Density Polyethylene - LDPE - Chapter

The first section of this chapter describes the most important high pressure process run under homogeneous conditions to manufacture Low Density PolyEthylene (LDPE). The radical polymerization of ethylene to LDPE is carried out in tubular reactors or in stirred autoclaves. Tubular reactors exhibit higher capacities than stirred autoclaves. The latter are preferred to produce ethylene copolymers having a higher comonomer content. 

The largest consumers of ethylene are the various types of polyethylene Low Density Polyethylene (LDPE), High Density Polyethylene (HDPE), and Linear Low Density Polyethylene (LLDPE). Chapter 15 gives detailed discussions of preparation of the various types of polyethylene. 

For process optimization with respect to several economic criteria such as net present worth, payback period and operating cost, the classical Williams and Otto (WO) process and an industrial low-density polyethylene (LDPE) plant are considered. Results show that either single optimal solution or Pareto-optimal solutions are possible for process design problems depending on the objectives and model equations. Subsequently, industrial ecosystems are studied for optimization with respect to both economic and environmental objectives. Economic objective is important as companies are inherently profit-driven, and there is often a tradeoff between profit and environmental impact. Pareto-optimal fronts were successfully obtained for the 6-plant industrial ecosystem optimized for multiple objectives by NSGA-ll-aJG. The study and results reported in this chapter show the need and potential for optimization of processes for multiple economic and environmental objectives. 

The above background provides the motivation for the study and applications described in this chapter. Here, two types of process optimization problems are described. The first type has only economic objectives the two examples considered for this are the classical Williams and Otto (WO) process used recently by Pintaric and Kravanja (2006), and an industrial low-density polyethylene (LDPE) plant based on our recent studies (Agrawal et al., 2006 and 2007). The economic objectives tried are PBP, NPW, IRR, profit before taxes, and/or operating cost. The second type has both economic and environmental indices for this, the industrial ecosystem with four plants employed by Singh and Lou (2006) is expanded to an ecosystem with six plants and then optimized for multiple objectives. 

A major example of the second branched polymer type is the polyethylene that is made by free radical polymerization at temperatures of 100-300°C and pressures of 1,000-3,000 atm. The extent of branching varies considerably depending on reaction conditions and may reach as high as 30 branches per 500 monomer units. Branches in polyethylene are mainly short branches (ethyl and butyl) and are believed to result from intramolecular chain transfer during polymerization (described later in Chapter 5). This branched polyethylene, also called low-density polyethylene (LDPE), differs from linear polyethylene (high-density polyethylene, HDPE) of a low-pressure process so much so that the two materials are generally not used for the same application. 

This chapter covers fundamental and applied research on polyester/clay nanocomposites (Section 31.2), which includes polyethylene terephthalate (PET), blends of PET and poly(ethylene 2,6-naphthalene dicarboxy-late) (PEN), and unsaturated polyester resins. Section 31.3 deals with polyethylene (PE) and polypropylene (PP)-montmorillonite (MMT) nanocomposites, including blends of low density polyethylene (LDPE), linear low density polyethylene (LLDPE), and high density polyethylene (HDPE). Section 31.4 analyzes the fire-retardant properties of nanocomposites made of high impact polystyrene (HIPS), layered clays, and nonhalogenated additives. Section 31.5 discusses the conductive properties of blends of PET/PMMA (poly (methyl methacrylate)) and PET/HDPE combined with several types of carbon... 

Clearly, the combinations of resins and fillers and the resulting property variations are endless (see Fig. 6-2). The point is that each combination is in fact a new material with its own trade-offs. Some properties will be improved, others unchanged, and still others diminished from those of the basic unfilled plastic. In this chapter there is no relationship, direct or implied, between any plastic in terms of the space given it and its performance or the size of its market. The largest consumption of these plastics is low-density polyethylene (LDPE) formulations, at about 25 percent weightwise, followed by high-density polyethylene (HDPE), then polypropylene, polyvinyl chloride, and polystyrene. These together total about two-thirds of all plastic consumption. 

Low-density polyethylene (LDPE) and copolymers, used primarily in films and packaging applications. LDPE has density of <0.94 g cm, and is produced via high-pressure free-radical polymerization polyethylenes of higher density (and polypropylene) are produced via transition metal catalysis, as described elsewhere (see Chapter 8). 

Figure 9.32b gives, for comparison, log G versus log G" plots for a low-density polyethylene (LDPE) specimen subjected to the thermal history as described in the temperature protocol given in Figure 9.32a. It is clearly seen in Figure 9.32 that the log G versus log G" plot shows temperature independence, regardless of the thermal history to which the specimen was subjected. Such an experimental observation is expected because LDPE is a flexible homopolymer. The point we try to make here is that for a TLCP with textures, its morphology changes with temperature. In the preceding chapter we made similar observations in microphase-separated block copolymers.

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. 

The polymer has a low cohesive energy density (the solubility parameter 8 is about 16.1 MPaU2) and would be expected to be resistant to solvents of solubility parameter greater than 18.5 MPa1/2 (Chapter 4). Since polyethylene is a crystalline hydrocarbon polymer incapable of specific interaction, there are no solvents at room temperature. Materials of similar solubility parameters and low molecular weight will however cause swelling, the more so in low-density polymers. LDPE has a gas permeability in the range normally expected with rubbery materials. HDPE has a permeability of about one-fifth that of LDPE. 

Bend made the composite that composed of PF and wood flour (WF). The first commercial application of this composite was used as a gearshift knob for Rolls Royce car in 1916 under the registered trademark of Bakelite [1]. On the contrary, thermoplastics can be repeatedly melted without any change in their inherent properties, including polypropylene (PP), high and low density polyethylene (HOPE and LDPE), polyvinyl chloride (PVC), and so on. In this chapter, the abbreviation of WPCs represents wood-thermoplastic composites, which are usually simply referred to as wood-plastic composites (WPCs). Similarly, the common understanding of plastics always refers to thermoplastics. 

Generally, there is a certain correlation between density, on the one hand, and flexural strength and modulus, on the other, for many other materials, and that correlation is not related to porosity. For example, there is a strong correlation (R = 0.984) between density of all 38 polyethylene materials, listed in Table 7.49 of Chapter 7, including LDPE, LLDPE, HDPE, and their flexural modulus (Figure 6.1). Besides, mineral fillers in WPC materials increase density of the final product and also increase its flexural modulus. However, this chapter is mainly concerned about relationships between density and properties of WPC having the same formulation but produced at different regimes. 

---