High-pressure LDPE processes

Because of the (chain-length dependent) restricted solubility of polymers in hydrocarbons, solution polymerization requires higher temperatures and / or higher pressures. Technologically, solution polymerization processes are similar to the high-pressure LDPE process, see chapter 3, and will not be discussed here. 

Figure 1. Schematic of high-pressure LDPE process.

Yields in all these processes are in the range 97-99% of theory overall, giving feedstock energy inputs of 72-73 GJ/tonne with ethylene at 2MJ/mol. Processing requirements add 10-12 GJ/tonne for the high pressure (LDPE) process and 6-7 GJ/tonne for the low pressure (HDPE, LLDPE) processes. 

A more recent development in ethylene polymerization is the simplified low pressure LDPE process. The pressure range is 0.7—2.1 MPa with temperatures less than 100°C. The reaction takes place in the gas phase instead of Hquid phase as in the conventional LDPE technology. These new technologies demand ultra high purity ethylene. 

PE produced by a high-pressure polymerization process (pressure 1000-3000 atm) using a free radical initiator is a highly branched material that contains both LCBs and SCBs. The polymer so produced is a low-density material (density up to about 0.925 g/cc) and is known as high-pressure low-density PE (HP LDPE). The LCBs are formed via intermolecular hydrogen transfer [19], whereas SCBs are formed by intramolecular hydrogen abstraction [16]. 

The different available high pressure polymerisation processes of polyethylene (PE) yield LDPE (low density PE), LLDPE (linear low density PE) and copolymer features of the same. The various process variations have been developed during recent decades and introduced a number of well developed steps and devices to achieve safe and economical operating conditions at the very high reaction pressures of 1500 to 3000 bar. 

The molecular weight distribution in the process is controlled by the choice of catalyst. The density (between 0.915 and 0.97 g/crr ) is controlled by the amount of comonomer added. This versatile process avoids using hydrocarbon dilutants or solvents. In comparison to conventional high pressure LDPE units, Union Carbide has reported significant savings in plant investment and energy costs with this gas-phase technology. 

The development of PE began in 1936 with the introduction of the high pressure polymerization process of ethylene to LDPE (0.915-0.94 g/cm3) which produced a... 

Low density polyethylene material has branched chains and limited crystallinity, which lead to an open structure and the low density. It is particularly soft and flexible, transparent to translucent, has good impact resistance and relatively low melting points, which give good heat sealability. Most LDPEs are made by a high pressure polymerisation process starting from ethylene gas. The proportion of crystallinity to amorphous is around 3 2 (i.e. 60-65% crystalline). Recently new linear polyethylene copolymers of 0.89-0.91 (ultra or very low densities) have been developed. Special antioxidant free grades are available for pharmaceutical applications. 

Discovery of Linear (High-Density) Polyethylene Use of Comonomers Linear Low-Density Polyethylene Stereospecific Polymerization Discovery of Polypropylene Manufacturing Processes High-Pressure LDPE Low-Pressure, Linear HOPE LLDPE... 

Much of the growth in LLDPE demand will be at the expense of other polyolefins, especially conventional high-pressure, LDPE. Its proponents claim that LLDPE can replace LDPE in 70 to 80% of its markets and that "the last conventional polyethylene plant has been built." Another sign of the times is the fact that ICI, originator of the high-pressure process, has now ceased production. 

High-Pressure LDPE. The original ICI (and, later, Du Pont) process uses steel autoclave reactors, operating at pressures ranging from... 

This technique, also called extruder degassing, is applied to remove residual hydrocarbon components in those processes in which the product is extruded starting from the melted form, for instance solution LLDPE and high pressure LDPE. This technique requires an extended extruder since the polymer needs to be compressed, vacuum devolatilised and then compressed again for the final pelletising step. The hydrocarbon vapours coming from the vacuum dome are processed in a vacuum/scmbber system. The non-condensable substances, mainly ethylene, are sent to a flare, due to the risk of undesirable contamination with oxygen. 

Materials scientists have taken other steps to improve processability such as blending high-pressure LDPE or more advanced resins such as eth-ylene/norbornene-based polyolefins into the base resin to improve shear thinning behavior. 

Using UNIPOL II process technology. Union Carbide introduced easy flow LLDPE products that had better processability than LLDPE with a narrow MWD and LDPE with an intermediate MWD. Union Carbide reported processability and product physical property data for these new resins [25]. Some of the data is summarized in Table 6.7 which compares a UNIPOL II easyflow resin with a high-pressure LDPE. 

Compared to other polymer production processes and even among polyolefin processes, the high-pressure LDPE production technology is highly suitable for... 

The concerned industries are chemical/petrochemical, power, nuclear, and armament industries, as well as high-pressure food processing. Autofrettage is common practice for equipment used in polyethylene (LDPE) plants, gun barrels, waterjet cutting pumps, and high-pressure pasteurization equipment. 

The high-pressure process was developed from the high-pressure polyethylene process (LDPE). The polymerization is carried out in bulk. By means of this process mainly copolymers with a vinyl acetate content of up to 45% are produced. The important range lies between 5% and 30% vinyl acetate content, giving copolymers with thermoplastic properties. The maximum molecular weight achieved by the high-pressure process is comparatively low due to the high chain-transfer activity of the vinyl acetate in bulk polymerization. Therefore, the vinyl acetate content is limited. 

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