solid-state microwave synthesis of melamine-formaldehyde …
TRANSCRIPT
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.
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.
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.
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
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.
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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