Solid State Polymerization Volatile Stripping Mass Transfer Limits in rPET
Solid state polymerization stripping rate is dictated by internal Fickian diffusion limits of condensation by-products and volatiles within rPET pellet cores.
Crystalline Core Skin Morphology
Meaning
Microstructural distribution profiles describe the variation in crystallinity from the rapid-cooled surface of a moulded part to its slower-cooled interior. In semi-crystalline polymers like polyamides, crystalline core skin morphology dictates the local density and orientation of the polymer chains. This graded structure forms because the material touching the cold steel mold wall solidifies instantly, preventing crystallization, while the hot core has time to form organized spherulites.
Thermal Profile
Cooling rates in the cavity control the relative thickness of the amorphous skin and the highly crystalline core. Rapid heat transfer yields a thicker, highly oriented skin layer that resists abrasion. Slower cooling in the center allows the growth of larger crystalline regions, which increases the modulus of the core.
This spatial variation can be analyzed using polarized light microscopy on microtomed cross-sections of the part.
Mechanical Behavior
The distribution of structural phases influences the overall flexural strength and toughness of the finished component. A part with an unbalanced crystalline core skin morphology is susceptible to warpage due to anisotropic shrinkage across the part thickness. Extreme transitions between the skin and core can also act as stress concentration zones where cracks initiate under cyclic loading.
Process Control
Moulders adjust the mold temperature and cooling time to optimize this microstructural layout for specific applications. High mold temperatures promote a thinner skin and a more uniform crystalline transition, which improves dimensional stability at the expense of cycle time.