Differential Scanning Calorimetry Protocol for Recycled Polyethylene Polypropylene Blends
Second-heat differential scanning calorimetry enthalpy integration at 10 K/min isolates recycled PE and PP fractions against reference crystalline values.
Sigmoidal Baseline
Meaning
S-shaped mathematical baseline constructs used in thermal analysis software adjust heat capacity shifts occurring across endothermic or exothermic phase transitions during differential scanning calorimetry. A step change in specific heat capacity naturally accompanies transitions like glass transition or melting, causing the post-transition thermal baseline to settle at a different level than the pre-transition baseline. Applying a sigmoidal baseline accounts for this proportional heat capacity shift by weighting the baseline position relative to the instantaneous fraction of transformed material.
Thermal analysts use this curved baseline construction to isolate true transformation enthalpy from background heat capacity changes.
Enthalpy Calculation
Integration of transformation peaks requires accurate subtraction of background heat flow signals across the transition temperature range. Linear baseline approximations introduce systematic errors when heat capacity differs substantially before and after the thermal event. Constructing a sigmoidal baseline connects the start and end points of the peak with a curve that mirrors the cumulative area of the transformation.
This variable background subtraction yields precise crystalline enthalpy calculations required to determine polymer crystallinity percentage.
Transition Analysis
Broad melting transitions in complex semi-crystalline polymers alter heat capacity continuously throughout the phase change. The mathematical shape of the sigmoidal baseline adjusts dynamically to match the progress of melting.
Integration Boundary
Defining accurate integration limits prevents skewing heat of fusion values during thermal characterization. Laboratory technicians select start and end temperatures where heat flow departs from and returns to stable baseline response. Misplacing baseline limits distorts calculated enthalpy values, leading to incorrect assessments of polymer blend ratios or crosslink density.