Abstract:
In this study the effectiveness of various chain extenders in increasing the molecular weight of unsaturated polyesters was investigated. For this purpose eight different chain extenders (with di or multi-functionalities) such as epoxides, isocyanates and bisoxazolines were added to short chain oligomeric polyesters with molecular weights of 500-700 and the desired molecular weight of 1500-2000 was reached within very short times. Diglycidyl ether of bisphenol A, 1,2;7,8-diepoxyoctane and epoxidized soybean oil reacted with carboxylic end groups of unsaturated polyesters within 30-60 minutes; two blocked isocyanates, namely diphenylmethane-bis-4,4’-N,N’-ethyleneurea and diphenylmethane-bis-4,4’-carbamoyl-Є-caprolactam reacted with both carboxylic and hydroxylic end groups of the unsaturated polyesters to reach the desired molecular weight within less than one hour. Isocyanated soybean oil reacted with hydroxyl end groups of the polyester chains. 1,3-phenylene-bis-oxazoline and 2,2’-bis-2-oxazoline gave a fast reaction with the carboxylic end groups of the oligomeric polyesters within a reaction time as short as 5 minutes. With the help of these extenders the conventional polyesterification reaction time can be reduced from 20 hours to 6-7 hours, diol loss can be eliminated and product yields can be increased by about 5 per cent. After dilution with styrene to give a styrene concentration of 35 per cent, the resins were cured in the usual manner and the mechanical and thermal properties of the cured resins were found to be comparable to that of the commercial reference product. Thus, the industrial use of the chain extended polyesters was verified. Gel times remained close to that of the commercial reference product (10-15 minutes). Tg values were found about 80-90 oC and the TGA data showed that the 5 per cent weight loss temperature in many cases was about 230-275 oC, very comparable to that of the commercial reference product.
