Cooling density polyethylene

The principal use of LDPE and LLDPE in building products is as a film water barrier under below-grade doors as a wall vapor barrier, though PVC is typically preferred and as temporary enclosure film during constmction. The film is made either by extmding a thin-waHed tube, which may be sHt or wound up direcdy, or by extmsion through a slot die and cast direcdy on to a cold roU, cooled, then wound up. The former method is more widely used. A much smaller use for low density polyethylene is in piping. 

The cooling requirements will be discussed further in Section 8.2.6. What is particularly noteworthy is the considerable difference in heating requirements between polymers. For example, the data in Table 8.1 assume similar melt temperatures for polystyrene and low-density polyethylene, yet the heat requirement per cm is only 295 J for polystyrene but 543 J for LDPE. It is also noteworthy that in spite of their high processing temperatures the heat requirements per unit volume for FEP (see Chapter 13) and polyethersulphone are, on the data supplied, the lowest for the polymers listed. 

Low density polyethylene is made at high pressures in one of two types of continuous reactor. Autoclave reactors are large stirred pressure vessels, which rely on chilled incoming monomer to remove the heat of polymerization. Tubular reactors consist of long tubes with diameters of approximately 2.5 cm and lengths of up to 600 m. Tubular reactors have a very high surface-to-volume ratio, which permits external cooling to remove the heat of polymerization. 

Free-radical polyolefin reactions form polymers with many mistakes in addition to the ideal long-chain alkanes because of chain-branching and chain-termination steps, as discussed. This produces a fairly heterogeneous set of polymer molecules with a broad molecular-weight distribution, and these molecules do not crystallize when cooled but rather form amorphous polymers, which are called low-density polyethylene. 

Fig. 28 Scanning electron micrograph of high density polyethylene first isothermally crystallised at 128 °C and then rapidly cooled to room temperature. The sample was etched with hot p-xylene to remove the material crystallising in the cooling phase. Scale bar represents 20 pm. From Gedde and Jansson [154] with permission from Elsevier, UK...

Fig. 9.24 Cross sections from cooling experiments. See Figs. 9.20 and 9.21. Material Low density polyethylene (LDPE).

Figure 3. Series of SAXS patterns of a low-density polyethylene taken during cooling at 10°C/min. The peak develops near93°C, indicating the onset of crystallization. Reprinted with permission of John Wiley Sons from Russell, T. P., and Koberstein, J. T., Simultaneous Differential Scanning Calorimetry and Small Angle X-Ray Scattering, J. Polym. Sci. Polym. Phys. Ed. 23, 1109 (1985) [19]. Copyright 1985, John Wiley Sons.

A similar technique is applied to low-density polyethylene reactors. Some of these systems operate in cooled tubular reactors at a very high pressure. Since the reactor has a thick tube wall, the temperature response to changes in the coolant is slow. Instead, the reaction rate (and thereby temperature.) is controlled by injecting initiator at select places along the length of the reactor tube (see Fig. 4.28). 

Plastic polymers make up a high proportion of waste and the volume and range used is increasing dramatically. The two main types of plastic are thermoplastics which soften when heated and harden again when cooled and thermosets which harden by curing and cannot be remoulded. The six main plastics in municipal solid waste are, high-density polyethylene (HDPE), low-density polyethylene (LDPE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC) and polyethylene terephthalate (PET). In addition there are... 

Optionally, in the case of Fe- and Co-containing nanoparticles, the mineral oil was substituted for a mineral oil-low density polyethylene (LDPE) solution-melt. The thermal destruction was carried out at vigorous stirring at a constant temperature in the argon flow. The MCC solution was introduced into the reaction system dropwise at a constant rate. The black material produced after the addition of all the MCC solution was stirred at the synthesis temperature for 0.5 h and cooled down to room temperature. The product was extracted via rinsing the mixture with hexane. The calculated concentrations of metal-containing nanoparticles in the product resulted varied from 1 to 50 wt.%. 

Several commercial processes are used to produce high-density polyethylene. All employ more moderate pressures and most also use lower temperatures than the low-density polyethylene processes. The Ziegler-developed process uses the mildest conditions, 200-400 kPa (2 atm) and 50-75°C, to polymerize a solution of ethylene in a hydrocarbon solvent using a titanium tetrachloride/aluminum alkyl-based coordination catalyst. After quenching the polymerized mixture with a simple alcohol, the catalyst residues may be removed by extraction with dilute hydrochloric acid or may be rendered inert by a proprietary additive. The product is almost insoluble in the hydrocarbon solvent, so is recovered by centrifuging and drying. The final product is extruded into uniform pellets and cooled for shipping to fabricators. 

If we consider the process of cooling molten polyethylene, there will be a progressive decrease in the volume that the chains occupy. This specific volume, V, is the reciprocal of the density and this is shown in Figure 1.7 for the case on cooling the polymer from 150 C to — 150 °C. It is seen that there is a linear decrease in with decreasing temperature, which is consistent with the coefficient of thermal expansion. 

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