Polypropylene crystallization half-times

Figure 39.4 shows several chart recorded bulk crystallization traces as a function of temperatures for isotactic polypropylene (iPP, Mw = 257,000). Avrami s analysis takes the lower values of each plot i.e., before impingement. The half-time is the point where half of the intensity is reached. Figure 39.5 presents Avrami s analysis for different crystallization temperatures, where it can be seen that all plots have similar slope, n, and different intercepts, k, in this case. 

Studies on a similar group of materials - polymeric composites reinforced with sisal fibers - were conducted by Manchado et al. [35]. They analyzed the presence of different fibers, such as sisal, on crystallization of polypropylene. The composites were prepared in special chamber for mixing where the matrix was plastified at 190°C. Obtained materials were subjected to thermal analysis by DSC. The analysis of thermograms allowed for a similar finding like in Joseph s studies [34], The presence of sisal fibers, as well as other fibers used in the study, accelerated crystallization of polypropylene. This was explained by the nucleating effect of sisal filler. Also, the half-time crystallization (ti/2) decrease was observed for polypropylene with the addition of sisal fibers in comparison with unfilled polypropylene. The analysis of nonisothermal crystallization showed that the degree of polypropylene crystallinity is higher for the composites filled with sisal fibers than for unfilled polymer. 

The high molecular weight unfilled polypropylene prepared by the same catalyst has a form stability of 48.7°C (Table 6). A composite polypropylene with 0.9 wt% of MWCNT shows form stability up to 60.4 C, and a composite polypropylene with 2.3 wt% of MWCNT a form stability of up to 71.5°C. Other important parameters are the crystallization temperature and the half time of crystallization. The addition of only 0.9 wt% of MWCNT led to an increase in crystallization temperature from 118 to 123°C. The half time of crystallization was significantly reduced (faster crystallization rate) by low amoimts of nanotubes. At 135°C it was 4.5 min for a composite with 0.9 wt% MWCNT and 2.4 min for a composite with 2.2 wt% MWCNT. The higher crystallization rate increases the economy of an industrial forming process. 

Kinetic analysis of the polymer crystallization by the DSC method in isothermal conditions can provide information about the effect of nanoparticles on the mechanism of nucleation and crystals growth. The reduction of the half-crystalliza-tion time ti/2) was considered an evidence of the crystallization rate of polypropylene (PP) at low OMMT contents [35]. ti/2 was lower than that of neat polymers for the crystallization process of many polymer matrices modified with OMMT [36,37]. 

Here r denotes some characteristic time of the crystallization process, for example, that at which half of the final crystallinity is reached. The exponential change of r with temperature tells that, as in the nucleation step, crystal growth is associated with an activation barrier. For polyethylene, where t changes by a decade within 4 K, this barrier is lower and more temperature sensitive than in s-polypropylene, where a shift in the crystallization temperature of 15 K is necessary for a comparable change. There are different views in the literature about the nature of this barrier one possible explanation is given later in Sect. 5.3.1. 

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