High pressure polyethylene

LDPE, also known as high pressure polyethylene, is produced at pressures ranging from 82—276 MPa (800—2725 atm). Operating at 132—332°C, it may be produced by either a tubular or a stirred autoclave reactor. Reaction is sustained by continuously injecting free-radical initiators, such as peroxides, oxygen, or a combination of both, to the reactor feed. 

Table 1. Properties of Low Density High Pressure Polyethylene...

Branching can to some extent reduce the ability to crystallise. The frequent, but irregular, presence of side groups will interfere with the ability to pack. Branched polyethylenes, such as are made by high-pressure processes, are less crystalline and of lower density than less branched structures prepared using metal oxide catalysts. In extreme cases crystallisation could be almost completely inhibited. (Crystallisation in high-pressure polyethylenes is restricted more by the frequent short branches rather than by the occasional long branch.)... 

Figure 2 Light permeability of polyolefins after quenching (1-4) and of nonquenched samples (l -4 ) l,l -poly-propylene (PP) 2,2 -high-pressure polyethylene (HPPE) 3,3 -low-pressure polyethylene (LPPE) 4,4 -medium-pres-sure polyethylene (MPPE). Film thickness-150 fic moulding time-10 minutes, moulding pressure HPPE-160°C LPPE, MPPE, PP-190-200X.

Figure 3 Light permeability of the high-pressure polyethylene sample as a function of thickness I-3-quenched polyethylene 4-6-normal polyethylene 1 and 4-50 jlc thick 2 and 5-200 /jlc thick 3 and 6-350 fic thick.

Thermal destruction of low-pressure polyethylene with molecular weight of 34,800 and of high-pressure polyethylene is completely retarded by potassium hydroxide. The molecular weight of high-molecular polyethylene decreases by a factor of 1.8, and without an... 

V. S. Shifrina and N. N. Samosatsky, High-Pressure Polyethylene, Goschimizdat Publishers (1958). 

The computer model consists of the numerical integration of a set of differential equations which conceptualizes the high-pressure polyethylene reactor. A Runge-Kutta technique is used for integration with the use of an automatically adjusted integration step size. The equations used for the computer model are shown in Appendix A. 

Since the anionic triblock copolymers are based on monomers susceptible to this mechanism, one recent approach to this synthesis has been to prepare butadiene-isoprene-butadiene triblock copolymers, which are then hydrogenated so that the high-1,4 polybutadiene end blocks become crystallizable, similar to high-pressure polyethylene (l -5 ). 

It is now recognized that a continuum of architecture and properties, which begins with the classical branched polymers, resides between these two classes. Typical branched structures such as starch or high pressures polyethylene are characterized by more than two terminal groups per molecule, possessing substantially smaller hydrodynamic volumes and different intrinsic viscosities compared to linear polymers, yet they often exhibit unexpected segmental expansion near the theta state . 

Simplest of the techniques requiring only monomer and monomer-soluble initiator, and perhaps a chain-transfer agent for molecular weight control. Characterized, on the positive side, by high polymer yield per volume of reaction, easy polymer recovery. Difficulty of removing unreacted monomer and heat control are negative features. Examples of polymers produced by bulk polymerization include poly(methyl methacrylate), polystyrene, and low-density (high pressure) polyethylene. 

Fig. 3-18 Flow diagram of high-pressure polyethylene process. After Doak [1986] (by permission of Wiley-Interscience, New York).

The polyethylene produced by radical polymerization is referred to as low-density polyethylene (LDPE) or high-pressure polyethylene to distinguish it from the polyethylene synthesized using coordination catalysts (Sec. 8-1 lb). The latter polyethylene is referred to as high-density polyethylene (HDPE) or low-pressure polyethylene. Low-density polyethylene is more highly branched (both short and long branches) than high-density polyethylene and is therefore lower in crystallinity (40-60% vs. 70-90%) and density (0.91-0.93 g cm 3 vs. 0.94-0.96 g cm-3). 

It is called polydispersity index (PDI). The value of PDI can range between approx. 1.01 (for anionically prepared polymers) up to more than 30 (high-pressure polyethylene). In general, it is between 2 and 5. 

A number of processes have been developed to obtain products of different physical properties. The nature of the product is affected by the addition of diluents or other additives before carrying out the polymerization. Autoclaves or stirred-tank reactors, and tubular reactors, or their combinations have been developed for the industrial production of high-pressure polyethylene.206,440 Pressures up to 3500 atm and temperatures near 300°C are typically applied. 

The inhibitory properties of the diisopropyl salicylato chelates were confirmed in high pressure polyethylene at elevated temperatures. A more detailed study of this application to polyethylene and other polymers is in progress. 

A.W. Guill, Safety in High Pressure Polyethylene Plants, American Inst, of Chem. Engng. 1973, 49-52. 

The major hazard that can occur in the high-pressure polyethylene process is a runaway of the reactor and decomposition of ethylene as well as fires, explosion, and disintegration of high-pressure parts. Although the last incidents are well understood, the reasons for runaway and ethylene decomposition have been evaluated only recently. Experience over twenty years has shown that decomposition mostly takes place in the reactor and in the high-pressure separator, but decompositions have also been reported from ethylene-feed and product lines. 

R.G. Ziefle, Designing Safe High Pressure Polyethylene Plants, Chem. Eng. Progress Technical Manual, American Institute of Chemical Engineers, New York, 1973. 

TABLE 2. PROPERTIES OF LOW DENSITY HIGH PRESSURE POLYETHYLENE... 

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