Hello Janitha Jeewantha

To my mind the explanation is very simple. After the first curing cycle glass transition appears slightly below 50 C so that above this temperature bisphenol is in viscous (liquid) state. Therefore its heat capacity varies slightly when temperature is elevated and DSC trace is almost horizontal.

Cycle II results in further transformation of viscous state into glassy state. It results in appearance of the second glass transition near 100 C which in turn results in further increase of heat capacity after transition. This is why you observe intersection of DSC traces.

Hope this info will help.

Regards

A. Kossoy

Hallo Janitha,

adding to Arcady explanation, you must remember that there will be a mass loss associated with the cure reaction. This mass loss will also contribute to the heat flow signal which in turn will shift the slope & position of the curve (2nd)

To understand (and prove) this you should run a TGA as well. (to see the mass loss of cure 1)

Trust this will help you

Solid line thermogram shows 2 glass transitions: a narrow one at about 50C and more broad at about 100C. Presence of two glass transitions may indicate coexistence of two distinct regions differing in cross link densities: regions with lower cross link density characterized by the lower Tg and regions with higher cross link densities characterized by higher Tg. Dashed line thermogram shows single glass transition at about 50C.

Janitha, just to add to my reply, overlay the 1st derivative of heatflow vs temp with your std curve. This helps a lot to understand the Tg signal.