The cured unsaturated polyester resins are often very brittle. In this paper, liquid carboxyl-terminated butadiene acrylonitrile (CTBN) copolymer has been selected to improve toughening behavior of unsaturated polyester resin. CTBN liquid rubber has acidic –OH of carboxylic group which cause to poor solubility in the unsaturated polyester resins and incompatible with unsaturated polyester. 1, 4-Butanediol diglycidyl ether (1, 4-BDGE) was selected to react with carboxylic group of the CTBN liquid rubber which converted to ester functional group. Four blend samples were prepared by mixing unsaturated polyester with different concentrations of modified CTBN from 0 to 3 %. The reactions between epoxy groups of 1, 4-BDGE and carboxyl groups of CTBN were followed by Fourier-transform infrared (FTIR) spectroscopy. Tensile tests of neat and blend of the modified CTBN- unsaturated polyester have been studied to observe the effect of modified CTBN additive. The results show that when 2wt% CTBN was added to polyester, the tensile strength and elongation at- break increased by 7.22 and 39.4 %, respectively compared to neat polyester. The maximum toughness of the prepared samples was achieved for 2w% CTBN/ 98wt% polyester. Morphological studies revealed that the rubber particles size was dependent to the rubber content.

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