Polyethylene Melt Processing

This oxidation step was carried out in air at 40°C using 10 Mrad gamma irradiation [14,15]. 

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There are a number of polymers which in fact cannot be melt processed because of their high molecular weights. These include PTFE, very high molecular weight polyethylene and most grades of cast poly(methyl methacrylate). In such cases shaping in the rubbery phase is usually the best alternative. 

Melt elasticity is of considerable importance in understanding much of the behaviour of polyethylene when processing by film extrusion techniques and when blow moulding. The complex relationships observed experimentally here have been summarised by the author elsewhere. ... 

Although plastics materials may in principle be processed in a variety of physical states (in solution, in emulsion, as a paste or as a melt), melt processing is used almost exclusively with polyethylene. The main features to be borne in mind when processing the polymers are ... 

Polycarbonate melts adhere strongly to metals and if allowed to cool in an injection cylinder or extrusion barrel may, on shrinkage, pull pieces of metal away from the wall. It is therefore necessary to purge all equipment free of the resin, with a polymer such as polyethylene, after processing. 

L. L. Blyler and T. K. Kwei processed their own experimental data in an incorrect way polyethylene melts with dissolved nitrogen have the same viscosity as pure melts. Dissolved nitrogen cannot change the value of the free volume of the PE melt hence, L. L. Blyler s and T. K. Kwei s formula is incorrect. 

Many authors studying the formation of ECC from melts and solutions suggested that preliminary unfolding and extension of macromolecules occurs. Keller and Maehin25 have shown that in all known cases (including such extreme variants as the crystallization of natural rubber under extension and a polyethylene melt under flow) the same initial process of linear nucleation occurs and fibrillar structures is formed by the macromolecu-lar chains oriented parallel to the fibrillar axes27. ... 

Crystallization in asymmetric diblocks with compositions = 0.35 and 0.46 was also investigated by Hamley et al. (19966). It was found that a lamellar structure melted epitaxially (i.e. the domain spacing and orientation were maintained across the transition) to a hexagonal-packed cylinder structure in the /PE = 0.35 sample. This is illustrated in Fig. 5.15, which shows SAXS patterns in the solid and melt states, with a schematic of the epitaxial melting process (Hamley et al. 1996a.b). The same epitaxial transition has been observed for a polyethylene oxide)-poly(buty)ene oxide) diblock (Ryan et at. 1997) vide infra). 

Commercially produced polyethylene and polypropylene have inherently limited stability to high temperatures and UV light and require stabilisation to survive the melt processing, fabrication and end-use conditions. 

Oxidative degradation of polyethylene (PE) and polypropylene (PP) can occur at all stages of their lifecycle (polymerisation, storage, processing, fabrication and in-service). The auto-oxidation process of polyolefins is best described by the classical free-radical-initiated chain reaction outlined in Scheme 1 [1]. Impurities initially present in the polymers during polymerisation or melt processing, exert profound effects on the behaviour of the final polymer article in service. 

Wenzhoug Tang, Michael H. Santare, Suresh G. Advani. Melt processing and mechanical property characterization of multi-walled carbon nanotube/ high density polyethylene (MWNT/HDPE) composite films. Carbon 2003 41 2779-2785. 

For one thing, remembering again the fact that an interval from p — 3normal distribution, the figure 6a is sometimes stated as the process capability. This usage would say that in the context of the polyethylene melt index example of Table 5.4 the a = 12.1 figure from (5-10) implies a melt index process capability... 

As symmetrical molecules approach within a critical distance during melt processing, crystals begin to form in the areas where they are the most densely packed. A crystallized area is stiffer and stronger, a non-crystallized (amorphous) area is tougher and more flexible. With increased crystallinity, other effects occur. As an example, with polyethylene (crystalline) there is increased resistance to creep. 

EVOH have superior gas barrier properties.80 89 They are often used as the internal layer in multilayer food packaging films, blow molded rigid containers, gasoline tanks for automobiles for a variety of purposes, etc. EVOH can be fabricated by the usual melt processing methods. The barrier properties of films decrease in the presence of moisture, so multilayer with protective polypropylene (especially biaxially oriented material), low-density polyethylene, nylons, or other moisture barrier films provides films or products that are useful even with liquids. The... 

An additive system was developed for poly(vinyl chloride) for medical applications. The additives include primary stabilisers (Ca-Zn stearate and Zn stearate), secondary stabilisers (epoxides) and lubricants (ethylene bisamide and high density polyethylene), to improve melt processing and heat stability. The use of the stabilisers resulted in reduced equipment down-time, increased the level of recycled material which could be incorporated, and enhanced the product characteristics, including colour, clarity, blush, aqueous extractables and particle generation. 5 refs. 

In materials that are often used in construction-wood, concrete and steel—there is still an enormous variation in modulus, by a factor of 15 or so. You can also see that most polymer materials, at least in their usual melt processed form, are not very stiff at all, polyethylene having a modulus of about 150 MPa, while even a glassy polymer like atactic polystyrene has a modulus of only about 3000 MPa (about l/20th that of window glass). 

Blown film extrusion is perhaps the most widely used extrusion technique, by production volume. Billions of pounds of polyethylene are processed annually by this method to make products such as grocery sacks and trash can liners. In a blown film system (Figure 14-30), the melt is generally extruded vertically upward through an annular die. The thin tube is filled with air as it travels up to a collapsing frame that flattens it before it enters the nip rollers, which pull the film away from the die. The flattened tube then travels over a series of idle rollers to a slitter,... 

The conformations of rigid chains will not be much different in the amorphous state near Tm than they are in the crystal lattice. This means that the melting process confers relatively little additional disorder on the system A5m is low and Tm is increased correspondingly. For example, ether units in polyfethylene oxide) (1-42) make this structure more flexible than polyethylene, and Tm of high-molecular-weight versions of the former species is only 66"C. By contrast, poly(/ -xylene) (11-1) is composed of stiff chains and its crystal melting point is 375°C. 

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