CROSS-LINK DENSITY OF OLIGOURETHANE DIOLS CURED WITH AMINO RESIN AT DIFFERENT TEMPERATURES

Oligourethane diols were crosslinked with melamine formaldehyde (N, N, N',N',N'',N''-hexakis (butoxymethyl)-2,4,6-triamino-1,3,5-triazine). T heir films were wetted with dichloromethane and the increase in dimens ions of the swollen film under equilibrium swelling conditions was use d to calculate crosslink density by using the Flory-Rehner equation. T he molar mass between two crosslinks (M) over bar c was also determine d. The crosslink density of the coating is found to be influenced by a trans-etherification reaction as well as by self-condensation of the crosslinker. In formulations containing primary hydroxylated diol a ma jor part of the crosslinker is consumed in a trans-etherification reac tion, whereas a part of the crosslinker undergoes self-condensation in formulations containing secondary hydroxylated diols. The aromatic na ture of 4-methyl-m-phenylene diisocyanate (TDI) imparts rigidity while the aliphatic nature of to-1-(isocyanatomethyl)-1,3,3-trimethylcycloh exane (IPDI) results in flexibility of the backbone. The formulations containing 30% crosslinker and cured at 140 degrees C for 20 min are f ound to undergo completion of the major part of the reaction, indicati ng ideal conditions for their crosslinking from a commercial point of view.