Meaning
Thermal analysis detects exothermic phase changes when quenched semi-crystalline polymers gain sufficient chain mobility during reheating above their glass transition temperature. Solid polymer chains undergo cold crystallization as frozen amorphous molecules rearrange into ordered crystalline structures without passing through a liquid melt state. Chain mobility increases abruptly as temperature rises, enabling polymer segments to fold into organized crystallite lattices.
Uncontrolled crystal formation during heating causes density shifts and internal stress release in polymer parts.
Exothermic Transition
Differential scanning calorimetry records an exothermic peak during controlled heating ramps of amorphous polymer samples. Latent heat release during cold crystallization indicates the degree of initial quenching present in processed resins. Rapid baseline shifts signal the onset of crystallization, allowing analytical technicians to quantify the un-crystallized polymer fraction.
Polymer grades with slow crystallization kinetics exhibit prominent exothermic peaks upon reheating.
Quench History
Rapid cooling inside mold cavities prevents polymer chain alignment by freezing molecules into metastable amorphous states. High cooling rates during injection moulding maximize the potential for cold crystallization when parts experience elevated service temperatures later.
Dimensional Instability
Volumetric contraction accompanies crystal lattice formation during thermal exposure of under-crystallized polymer components. Part dimensions shift unpredictably when cold crystallization increases local density within moulded walls during secondary drying or painting cycles. Mold heat settings must exceed the glass transition temperature to ensure full crystalline development before ejection from the cavity.
