Abstract
Three kinds of HPMI/n-BuA (monomer ratio≅1/4) copolymers which had different average molecular weights were synthesized. It was found that these copolymers had lower Tg and higher thermal decomposition temperatures than those of novolac. Molding compounds were prepared by hot roll-kneading from a novolac, the copolymer, hexamine, and glass fiber. Test pieces of modified phenolic resins were prepared by transfer molding from the molding compounds. Thermal and mechanical properties of the modified phenolic resins were examined by dynamic thermomechanometry, the test of flexural strength, and impact strength. Fracture toughness was examined using test piece without containing glass fiber.
It was found that phenolic resins modified with the copolymers had heat resistance and better impact strength than an unmodified phenolic resin, though the flexural strength of the modified phenolic resin was a little inferior to that of unmodified phenolic resin. There was no effect of average molecular weight of the copolymer upon these properties. But fracture toughness of the modified phenolic resin was inceased with the increase of an average molecular weight of HPMI/n-BuA copolymer and with the decrease of distribution range in their molecular weights. When content of the copolymer was 10phr, those showed the maximum value which was about 1.5 times as much as that of the unmodified phenolic resin. By observation with scanning electron microscope, many domains consisting of the copolymer were observed over the fracture surface. On these domains, size was smallest, distribution was narrowest, and density was highest, when content of the copolymer was 10phr.
