the DSC results for a blend containing 90% HDPE and 10% polyethylene wax shows two separate peaks during the first heating, one for the wax phase and one for the HDPE phase. but the peak associated with the wax phase disappears during the second heating run. why does this happen?
the DSC is performed with these conditions: rate 10 degrees of Celsius per minute, first heating from 20 to 180 degrees, 5 min isothermal heating in 180, cooling with the same rate back to 20 degrees, held isothermally for 5 min and then heating to 180 with the same rate.
I think it's due to high heating speed. At high heating rates, DSC can sometimes fail. Under normal conditions, the polyethylene wax should have a peak at about 150 °C. I think the problem will be solved when your heating speed is 3 min/°C.
Probably both components have a significant but limited mutual solubility. In the 1st heating the sample is inhomogeneous, and at the contact points(s) of both components there might form a eutectic, or an azeotrope, with correspondingly lower melting temperature. This is the 1st peak that disappears later. After the 1st heating the sample might be homogeneous, and formed a mixed crystal (solid solution). This has a clear liquidus and solidus, and in between you have one single (probably smeared?) melting process. The eutectic/azeotrope no longer appears.
I agree with Detlef Klimm (with the exception of the "azeotrope" part - eutectic [melting solids] yes, azeotrope [boiling liquids] no).
It is even possible - if the wax melting point is much lower than that of HDPE - that it will melt independently in the first run, without initially forming a eutectic. Runs at different heating rates, as suggested by Behzat Yildirim, should help figure it out.
Indication of the wax being dissolved in PE during the second heating is lowering of PE melting onset, peak and end temperatures.
because wax crystallizes slowly and can not crystallize during the cooling step
By experience, I think all these previous suggestions you got are pertinent and logical, because the reasons of a change in thermograms of a blend can be several ones.
Maybe you could make some punctual tests (e.g. DSC with different heating - and cooling - rates; or increase of blending time/temperature of the two components, or crystallization kinethics measurements etc....). You should understand whether the phenomenon is linked to the DSC test conditions, or to maybe a bad premixing, or to a plascitization effect byt the wax...or else.
Good luck.
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