Meaning
Thermal properties of amorphous or semi-crystalline polymers change when dissolved low-molecular-weight molecules increase chain mobility. A glass transition depression occurs when plasticizers, absorbed water or residual solvents lower the temperature at which a polymer transitions from a hard state to a rubbery state. This shift alters the mechanical performance of the material at its planned operating temperature.
Material Plasticization
Molecular interactions between polymer chains are disrupted by the introduction of smaller, flexible molecules. This effect leads to a glass transition depression as the free volume within the polymer matrix increases. Moisture absorption in nylon is a common cause of this phenomenon during service.
While this increases the toughness of the molded parts, it reduces their stiffness and tensile strength.
Process Modification
Melt temperatures and injection pressures must be adjusted to account for changes in polymer thermal properties. During injection moulding, a glass transition depression allows for lower processing temperatures and reduced cycle times. However, if the transition temperature drops below the demoulding temperature, the parts may deform during ejection.
Toolmakers must adjust cooling cycles to prevent warped parts. This thermal change can also affect the post-moulding shrinkage behavior of semi-crystalline plastics.
Resin Specification
Calorimetric measurements using differential scanning calorimetry verify the thermal limits of plastic materials. High concentrations of low-molecular-weight oligomers in regrind material trigger this depression and reduce part quality.
