Polyethene production processes

In terms of production volume, polyethene (PE) is the most important polymer. The production capacity for PE was 94 mio. tons in 2008 and is expected to rise to 128 mio. tons in 2015 (Plastemart, 2010). Despite the simple structure of the ethene monomer, many different polyethene production processes are in operation, leading to different classes of products with specific physicochemical properties and application fields. 

The main three polyethene classes are low-density polyethene (LDPE), high-density polyethene (HDPE), and linear low-density polyethene (LLDPE) (Scheme 6.20.1). Table 6.20.1 gives an overview of these classes and indicates the differences in density, production processes, and chemical structure. 

Transesterification is a crucial step in several industrial processes such as (i) production of higher acrylates from methylmethacrylate (for applications in resins and paints), (ii) polyethene terephthalate (PET) production from dimethyl terephthalate (DMT) and ethene glycol (in polyester manufacturing),... 

By far the most important industrial coordination polymerization processes are Ziegler-Natta polymerizations of 1-olefins [107-110], most notably the production of high-density polyethene [111] and stereo-specific olefin polymers and copolymers [108], However, these processes employ solid catalysts, and the complex kinetics on their surfaces have no place in a book on homogeneous reactions. 

The earliest Ziegler-Natta catalysts were insoluble bimetallic complexes of titanium and aluminum. Other combinations of transition and Group I-III metals have been used. Most of the current processes for production of high-density polyethene in the United States employ chromium complexes bound to silica supports. Soluble Ziegler-Natta catalysts have been prepared, but have so far not found their way into industrial processes. With respect to stereo-specificity they cannot match their solid counterparts. 

Only a limited number of building blocks are required by the large scale chemical industry. Of these ethene occupies a prime position. It is the principal feedstock for the manufacture of about 30% of all petrochemicals. In 1985 about 13.8m tonnes were produced in the USA alone. Of this 44% was converted into polyethene, 18% into ethene oxide and 14% into vinyl chloride (for PVC). Ethene oxide is the precursor to 1,2-ethane diol (ethylene glycol) which is used as an antifreeze and as a component for polyester production. These figures emphasize the dominant position of polymers in the petrochemical industry. Some of the processes, described below, which employ homogeneous catalysts are carried out on a much smaller scale and may represent only a small fraction of the total usage of chemical feedstocks. 

For the production of isotactic polypropene, different processes can be used (see production of polyethene). In the laboratory, the catalyst is suspended in dry pure heptane (or other hydrocarbons) in inert gas atmosphere and then bubble in propane gas at 30 to 60 °C. Polymers form a while solid permeating the catalyst particles. 

Monomer. Ethylene (which according to lUPAC rules should be called ethene and the corresponding polymer, from source-based rules, polyethene) is a gas Tb = - 0A°C) obtained from the thermal cracking (free radical process) of oil products. The initial reaction is a homolytic rupture of the covalent bonds of hydrocarbons that generate primary free radicals but the subsequent reactions are extremely varied (H abstractions, additions, decompositions, and isomerizations of radicals, etc.) and lead to a complex mixture that must be fractionated. Ethylene can also be produced either by dehydration of ethanol... 

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