The crystallization kinetics of polyethylene under isothermal and non-isothermal conditions

Chew, S.; Griffiths, John R.; Stachurski, Zbigniew

doi:10.1016/0032-3861(89)90185-7

Cited by 46 publications

(16 citation statements)

References 21 publications

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“…A compression press was used to mould the polymers into thin films (100 mm × 40 mm × 1.5 mm). The batch process consists of moulding 6 g of resin at 180°C under 3 tons of pressure for 10 min, long enough to remove any previous thermal history (Chew et al, 1989; Albano et al, 2003). The mould was then water‐cooled to release the sample.…”

Section: Methodsmentioning

confidence: 99%

On‐line prediction of crystallinity spatial distribution across polymer films using NIR spectral imaging and chemometrics methods

2008

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A rapid and non-intrusive on-line NIR imaging sensor is developed for monitoring spatio-temporal crystallinity variations across the surface of polymer films. A multivariate image analysis and regression (MIA/MIR) approach is proposed and compared with standard NIR calibration techniques using averaged spectra or second order derivatives combined with PLS regression. Predictions of both the local and global crystallinity variations of HDPE, LDPE, and PP polymer samples were obtained with each approach. Our results show that small variations in crystallinity introduced by changes in cooling rates can be predicted within experimental errors. Crystallinity spatial distributions were also validated and found consistent with processing conditions. Un capteur d'imagerie en proche infrarouge (NIR) rapide et non intrusif est mis au point pour faire le suivi des variations de cristallinité spatiotemporelleà la surface des films de polymères. Une approche par régression et analyse d'images multivariées (MIA/MIR) est proposée et comparée aux techniques de calibration NIR utilisant des spectres moyens ou des dérivées du second ordre combinésà la régression PLS. Des prédictions des variations de cristallinité locales et globales d'échantillons de polymères HDPE, LDPE et PP sont obtenues avec chacune des approches. Nos résultats montrent que de faibles variations dans la cristallinité introduites par des changements dans les vitesses de refroidissement peuventêtre préditesà l'intérieur des erreurs expérimentales. Les distributions spatiales de cristallinité sontégalement validées et s'avèrent cohérentes avec les conditions de traitement.

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Section: Methodsmentioning

confidence: 99%

On‐line prediction of crystallinity spatial distribution across polymer films using NIR spectral imaging and chemometrics methods

2008

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“…This was done with the aid of several independent methods. Particularly translucent was the method in which ultrathin slices (5 lm) were used according to Chow et al [20]. With this method spherulites, which in the bulk would start in a neighboring layer, did not interfere.…”

Section: Number Of Athermal Nuclei Per Unit Volume As a Function Of Tmentioning

confidence: 99%

The physics of athermal nuclei in polymer crystallization

1999

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IntroductionStructure formation in crystallizable polymers under the in¯uences of heat transfer and¯ow is a subject of major interest in polymer processing and in the development of`t ailored resins''. The ®rst author of this paper observed in 1982 ± to his own surprise ± that no serious attempt had been made to understand the underlying processes. So it is also not surprising that the required mathematical tools had not been developed. For instance, the path was not paved for a description of the overall crystallization speed in terms of the increasing surface of the crystalline areas on which further growth takes place. In order to make use of the developments in mathematics, one needs several physical parameters, which ± if one wants to avoid speculations ± can only be determined by experiment. Traditionally, the rate of primary nucleation and the growth speed of spherulites, both as functions of temperature, were required in a wide range of temperatures for a description of structure development during solidi®cation even for the simplest case of a quiescent melt.The pertinent experimental diculties, however, are tremendous, because of the fact that the relevant undercoolings are considerable. The experiments had to be extended over quite a large temperature range of undercooling. For i-PP a range of no less than 127°C could be covered in our laboratory. In a recent review [1] the results of this extended research are compiled up to early 1995. Some more recent results were published afterwards in separate papers (growth speed measurements with four independent methods [2] and further ®ndings in shear-induced crystallization [3,4]).The reader will understand that for all these experiments, in which heat transfer problems play a dominant role, some guidelines with respect to expected phenomena are rather desirable. In other words, a classi®cation of the temperature range between the Abstract The lower temperature limit for sporadic``thermal'' nucleation, as estimated in a previous paper (Janeschitz-Kriegl (1997) Colloid Polym Sci), can be veri®ed by a variety of earlier experiments which are compiled in this paper. Below the said limit only``athermal'' nuclei are of importance in samples containing no large numbers of hetero-nuclei. The number of athermal nuclei increases tremendously (by many decades) with decreasing temperature. Why these nuclei are dormant and``awake'' only at`t heir'' temperature is explained in terms of their growth conditions, which depend on the length of the incorporated (helical) sequences. Also the sluggishness of molecular processes, occurring in the temperature range of metastable conditions, where sporadic nucleation can occur is demonstrated by some of the recalled experiments. Except for very fast industrial processes, where¯ow plays a dominant role, this sluggishness makes a noticeable in¯uence of the sporadic nucleation on structure formation very improbable.

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“…From the point of view of the thermal equilibration the extremely low sample thickness, which was achieved by the evaporation of a dilute solution, was in vain because of the presence of the cover glasses. This low thickness of the polymer sample only enabled us to look at separate spherulites and count their number according to a proposal by Stachurski and coworkers [25]. (Obviously these spherulites were disk-like, their radii being much larger than the layer thickness.)…”

Section: Discussionmentioning

confidence: 99%

A Process Classification Number for the Solidification of Crystallizing Materials

Janeschitz‐Kriegl

Eder

Ratajski

2006

International Polymer Processing

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A critical number, which one may call a process classification number, has been introduced more than twenty years ago [1 to 3]. It has been called the Janeschitz-Kriegl number Jk [4]. It gives the ratio of two times governing the solidification process of a crystallizing material: one for the thermal equilibration and the other for the crystallization process itself. With one class of materials (mainly metals) the thermal equilibration time is usually the longest time, dependent of course on the sample thickness. With another class (glass forming minerals) always the crystallization time is the longest time. So, one obtains (as extreme cases) purely heat diffusion controlled and purely crystallization kinetics controlled processes. Only polymers (and probably also other crystallizing soft condensed matters) show an intermediate behavior, as can be characterized by the announced number.So far, however, this number could not be made operational because of a lack of crystallization kinetics data. This shortcoming could now be cleared away. It turns out that between the values of Jk for HDPE and for i-PS a gap exists of more than six decades. In their behavior all other known industrial polymers lie between these limiting cases. In this way the transition from heat diffusion controlled to crystallization kinetics controlled processes is clearly marked. For the now relevant interaction between cooling and crystallization a new mathematics was required [2]. It enables also the description of the development of structures, which are of particular interest for the quality of the product.

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The crystallization kinetics of polyethylene under isothermal and non-isothermal conditions

Cited by 46 publications

References 21 publications

On‐line prediction of crystallinity spatial distribution across polymer films using NIR spectral imaging and chemometrics methods

On‐line prediction of crystallinity spatial distribution across polymer films using NIR spectral imaging and chemometrics methods

The physics of athermal nuclei in polymer crystallization

A Process Classification Number for the Solidification of Crystallizing Materials

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