Cooling spherulites

In hot-filled PET bottles, distortion of the finish may cause probiems with success-fui sealing of the bottle. If the finish of the PET preform is heated white the body of the preform is kept cool, spherulitic crystals result that turn the finish an opaque white and greatly increase its rigidity. Such crystallized finishes have iess tendency to deform during hot-fiiiing of the bottie. This operation is typicaiiy done in the preform stage. 

The polymer is liable to depolymerisation at temperatures just above T. In the case of pure polymer there is a tendency for the few spherulites to grow to sizes up to 1mm diameter. Spherulite size may be reduced by the use of nucleating agents and by fast cooling. 

Figure 11 Left Spherulites of a Ziegler-Natta isotactic poly(propylene) with Mw = 271,500 g/mol and mmmm — 0.95, isothermally crystallized at 148°C. Right Banded spherulites of a linear polyethylene with Mw = 53,600 g/mol slowly cooled from the melt.

Slow cooling has also been reported to enhance the formation of non-spherulitic morphologies in stareh gels (Nordmark and Ziegler, 2002). 

Figure 15.8 SEM image of spherulite aggregates of jet cooked high amylose starch dispersion slowly cooled without stirring. (Reprinted from Fanta et al., 2008, with permission from Elsevier).

Previous work hod shown that low temperature coke is formed from cools hooted to between 450° and 500° C. by a process of nudeation and growth of spherical bodies in the plastic vitrinite. An essentially similar process has now been found to occur with coke-oven and petroleum pitches, with polyvinyl chloride, and with some polynuclear hydrocarbons, all of which yield carbons which grophitize readily at high temperatures. The process is probably general for the initial stages of formation of such carbons from the liquid phase. Some control of the solidification process has been achieved on the laboratory scale, and the physical and chemical structure of the spherulites has been investigated. 

Polymerization on heterogeneous catalysts differs from other catalytic reactions in the sense that the product remains on the catalyst. Several techniques can be used to study the polymer product after reaction. Figures 9.29 and 9.30 show several examples of polymer that was formed at 160 °C (i.e., above its melting point), and subsequently cooled to room temperature. During cooling, polyethylene crystallizes and is expected to develop its well-known spherulite morphology [101]. 

Figure 12.25 The 129Xc diffusion coefficient for Xe absorbed in the copolymer material PBT/PTMO as a function of the diffusion time. One sample was slowly cooled down from the melt and contained many large spherulites. The other was quickly cooled down and contained only small spherulites...

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