Solid-State Microwave Synthesis of Melamine-Formaldehyde Resin

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.

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][14][15][16][17] has attracted attention in recent years due to enhance reaction rates, high yields, improve purity, ease of work up after the reaction.
The present work reveals the comparative aspects of synthesis of melamineformaldehyde 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.

Equipments
Reagent grade chemicals were used without purifications.Viscosity was measured according to ASTM D1084 method 18 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 MH Z frequency.Reaction mixture internal temperature was measured on Mine Gun type Non Contact I.R. thermometer.Resin solid content 19 were determined by heating a 1 g. of resin sample at 125 0 C for 1.50 h.The percentage of free formaldehyde content was determined using a modified version of Walker's 20 hydroxylamine hydroxide and sodium sulfide methods.FTIR spectra were recorded with a Perkin-Elmer 16 PC spectrometer.The 1 H NMR spectra were obtained with a Bruker WM 250 spectrometer (68.69 MH Z ) 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 earlier 21,22 .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 Al 2 O 3 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.

H 1 NMR spectra
A single appears as singlet at 6.2-6.8 ppm due to NH 2 in the melamine.In the H 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 simplified procedure was perceived, in which melamine and para formaldehyde could be made to react without and with Al 2 O 3 and silica gel (as solid support) under solvent free microwave irradiation.The use of Al 2 O 3 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 solution 23 .So under the microwave irradiation condition using Al 2 O 3 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

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.
NMR spectra of the resin the signal appears at 4.6-4.7 ppm correspond to -CH 2 O-; 5.1-5.2ppm to -NCH 2 OH; 6.0-6.3 ppm to -NH 2 -.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-CH 2 -N) and (dimethylene) ether (-CH 2 -O-CH 2 -) bonds are exist, from which the signals appear at 4.5 ppm and 4.6-4.8ppm, respectively.The intensities of these signals show that the latter bond is in 4-fold excess.

Table 1 .
Comparative study for the synthesis of melamine-formaldehyde resin Conventional and microwave irradiation method (without solid support)

Table 2 .
Comparative study for the synthesis of melamine-formaldehyde resin microwave irradiation with and without solid support.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 Al 2 O 3 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.