Polyethylene Product Space

Until the mid-1990s, the polyethylene product space consisted of the manufacture of ethylene-based homopolymers and copolymers over a density range of about 0.910-0.965 g/cc with Melt Index values between about 0.05-100. 

The introduction of single-site catalyst technology has expanded the polyethylene product space to densities as low as about 0.86 g/cc with traditional comonomers such as 1-butene, 1-hexene and 1-octene. In addition, single-site catalysts have provided new ethylene-based copolymers with comonomers based on styrene and cyclic olefins such as norbomene. 

Figure 6.1 Polyethylene product space identifing various grades of polyethylene used in various fabrication techniques.

Radiation cross-linking of polyethylene requires considerably less overall energy and less space, and is faster, more efficient, and environmentally more acceptable. Chemically cross-linked PE contains chemicals, which are by-products of the curing system. These often have adverse effects on the dielectric properties and, in some cases, are simply not acceptable. The disadvantage of electron beam cross-linking is a more or less nonuniform dose distribution. This can happen particularly in thicker objects due to intrinsic dose-depth profiles of electron beams. Another problem can be a nonuniformity of rotation of cylindrical objects as they traverse a scanned electron beam. However, the mechanical properties often depend on the mean cross-link density. ... 

Removal of cationic impurities from water. Careful analysis of water purified by various methods (see Table 7.10) indicates that the water that is obtained by passing ordinary distilled water through a small monobed deionizer (contained in polyethylene) and a submicrometer filter is equal or superior (with respect to cations) to water obtained by distillation in conventional quartz stills, and is distinctly superior to the product from systems constructed of metal.70 From the data available in the literature, simple distillation clearly does not produce high-purity water. In practice, two effects cause contamination of the distillate. Entrainment is the major factor that prevents the perfect separation of a volatile substance from nonvolatile solids during distillation. Rising bubbles of vapor break through the surface of the liquid with considerable force and throw a fog of droplets (of colloidal dimensions) into the vapor space... 

Treatment of solid wood over the years for increased utility included many chemical systems that affected the cell wall and filled the void spaces in the wood. Some of these treatments found commercial applications, while some remain laboratory curiosities. A brief description of the earlier treatments is given for heat-stabilized wood, phenol-formaldehyde-treated veneers, bulking of the cell wall with polyethylene glycol, ozone gas-phase treatment, ammonia liquid- and gas-phase treatment, and p- and y-radiation. Many of these treatments led to commercial products, such as Staybwood, Staypak, Im-preg, and Compreg. This chapter is concerned primarily with wood-polymer composites using vinyl monomers. Generally, wood-polymers imply bulk polymerization of a vinyl-type monomer in the void spaces of solid wood. 

Because cellular polyethylene is comprised of roughly equal volumes of resin and gas, its properties are different from those of ordinary unfoamed polyethylene. The cellular product has a much lower dielectric constant and therefore lower electrical losses. The composition of polyethylene (dielectric constant 2.3) and an inert gas (dielectric constant 1.0) has a dielectric constant of 1.5. In terms of electrical insulation the lower dielectric constant permits a reduction in space between inner and outer conductors without changing the characteristic impedance. For this reason it is possible to reduce the attenuation by... 

The LLDPE product represents the outcome of a method developed to produce a low-density polyethylene but by using the more moderate, and therefore less costly, conditions employed by the processes used to produce high-density polyethylene. It is not, strictly speaking, a polyethylene since it is not a homopolymer of ethylene. An LLDPE is actually a copolymer of ethylene, which includes traces of 1-octene (Dow and Du Pont), 1-hexene (Phillips), or 1-butene (Union Carbide). This results in a polymer that has entirely short branches, and these are more uniformly spaced along the backbone than in LDPE. The spacing of the branches obtained in these cases can be closely controlled by the proportion of a-olefin to ethylene used in the feed, and the lengths by the choice of the a-olefin comonomer. Properties intermediate to those of low- and high-density polyethylene are obtained for the product (Table 23.2). 

Even in XLPE cables cured under dry conditions some voids exist. These are thought to be due to the coalescence of catalyst by-products or interspherulitic space caused by crystallization of the cooling polyethylene. 

With a moisture sensitive product, moisture pick by this means can be quite rapid. Alternative materials to wool are either plastic shapes or plugs of expanded polystyrene, polyethylene or polyurethane foams. Closures can also be obtained which incorporate spring extensions. Such closures may have chambers which hold desiccants. Transit tests (vibration/drops) would be advised to check whether space fillers are necessary. 

Where the ultimate in protection is required, aluminium containers can be flanged and fitted with seamed-on aluminium easy-open ends. These containers are tamperevident, but once opened the product must be used within a short space of time, unless polyethylene overcaps can be used during the use of the product. 

Pyrolysis and reforming of several types of common plastics (polyethylene, polypropylene, polyvinyl chloride, polyethylene terephthalate, polyurethane, and polycarbonate) were studied qualitatively, using a micro-reactor interfaced with a MBMS. Each type of plastic pyrolyzed at 550-750°C. This was followed by steam reforming of vapors in a fixed bed of C-11 NK catalyst at 750-800°C. The composition of the product gas (mass spectrum) was observed for different values of the steam-to-carbon mtio and space velocity that changed depending on the size of plastic samples. Preliminary tests showed that at process conditions similar to those used for reforming natural gas, polymers were almost completely converted to hydrogen and carbon oxides. 

A fixed bed flow reactor study carried out by Ogawa et al was consistent with the results of Lin and White. Using a 13 wt% alumina SA catalyst, the space time was varied, and its effect on the product distribution determined. As the contact time increased, the yield of styrene decreased and the yields of ethylbenzene and methylphenylindans increased. The weight of total liquid products decreased as contact time increased, but the yield of gases remained essentially constant at a value close to zero. This suggests that carbon residue remained on the catalyst, and its value increased with increasing space time. This is similar to the behavior observed for polyethylene conversion over SA catalysts. 

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