, no work has been carried out on the synthesis, characterization and thermal degradation studies of the copolymer from 2,4-dihydroxybenzoic acid, melamine and formaldehyde.Experimental 2,4-Dihydroxybenzoic acid, melamine and formaldehyde (37%) were purchased from the market and are from Merck, India.Solvent like N, N-dimethyl formamide and dimethyl sulphoxide were used after distillation.All other chemicals used were of chemically pure grade.
The mixture was heated at 126 ± 2 0 C in an oil bath for 5 h.The solid product obtained was immediately removed from the flask as soon as the reaction period was over.It was repeatedly washed with hot water to remove unreacted monomers.The air dried copolymer was extracted with ether to remove excess of 2,4-dihydroxybenzoic acid -formaldehyde copolymer, which might be present along with 2,4-DHBAMF copolymer.It was further purified by dissolving in 8% NaOH solution and filtered.It was then precipitated by drop wise addition of 1:1 (v/v) conc.HCl / water with constant stirring and filtered.The process was repeated twice.The resulting polymer sample was washed with boiling water and dried in vacuum at room temperature.The purified copolymer was finally ground well to pass through a 300 mesh size sieve and kept in vacuum over silica gel.The yield of copolymer found to be 82.37%(Table 1

Characterization of copolymers
The copolymer was subjected to elemental analysis for C, H, N on a Colemann C, H, N analyzer.
The number average molecular weight ( Mn ) was determined by non-aqueous conductometric titration in DMF using ethanolic KOH as the titrant.A plot of the specific conductance against the milliequivalent potassium hydroxide required for neutralization of 100 g of copolymer was made.Electronic absorption spectrum of the copolymer in DMF was recorded on Shimadzu double beam spectrophotometer in the range of 190-700 nm.Infrared spectrum of copolymer was recorded in nujol mull on Perkin-Elmer-spectrum RX-I spectrophotometer in the range of 4000 -500 cm -1 . 1 H NMR spectrum of all the newly prepared copolymer has been scanned on a Bruker Advance -II 400 MHz NMR spectrophotometer, DMSO-d 6 was used as a solvent.

Thermal analysis
Dynamic (non-isothermal) thermogravimetric analysis of the copolymer prepared has been carried out in air atmosphere with a heating rate 10 0 C/min. in a platinum crucible.

Theoretical considerations
To provide further evidence regarding the degradation system of analyzed compounds, we derived the TG curves by applying an analytical method proposed by Freeman-Carroll 11 and Sharp-Wentworth 12 .

Results and Discussion
The newly synthesized and purified 2,4-DHBAMF copolymer was found to be brownish yellow in color.The copolymer was soluble in DMF, DMSO, THF, concentrated H 2 SO 4 and NaOH solution and insoluble in almost all other organic solvents.The melting point of this copolymer is in the range of 350 0 C-400 0 C.This copolymer was analyzed for carbon, hydrogen, nitrogen content.The molecular weight for 2,4-DHBAMF copolymer was found to be 5706 (Table 1).
The UV-visible spectra (Figure 1) of the 2,4-DHBAMF copolymer in pure DMSO was recorded in the region 200 -850 nm at a scanning rate of 100 nm min -1 and at a chart speed of 5 cm min -1 .The copolymer sample displayed two characteristic broad bands at 250 -280 and 290 -340 nm.These observed positions for the absorption bands indicate the presence of a carbonyl group (ketonic) having a carbon -oxygen double bond which is in conjugation with the aromatic nucleus 13 .The latter band (more intense) can be accounted for п→п* transition while the former bond (less intense) may be due to n→п* electronic transition 13 .The bathochromic shift (shift towards longer wavelength) from the basic values of the C=O group viz.320 and 240 nm respectively, may be due to the combined effect of conjugation and phenolic hydroxyl group (auxochromes) 14 .
The IR-spectra of 2,4-DHBAMF copolymer resin was shown in Figure 2. A broad band appeared in the region 3320 -3324 cm -1 may be assigned to the stretching vibration of the phenolic hydroxyl groups exhibiting intermolecular hydrogen bonding 15 .The presence of weak peak at 2362 -2379 cm -1 describes the -NH-in the melamine moiety may be ascribed in the polymeric chain 13,15 .The sharp band displayed at 1573 -1620 cm -1 may be due to the stretching vibration of carbonyl group.A weak band at 1447 -1449 cm -1 is ascribed to aromatic ring.The sharp and weak band at 1279 to 1344 cm -1 suggests the presence of -CH 2methylene bridges 14 .In the polymer chain 1, 2, 3, 4, 5-penta substitution of aromatic ring is recognized from the bands appearing 13 3.There is a weak signal appearing at 7.2 -7.8 ppm may be due to aromatic proton.The intense singlet signal appeared in the region 4.5 -5.5 ppm can be assigned to phenolic proton of Ar-OH 15 .The medium triplet signal appeared at 3.6 -4.2 ppm may be due to amido protons -CH 2 -NHpolymer chain 16 .Also the medium doublet signal in the range of 1.9 to 2.5 ppm is attributed to the protons of methylenic bridge Ar-CH 2 -NH-of polymeric chain 15 .

TG of 2,4-DHBAMF copolymer
Thermogram of this copolymer is shown in Figure 4. Thermogram of copolymer depicts three step decomposition in the temperature range 40 0 C -800 0 C. The first step is of slow decomposition between 40 0 C to 340 0 C corresponds to 36.49% loss which may attribute to loss of side chain of aromatic nucleus and methylene bridges along with loss of water molecule against calculated 37.31% present per repeat unit of the polymer.The second step decomposition starts from 340 0 C to 480 0 C that represents degradation of aromatic ring and amido groups attached to aromatic ring (76.06% found and 76.71% calculated).The third step of decomposition starts from 480 0 C to 800 0 C corresponds to 99.01% loss of remaining melamine moiety against calculated 100% loss (Table 2).

Thermoanalytical data
A plot of percentage mass loss versus temperature is shown in the Figure 4 for a representative 2,4-DHBAMF copolymer.To obtain the relative thermal stability of the copolymer, the method described by Freeman-Carroll and Sharp-Wentworth was adopted.By using thermal decomposition data and then applying above methods the activation energy (Ea) is calculated.The activation energy calculated by these methods is depicted in Table 2.The thermal stability of polymer predicted on the basis of the initial decomposition temperature is in concurrence with that predicted from the activation energy values.By using the data of Freeman-Carroll method various thermodynamic parameters have been calculated (Table 2).Fairly good straight-line plots are obtained using the two methods.This is expected since the decomposition of terpolymer is known not to obey first order kinetics perfectly 7,9,17 .

Conclusion
The 2, 4-DHBPOF copolymer based on the condensation polymerization of 2,4dihydroxybenzophenone and oxamide with formaldehyde in the presence of acid catalyst has been prepared and proposed structure have been determined by various physicochemical techniques.Thermoanalytical data was used to calculate activation energy by Freeman-Carroll and Sharp Wentworth methods.The results indicate that given copolymer have potential as matrix resin for long term applications at temperature up to 350 o C.
The straight-line equation derived by Freeman and Carroll, which is in the form of /dt = rate of change of weight with time, Wr = Wc-W, Wc = wt.loss at completion of reaction, W= total wt loss up to time t, Ea= energy of activation, n= order of reaction.line from which slope we obtained energy of activation (Ea) and intercept on y-axis as order of reaction (n).The change in entropy (∆S), frequency factor (z), apparent entropy (S*) can also be calculated by further calculations.
/dT = rate of change of fraction of weight with change in temperature, β = linear heating rate dT/dt.the straight line which give energy of activation (Ea) from its slope.

Figure 2 .
Figure 2. Infrared spectra of 2,4-DHBAMF copolymer.1 H NMR spectra of 2,4-DHBAMF copolymer resin is shown in Figure3.There is a weak signal appearing at 7.2 -7.8 ppm may be due to aromatic proton.The intense singlet signal appeared in the region 4.5 -5.5 ppm can be assigned to phenolic proton of Ar-OH15 .The medium triplet signal appeared at 3.6 -4.2 ppm may be due to amido protons -CH 2 -NHpolymer chain16 .Also the medium doublet signal in the range of 1.9 to 2.5 ppm is attributed to the protons of methylenic bridge Ar-CH 2 -NH-of polymeric chain15 .
a First degradation temperature range, b Second degradation temperature range, c Third degradation temperature range, d First maximum loss, e Second maximum loss, f Third maximum loss.