Gel chromatography of melamine-formaldehyde resins: Gel chromatography of amino resins. I

The determination of the apparent number and weight average molecular weights (Mn, and Mw, ) and molecular weight distributions of melamine-formaldehyde resins (MFR) was studied by gel chromatography. Samples studied were analyzed in an open column gravity feed system by gel chromatography. The operating parameters were as follows: solvent, dimethylformamide; temperature, 25°C±2°C; gel, Sephadex LH-20; Column, Sephadex Column Model SR 25/100 purchased from Pharmacia Co., 80 cm; detector, Waters Associates, Differencial Refractomer Model 403; sensitivity, 16X; sample concentration, about 80 mg/2 ml; flow rate, 1 ml/min controlled with Mariotte flask system. The calibration curve was obtained from free melamine, isomers of mononuclear methylol melamines, and the standard monodispersed polystyrene and polydispersed polyethylene glycols purchased from Pressure Chemical Co. The separation o f isomers of mononuclear methylol melamines in MFR was considerably successful. The mechanism of MFR separation is attributed to the molecular sieving and absorption effects of Sephadex LH-20. It was found that Mn of commercial MFR was between from 200 to 400 and their molecular weight distributions were generally narrow. The calibration curve used is not applicable to MFR etherified with alcohols and to that modified with urea-formaldehyde resins.