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ISSN: 0973-4945; CODEN ECJHAO E-Journal of Chemistry http://www.e-journals.net 2009, 6(1), 120-124 Solid-State Microwave Synthesis of Melamine-Formaldehyde Resin SUBHASH BAJIA, RASHMI SHARMA and BIRBAL BAJIA Department of Pure and Applied Chemistry, Maharshi Dayanand Saraswati University, Ajmer-305009, India. Department of zoology, Govt. College, Ajmer. Department of Chemistry M. P. Government P. G. College, Chittorgarh (Rajasthan), India. [email protected] Received 7 January 2007; Revised 24 August 2007; Accepted 6 January 2008 Abstract: An efficient synthesis of melamine-formaldehyde resin has been achieved using conventional as well as microwave irradiations (without and with solid support) in different molar ratio. Resin samples were tested for their chemical as well as physical properties. The structure of all the resin has been supported by their spectral data. Keywords: Melamine-formaldehyde resin, Solid-state microwave synthesis, Free-formaldehyde. Introduction Melamine-formaldehyde resins are the most useful thermosetting materials 1 for the manufacture of the composite and panels 2-3 . Distinct advantages of the amino resin are transparent, better hardness, thermal stability 4-5 , excellent boil-resistance, chemical resistance, scratch resistance 6 , abrasion resistance, flame retardant 7-9 , moisture resistance 10 , surface smoothness and electrostatic properties. The melamine-formaldehyde resins are used in textile coatings treatment. Microwave irradiation is well known technique to promote the synthesis of a variety of compounds, where chemical reactions are accelerated because of selective absorption of microwaves by polar molecules 11 . Recently, the coupling of some MWI with solid support under solvent free conditions has received notable attention 12 . As a part of our program towards the non-traditional approaches to the experimental set up of organic reactions, the concept of “Microwave induced Organic Reaction Enhancement” (MORE) chemistry has been utilized for the rapid and efficient synthesis. Microwave assisted organic synthesis 13-17 has attracted attention in recent years due to enhance reaction rates, high yields, improve purity, ease of work up after the reaction.
Transcript
Page 1: Solid-State Microwave Synthesis of Melamine-Formaldehyde …

ISSN: 0973-4945; CODEN ECJHAO

E-Journal of Chemistry

http://www.e-journals.net 2009, 6(1), 120-124

Solid-State Microwave Synthesis of

Melamine-Formaldehyde Resin

SUBHASH BAJIA, RASHMI SHARMA and BIRBAL BAJIA

Department of Pure and Applied Chemistry,

Maharshi Dayanand Saraswati University, Ajmer-305009, India.

Department of zoology, Govt. College, Ajmer.

Department of Chemistry M. P. Government P. G. College,

Chittorgarh (Rajasthan), India.

[email protected]

Received 7 January 2007; Revised 24 August 2007; Accepted 6 January 2008

Abstract: An efficient synthesis of melamine-formaldehyde resin has been

achieved using conventional as well as microwave irradiations (without and with

solid support) in different molar ratio. Resin samples were tested for their

chemical as well as physical properties. The structure of all the resin has been

supported by their spectral data.

Keywords: Melamine-formaldehyde resin, Solid-state microwave synthesis, Free-formaldehyde.

Introduction

Melamine-formaldehyde resins are the most useful thermosetting materials1 for the

manufacture of the composite and panels2-3

. Distinct advantages of the amino resin are

transparent, better hardness, thermal stability4-5

, excellent boil-resistance, chemical

resistance, scratch resistance6, abrasion resistance, flame retardant

7-9, moisture resistance

10,

surface smoothness and electrostatic properties. The melamine-formaldehyde resins are used

in textile coatings treatment.

Microwave irradiation is well known technique to promote the synthesis of a variety of

compounds, where chemical reactions are accelerated because of selective absorption of

microwaves by polar molecules11

. Recently, the coupling of some MWI with solid support

under solvent free conditions has received notable attention12

. As a part of our program

towards the non-traditional approaches to the experimental set up of organic reactions, the

concept of “Microwave induced Organic Reaction Enhancement” (MORE) chemistry has

been utilized for the rapid and efficient synthesis. Microwave assisted organic synthesis13-17

has attracted attention in recent years due to enhance reaction rates, high yields, improve

purity, ease of work up after the reaction.

Page 2: Solid-State Microwave Synthesis of Melamine-Formaldehyde …

Solid- State Microwave Synthesis 121

The present work reveals the comparative aspects of synthesis of melamine-

formaldehyde resin using conventional as well as microwave irradiation (with or without

solid support) and the resin properties such as solid content, gel time, viscosity and free

formaldehyde content.

Experimental

Equipments

Reagent grade chemicals were used without purifications. Viscosity was measured

according to ASTM D1084 method18

with a Brook field digital viscometer. An efficient

synthesis was carried out in Microwave oven model LG MS 194 W operating at 160 W

generating 2450 MHZ frequency. Reaction mixture internal temperature was measured on

Mine Gun type Non Contact I.R. thermometer. Resin solid content19

were determined by

heating a 1 g. of resin sample at 125 0C for 1.50 h. The percentage of free formaldehyde

content was determined using a modified version of Walker’s20

hydroxylamine hydroxide

and sodium sulfide methods. FTIR spectra were recorded with a Perkin-Elmer 16 PC

spectrometer. The 1H NMR spectra were obtained with a Bruker WM 250 spectrometer

(68.69 MHZ) using d6-DMSO as internal standard.

Investigation of the synthesis of melamine formaldehyde resin

Under conventional method

The melamine-formaldehyde resins were synthesized using conventional methods reported

earlier21, 22

. Melamine and formaldehyde (37% aqueous solution) were added in three-neck

flask. The pH (8.5) was adjusted using aqueous caustic (40%) solution. The mixture was

refluxed for 3.5 h.

Under microwave conditions

The melamine-formaldehyde resins were synthesized by condensing melamine and para

formaldehyde without and with Al2O3 and silica gel (as solid support) in basic medium. The

reaction was carried out in Erlen Meyer flask capped with a funnel under microwave

irradiation at 160 watt in microwave oven. The reaction completion was monitored by

checking the free formaldehyde content in the reaction mixture. To insure the reproducibility,

the reaction was carried out three times in microwave. The final temperature of the reaction

was measured by non-contact IR thermometer especially fitted for measuring the internal

temperature. After cooling the resin was separated by dissolving in proper solvent.

Determination of free formaldehyde

The percentages of the free formaldehyde in the resin were determined using a modified version

of Walker’s hydroxylamine hydrochloride method. Approximately 3 g of resin were diluted with

25 mL of water and pH was adjusted to 4.0 using 0.1 M HCl. Thirty mL of hydrolamine

hydrochloride (0.5 M, pH = 4.0) were stirred for 10 minutes and free formaldehyde was

determined by back titration at pH = 4.0 using 0.1 M aqueous sodium hydroxide.

Spectral data of synthesized resin

Fourier transform infrared spectroscopy

In the IR spectrum of compound (A), there are two N-H stretching band appeared in the

region 3200-3500 cm-1

along with other peak in the region 1400-1500 cm-1

(N-H bend).

There is a band at 1470 cm-1

for symmetric s-triazyne ring. There are two bands in the region

1050-1250 cm-1

due to C-O-C stretching.

Page 3: Solid-State Microwave Synthesis of Melamine-Formaldehyde …

122 SUBHASH BAJIA et al.

H1NMR spectra

A single appears as singlet at 6.2-6.8 ppm due to NH2 in the melamine. In the H1NMR spectra

of the resin the signal appears at 4.6-4.7 ppm correspond to –CH2O-; 5.1-5.2 ppm to –

NCH2OH; 6.0-6.3 ppm to –NH2-. The signal intensities indicate that in the resin about 19.5%

of unsubstantiated amine groups are present, probably as a result of a steric hindrance. In the

structure (Scheme 1) the methylene (N-CH2-N) and (dimethylene) ether (-CH2-O-CH2-) bonds

are exist, from which the signals appear at 4.5 ppm and 4.6-4.8 ppm, respectively. The

intensities of these signals show that the latter bond is in 4-fold excess.

NH2

H2N N

N N

NH2

HCHO

HN

NH

H2N N

N N

NH2

OH

NH

H2N N

N N

OH

HCHO

NH

HN

OH

NH

N

N N

OH

NH

OH

HO

AB

C

D

NH

H2N N

N N

NH2

OH

N

HO

H2N N

N N

OH

N

HO

OH

HCHO

HCHO

NHN

N N

OH

N

HOOH

HCHO

HO

HN

NH

N

N N

OH

NHO

NH

H2N N

N N

OH

NH

+ D +

HN

NH

N

N N

OH

NH

OH

NH2

N

N N

A

H2N

HO

NH

REACTION SCHEME-1

REACTION SCHEME- 2

NH

N

N N

NH2

OH

H2N

2

NH

N

N N

NH2H2N

NH

N

N

N

NH2

NH2

O

HCHO

Scheme 1.

Results and Discussion

Melamine-formaldehyde resins were prepared by condensing melamine and aqueous

formaldehyde solution (37%) in the basic medium using aqueous sodium hydroxide. In view of

the long reaction time, moderate yields (Table 1 & 2) tedious work up after the reaction and

requirement of large quantity of solvent associated with conventional method, a relatively

Page 4: Solid-State Microwave Synthesis of Melamine-Formaldehyde …

Solid- State Microwave Synthesis 123

simplified procedure was perceived, in which melamine and para formaldehyde could be made

to react without and with Al2O3 and silica gel (as solid support) under solvent free microwave

irradiation. The use of Al2O3 and silica gel as solid support provided easy work up. Microwave

irradiation coupled with the solid support has received attention as a new with strategy for

organic synthesis due to the fact that many reactions seem to proceed with much alacrity under

such conditions as opposed to corresponding thermal assisted reaction. Solid reactions under

solvent free condition are also of great current interest especially in relation to environmental

concerns associate with the use of volatile solvent. Compare to traditional methods, many organic

reactions occur more efficiently in the solid state than in solution23

. So under the microwave

irradiation condition using Al2O3 and silica gel as solid support the desired product was formed in

maximum yields in significantly lower reaction time as compared to both the methods. The result

obtained under the microwave irradiation was extrapolated to conventional heating. Reactions

mentioned in Table 1 were examined by simply heating in a preheating oil-bath under the same

conditions (time, temperature and pH) as mentioned in the typical procedure with microwave

irradiation. The rate acceleration under microwave irradiation was due to specific microwave

effect. Under microwave irradiation enhanced dipole-dipole interactions caused the instantaneous

condensation between melamine-para formaldehyde

Table 1. Comparative study for the synthesis of melamine-formaldehyde resin Conventional

and microwave irradiation method (without solid support)

Conventional method Microwave method

(without solid support)

Molar

ratio

Time

h

Temp. 0C

F.F.

cont. %

Yield

%

Solid

cont. %

Time

min

Temp.a

0C

F.F.b

cont. %

Yieldc

%

Solid

cont. %

1:1.0

1: 2.0

1: 2.5

1: 3.0

2.5

2.3

2.4

2.2

Reflux

Reflux

Reflux

Reflux

1.3

2.4

2.7

3.0

66.0

65.4

76.0

71.5

40

43

45

48

4.7

5.5

4.6

4.9

93

97

92

91

0.2

0.2

0.3

0.1

71

70

77

80

47

49

58

67

a- The final internal temperature of the reaction mixture was measured by Non-Contact I.R.

Thermometer.

b- Free formaldehyde content in the resin.

c- Yield of final compound that exhibited physical and spectral properties in accordance with

assigned structure.

Table 2. Comparative study for the synthesis of melamine-formaldehyde resin microwave

irradiation with and without solid support.

Conventional method Microwave Irradiation

(Without solid support)a

Microwave

irradiation

(Silica Gel) b

Microwave

irradiation

(Al2O3)c

Time

h

Temp. 0C

Yield

%

Time

min

Temp.d

0C

Yield e

%

Time

min

Yield

%

Time

min

Yield

%

4 Reflux 60 3.0 80 83 2.5 87 1.0 88

a -Reaction mixture in microwave oven was irradiated at power output of 160 W.

b -Reaction mixture on silica gel solid support in microwave oven.

c- Reaction mixture on Al2O3 solid support in microwave oven.

d -The final temperature of the reaction mixture was measured by Non-Contact I.R. thermometer

e- Isolated yield of purified compounds.

Page 5: Solid-State Microwave Synthesis of Melamine-Formaldehyde …

124 SUBHASH BAJIA et al.

Conclusion

We have developed an economical, solid-state microwave assisted protocol for the synthesis

of melamine-formaldehyde resin, which can be a viable alternative to the conventional

synthesis. In all cases, a comparison of the reactions using conventional and microwave

irradiation (neat and solid supported) under same conditions. The resin synthesized by

microwave irradiation has less free formaldehyde, more solid content.

Acknowledgement

Authors are thankful to the Head, Department of Pure and Applied Chemistry, M. D. S.

University, Ajmer for providing necessary laboratory facilities, to the Director CDRI

Lucknow India, for providing spectral data. Authors also express thanks to UGC, New

Delhi, India for providing financial support.

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