PP Copolymer against Homopolymer for Cold Chain Drop Impact
Polypropylene impact copolymers containing disperse ethylene propylene rubber phases maintain cold drop impact strength down to minus 40 degrees Celsius where homopolymers shatter.
Ductile to Brittle Transition Temperature
Meaning
Thermal thresholds where a polymer switches from a tough, energy-absorbing state to a fragile state define a specific design constraint. Identifying the ductile to brittle transition temperature allows engineers to predict when a part will fail under impact. This value depends on the molecular structure of the resin and any additives used for toughening.
Molecular Mechanism
Polymer chains move less freely as the temperature drops, reducing the ability of the material to deform plastically. When the environment reaches the ductile to brittle transition temperature, the material no longer has the time to dissipate energy through molecular sliding. Instead, the energy goes into breaking chemical bonds and creating new surfaces, resulting in a clean crack.
This shift occurs because the relaxation time of the polymer becomes longer than the time of the impact event.
Design Limitation
Parts used in outdoor environments or cold storage must be made of materials with a transition point well below the lowest expected operating temperature. Knowing the ductile to brittle transition temperature prevents the use of standard polypropylene in freezing conditions where it would become glass-like. Impact copolymers are often selected to push this limit lower.
Testing Reliability
Laboratory measurements often vary based on the speed of the impact applied to the specimen. Because the ductile to brittle transition temperature is sensitive to the strain rate, testers must use consistent speeds to generate useful data. A faster impact effectively raises the temperature at which the transition occurs.