study of non-isothermal decomposition and kinetic analysis...

ISSN: 0973-4945; CODEN ECJHAO

E-Journal of Chemistry

http://www.e-journals.net 2010, 7(3), 1101-1107

Study of Non-Isothermal Decomposition and

Kinetic Analysis of 2,4-Dihydroxybenzoic Acid-

Melamine-Formaldehyde Copolymer

S. S. BUTOLIYA§*

, W. B. GURNULE

and A. B. ZADE

§

§Department of Chemistry, Laxminarayan Institute of Technology,

R.T.M. Nagpur University, Nagpur-440010, India. Department of Chemistry, Kamla Nehru College,

Sakkardara Square Nagpur-440009, India.

[email protected]

Received 27 February 2010; Accepted 20 April 2010

Abstract: 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 non-

aqueous medium. The copolymer has been characterized by UV-Visible, IR and 1H 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 oC.

Keywords: Copolymer, Thermogravimetry, Degradation, Kinetics, Activation energy.

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 al1 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 0C/min. in nitrogen atmosphere. Bonde and

1102 S. S. BUTOLIYA et al.

coworkers2 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 temperature3-6

. 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 al7 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 formaldehyde8-10

. 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.

Preparation of 2,4-DHBAMF copolymer

The 2,4-DHBAMF copolymer was prepared by condensing 2,4-dihydroxybenzoic acid (1.54 g,

0.1 mol) and melamine (1.26 g, 0.1 mol) with formaldehyde (11.1 mL, 0.3 mol) with the molar

ratios of 1:1:3 in the presence of 2 M HCl as a catalyst (Scheme 1).

Scheme 1. Synthesis of 2,4-DHBAMF copolymer.

Study of Non-Isothermal Decomposition 1103

The mixture was heated at 126 ± 2 0C 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).

Table 1. Synthesis and physical data of 2,4-DHBAMF copolymer resin.

Elemental analysis, % Reactant

% C % H % N

2,4

-Dih

ydro

xy

ben

zoic

aci

d

(2,4

-DH

BA

), m

ol

Mel

amin

e (M

),

mo

l

Fo

rmal

deh

yd

e

(F),

mo

l

Mo

lar

rati

o

Em

pir

ical

fo

rmu

la

of

rep

eati

ng

un

it

Em

pir

ical

fo

rmu

la

wei

gh

t DP

Mn

Cal

.

Fo

un

d

Cal

.

Fo

un

d

Cal

.

Fo

un

d

0.1 0.1 0.3 1:1:3 C13H13O4N6 317 18 5706 49.2 48.61 4.1 4.02 26.4 26.87

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

. 1H NMR spectrum of all the newly prepared copolymer has been scanned

on a Bruker Advance –II 400 MHz NMR spectrophotometer, DMSO-d6 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 0C/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- Carroll11

and

Sharp-Wentworth12

.

Freeman–Carroll method

The straight-line equation derived by Freeman and Carroll, which is in the form of

Wr

T

R

En

Wr

dt

dw

a

log

1

.303.2log

log

∆

∆

−=∆

∆

(1)


Where, dw/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.

The plot between the terms Wr

TvsWr

dt

dw

log

1

.log

log

∆

∆

∆

∆

gives a straight 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.

Sharp–Wentworth method

Using the equation derived by Sharp and Wentworth,

TR

E

B

A

C

dT

dC

a 1.

303.2log

1

log

−=−

∆

(2)

Where, dC/dT = rate of change of fraction of weight with change in temperature, β = linear

heating rate dT/dt.

By plotting the graph betweenT

vsC

dT

dC

1.

1

log

−

∆

, we obtained the straight line which give

energy of activation (Ea) from its slope.

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 H2SO4 and NaOH

solution and insoluble in almost all other organic solvents. The melting point of this copolymer is

in the range of 350 0C-400

0C. 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 nucleus13

. 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 transition13

.

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 bonding15

. The presence of

weak peak at 2362 - 2379 cm-1

describes the -NH- in the melamine moiety may be ascribed

in the polymeric chain13,15

. 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 -CH2-

methylene bridges14

. In the polymer chain 1, 2, 3, 4, 5- penta substitution of aromatic ring is

recognized from the bands appearing13

at 812.3 - 813.7.

Tra

nsm

itta

nce

, %


wavelength Wavenumber cm-1

Figure 1. UV visible spactra of 2,4-

DHBAMF copolymer.

Figure 2. Infrared spectra of 2,4-

DHBAMF copolymer. 1

H NMR spectra of 2,4-DHBAMF copolymer resin is shown in Figure 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-OH15

.

The medium triplet signal appeared at 3.6 - 4.2 ppm may be due to amido protons –CH2-NH-

polymer 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-CH2-NH- of polymeric chain15

.

Figure 3.

1H NMR spectra of 2,4-DHBAMF copolymer.

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 0C - 800

0C. The first step is of slow

decomposition between 40 0C to 340

0C 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 0C to 480

0C 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 0C to 800

0C 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.

Ab

sorb

ance

Chemical Shift δ, ppm

% W

eig

ht

loss

Temperature, oC


Figure 4. Thermogram of 2,4-DHBAMF copolymer.

Table 2. Thermogravimetric data of 2,4-DHBAMF copolymer corresponding to heating rate

of 10 0C/min.

First stage Second stage Third stage Ea

kJ/mol.

Trange1a,

0C

W1d

Trange1b,

0C

W1e

Trange1c,

0C

W1f T

hal

f, 0

C

FW SW

∆S Z S* n

40-340 36.49 340-480 76.06 480- 800 99.01 480 21.37 20.97 8.45 1238.7 -22.56 0.9 aFirst degradation temperature range, bSecond degradation temperature range, cThird degradation

temperature range, dFirst maximum loss, eSecond maximum loss, fThird maximum loss.

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 perfectly7,9,17

.

Conclusion

The 2, 4-DHBPOF copolymer based on the condensation polymerization of 2,4-

dihydroxybenzophenone 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 oC.

Acknowledgment

Authors are thankful to the Director, Laxminarayan Institute of Technology, R.T.M. Nagpur

University, Nagpur for providing laboratory facilities and also thankful to RSIC, Punjab

University, Chandigarh for carrying out spectral analysis.


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12. Gurnule W B, Juneja H D and Paliwal L J, Oriental J Chem., 1999, 15, 283.

13. Tarase M V, Zade A B and Gurnule W B J, App Polym Sci., 2008, 108, 738.

14. Gupta R H, Zade A B and Gurnule W B, J App Polym Sci., 2008, 109, 3315- 3320.

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