In this study, the effect of rheological behavior of epoxy during precuring on foaming was investigated. Dynamic time sweep test of epoxy/curing agent (100/1, w/w) was conducted. The viscosities as a function of time showed extremely rapid increase from the order of 10(2)-10(3) to 10(6)Pa . s at a certain time, followed by slow increase of the viscocities. Dynamic frequency sweep test of precured epoxy with curing agent was conducted at 90 C. The critical gelation time was obtained by using rheological criterion proposed by Winter and Chambon. We found that the slopes of G'(omega) and G(n)(omega) decreased with increasing precuring time. Correspondinly, tan delta showed a change from negative to positive slope at a critical time. By using the results, the critical gelation time was determined as t - 895-935 s. Samples of epoxy/curing agent/blowing agent (100/1/0.5) were precured for 960-1620 s. And then precured samples were foamed at 230 C for 300 s to decompose chemical blowing agent. The formed bubble size distribution becomes sharp with increase of the precuring time. There are roughtly two sizes of bubbles when precured for relatively short time (t < 1080 s) before foaming large bubbles (>100 mu m) and small once (congruent to 30 mu m). On the other hand, foams precured for long time (t > 1200 s) before foaming, large bubbles disappear, and the average diameter of the bubble becomes small while the porosity is low. (C) 2008 Wiley Periodicals, Inc.