Correction of melting peaks of different PE grades accounting for heat transfer in DSC samples

In this work, the effect of heat transfer on the apparent melting behavior of linear low-density polyethylene (LLDPE) and recycled high-density polyethylene (rHDPE) was studied. Experimental melting temperature distributions (MTD) were shown to be significantly affected by the heating rate of the scan and the sample mass. The sample mass dependence of the MTD is attributed to thermal inertia of differential scanning calorimetry (DSC) samples. On the other hand, the heating rate dependence of MTD is in apparent contrast with the thermodynamic nature of the melting process. The existence of temperature gradients, responsible for heat transfer in DSC samples, was explained by adimensional analysis and evaluation of the Biot and Deborah numbers. A previously introduced equation was used to model melting behavior of the polymers. The parameters from non-linear regression were shown to be dependent on the product between the scanning rate and the square of sample mass. Regression parameters were extrapolated at infinite Deborah number. The MTD obtained at infinite Deborah number was considered the real MTD of the polymer, and was coupled with a heat transfer model to calculate the actual temperature of the sample. The local rate of heating, multiplied by the true MTD, provided the local rate of melting. The average rate of melting was compared with the experimental DSC signal. The very close agreement of the experimental and numerical prediction results evidence that the observed differences between melting curves obtained at different heating rates and sample mass can be attributed to thermal gradients in the DSC sample.