Crystallization, cold spherulitic

Additionally, Quero et al studied the isothermal cold-crystallization of PLA/PBAT blends with and without acetyl tributyl citrate (ATBC). SEM results showed that the blends exhibited two phases, but partial miscibility changes in both, Tg and Tn, of the PLA phase, a relationship of the spherulitic growth rate with the blend eomposition and the occlusion of PBAT droplets within PLA spherulites. ATBC acted like a plasticizer for both phases (neat PLA and PBAT). In the ease of blends, ATBC prefers to be included inside the PBAT-rieh phase. There was a synergistic effect on the overall crystallization rate of PLA when both ATBC and PBAT were present in the blend. 

In the case of acetal, the use of a cold mold results in fast dissipation of heat from the melt into the mold wall. Consequently, the threshold limit for the formation of crystallization nuclei is quickly reached and a skin is formed on the parts that has an amorphous appearance but is actually crystalline, although to a much lower extent than the spherulitic region that is formed below the skin. The thickness to the amorphous zone is dependent on the mold and melt temperatures and other factors. 

The orientation process aligns the molecules and makes it easier to form crystalline domains. The crystalline structure is the preferred state to which PET will revert unless molecular motion is restricted. The typical level of crystallinity in cold mold blown bottles, like CSD bottles, is 20-25%. This crystallization is formed as the material is being stretched and is referred to as strain-induced crystallization (SIC). The crystallites formed under SIC conditions are numerous and remain small and therefore different from those formed under thermal crystallization conditions, which result in large spherulites. Spheru-lites will grow until they impinge on one another. At a certain size, the spherulites start to scatter incident light and will appear opaque white rather than the clear colorless amorphous PET. To preserve bottle clarity, the crystallites must remain small. 

Wang et al. [33] studied the spherulitic growth rates and the microstructure of SPS cold-crystallized isothermally at various temperatures in the range 115-240 °C, by small-angle light scattering (SALS), optical microscopy, and transmission electron microscopy (TEM Fig. 9.2). 

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