Determination of temperature and enthalpy of melting-crystallisation
Semi-crystalline plastics, when heated, pass from the crystalline (solid) stage to the molten (soft) phase. Â Due to their different molecular structures, plastics melt at different temperatures. Â The most precise method to determine the temperature and enthalpy of melting (the melting properties) of the plastics is by differential scanning calorimetry (ISO 11357-3, ASTM D3418).
A small piece of plastic (approximately 10 mg) is heated and cooled at a controlled rate under nitrogen. During the heating and the cooling cycles, the following parameters are obtained:
- melting of semi-crystalline plastics: transition stage between a fully crystalline or partially crystalline solid state and an amorphous liquid of variable viscosity
- crystallisation: transition stage between an amorphous liquid state and a fully crystalline or partially crystalline solid state
- enthalpy of fusion: heat required to melt material at a constant pressure
- enthalpy of crystallisation: heat released by the crystallisation of a material at a constant pressure
