Polyethylene melt crystallisation/melting

A further approach is used by Bayer with their polyesteramide BAK resins. A film grade, with mechanical and thermal properties similar to those of polyethylene is marketed as BAK 1095. Based on caprolactam, adipic acid and butane diol it may be considered as a nylon 6-co-polyester. An injection moulding grade, BAK 2195, with a higher melting point and faster crystallisation is referred to as a nylon 66-co-polyester and thus presumably based on hexamethylene diamine, adipic acid and butane diol. 

Processing temperatures should not exceed 180°C, and the duration of time that the material is in the melt state should be kept to a minimum. At the end of a run the processing equipment should be purged with polyethylene. When blow moulding, the blow pin and mould should be at about 60°C to optimise crystallisation rates. Similarly, injection moulds are recommended to be held at 60 5 C. 

Figure 5 Electron micrograph of a portion of melt crystallised polyethylene spherulite by transmission electron microscopy (TEM) showing lamellae. Reproduced from Ref. [3] with permission of John Wiley Sons, Inc.

Due to both kinds of branching leading to chain irregularities, the crystallisation of radical chain-polymerised polyethylene is strongly hindered. Its maximum degree of crystallinity is limited to about 50%, its melting temperature ranges from 80°C to 115°C and its density remains low ( 0.92). From this latter property, it received the name of low-density polyethylene (LDPE). 

Journal of Cellular Plastics 36, No.l, Jan./Feb.2000, p.29-44 NONISOTHERMAL MELTING AND CRYSTALLISATION OF FOAMED CROSSLINKED POLYETHYLENE Kotzev G Touleshkov N Christova D Nedkov E Bulgarian Academy of Sciences... 

The existence of crystal lamellae in melt-crystallised polyethylene was independently shown by Fischer [28] and Kobayashi [39]. They observed stacks of almost parallel crystal lamellae with amorphous material sandwiched between adjacent crystals. At the time, another structure was well known, the spherulite (from Greek meaning small sphere ). Spherulites are readily observed by polarised light microscopy and they were first recognised for polymers in the study of Bunn and Alcock [40] on branched polyethylene. They found that the polyethylene spherulites had a lower refractive index along the spherulite radius than along the tangential direction. Polyethylene also shows other superstructures, e.g. structures which lack the full spherical symmetry referred to as axialites, a term coined by Basset et al. [41]. 

The structural hierarchy of melt-crystallised polyethylene is schematically displayed in Fig. 8. Crystal thickness and its controlling factors are discussed in Sect. 2. It includes one phenomenologically resolved issue, the initial crys-... 

Polyethylene crystals change shape with time at temperatures between the crystallisation temperature and the final melting point [42]. The process is commonly referred to as crystal thickening because the major effect is that the crystals increase their dimensions along the c axis. These changes may occur without any change in crystal volume, i.e. crystal thickening is associated with a lateral contraction of the crystals (Fig. 9). A second option is that... 

The morphology of a polyethylene blend (a homopolymer prepared from ethylene is a blend of species with different molar mass) after crystallisation is dependent on the blend morphology of the molten system before crystallisation and on the relative tendencies for the different molecular species to crystallise at different temperatures. The latter may lead to phase separation (segregation) of low molar mass species at a relatively fine scale within spherulites this is typical of linear polyethylene. Highly branched polyethylene may show segregation on a larger scale, so-called cellulation. Phase separation in the melt results in spherical domain structures on a large scale. 

Fig. 26 Critical molar mass of melt-crystallised linear polyethylene as a function of crystallisation temperature. Filled circles data for a broad molar mass sample with Mn=8500 g mor1, Mw=153,000 g mor1 of Mehta and Wunderlich [149], Open circles data for a sample with Mn=12,900 g mor1, Mw=108,000 g mol-1 from Gedde et al. [152]. From [91] with permission from Kluwer, Doordrecht, Netherlands...

Fig. 29 Cumulative melting and dissolution (in p-xylene) curves of a linear polyethylene crystallised at 401 K to completeness and then rapidly cooled to room temperature. Drawn after data of Gedde et al. [152]...

At room temperature, PE is a semi-crystalline plastomer (a plastic which on stretching shows elongation like an elastomer), but on heating crystallites melt and the polymer passes through an elastomeric phase. Similarly, by hindering the crystallisation of PE (that is, by incorporating new chain elements), amorphous curable rubbery materials like ethylene propylene copolymer (EPM), ethylene propylene diene terpolymer (EPDM), ethylene-vinyl acetate copolymer (EVA), chlorinated polyethylene (CM), and chlorosulphonated polyethylene (CSM) can be prepared. 

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