Polymerisation Phillips process

Figure 27 Propagation, termination and transfer reactions in ethylene polymerisation by the Phillips process.

A major characteristic of the Phillips process chain polymerisation of ethylene is that it leads to very limited branching. The resulting polymer is thus highly linear and can reach high levels of crystallinity, hence high densities approaching 0.96-0.97. Such a polyethylene is known as HDPE for "High-density polyethylene". 

Fig. 2.2. Simplified flow chart of the Phillips process (solution phase polymerisation) (Elias 1992). The solvent (for example isobutane), ethylene, co-monomers and the catalyst are fed into the reactor. The polymer solution is de sed (flashed) from the reactor through a gas separator after a certain reaction time. In a cleaning nnit (centrifuge, washing device, drier) the polymer is separated out and ethylene and solvent residues are processed. The polymer emerges in the drier in the form of snow white flakes. Flakes, carbon black, stabiliser and further additives, for example Ca-stearate, are mixed in a mixer and this mix is fed into an ex-tmder at an adequate mix raho with the polymer flakes. Here the mix is melted, homogenised and finally granulated. The black pellets are then transported to the storage facihties...

Traditionally, PE has tended to be classed as low, medium, or high density. These sub-divisions have never been precisely defined. High-pressure polymerisation of ethylene takes place by a free radical process, medium-pressure polymerisation occurs in the presence of molybdenum oxide or chromium oxide (Phillips process), while for low- and medium-pressure polymerisation transition metal halide and alkylaluminium compounds (Ziegler process) are used. Some properties of PE obtained from various ethylene processes are presented in Figure 1.2. 

High density polyethylene (HDPE) is defined by ASTM D1248-84 as a product of ethylene polymerisation with a density of 0.940 g/cm or higher. This range includes both homopolymers of ethylene and its copolymers with small amounts of a-olefins. The first commercial processes for HDPE manufacture were developed in the early 1950s and utilised a variety of transition-metal polymerisation catalysts based on molybdenum (1), chromium (2,3), and titanium (4). Commercial production of HDPE was started in 1956 in the United States by Phillips Petroleum Company and in Europe by Hoechst (5). HDPE is one of the largest volume commodity plastics produced in the world, with a worldwide capacity in 1994 of over 14 x 10 t/yr and a 32% share of the total polyethylene production. 

With the exception of LDPE, polyolefins like other polyethylenes and polypropylene, which represent the largest amount of vinyl-type polymers produced in the world, are neither synthesized by radical nor by classical ionic polymerisation processes. Different types of polymerisation catalysts are in use for these purposes. The Cr-based Phillips catalyst, Ziegler-Natta type catalysts, metallocene or other more recently discovered catalysts, including late transition metal catalysts, are all characterized by their propagation step where the olefin monomer inserts into a carbon-transition metal link. ... 

Another, highly selective oligomerisation reaction of ethene should be mentioned here, namely the trimerisation of ethene to give 1-hexene. Worldwide it is produced in a 0.5 Mt/y quantity and used as a comonomer for ethene polymerisation. The largest producer is BP with 40 % market share utilizing the Amoco process, formerly the Albemarle (Ethyl Corporation) process. About 25 % is made by Sasol in South Africa where it is distilled from the broad mixture of hydrocarbons obtained via the Fischer-Tropsch process, the conversion of syn-gas to fuel. The third important process has been developed by Phillips. 

The selection and treatment of the support is fundamental to the process, and a plant may use catalysts made from a variety of supports to produce a whole range of products. Catalyst productivities are of the order of 5 kg of polyethylene per gram of catalyst or higher, with a corresponding chromium content of 2 ppm or less. The percentage of Cr atoms that form active polymerisation centres has been estimated as 12% [43]. Typically, commercial Phillips catalysts contain ca 1 % total Cr and have particle sizes of 30-150 pm [224]. 

These catalysts were developed from the generation. At low temperatures (below 100 °C) the active violet y or 5 form of the brown P-TiCls is formed. Through the smaller size of the primary crystallites, the surface area and activity of the catalyst was increased. The and generation catalysts (unsupported catalysts) were used in suspension processes with hexane as a solvent, in mass polymerisation processes (Rexene, Phillips), in the BASF gas phase process (vertical agitation) and in the solution process (Eastman). 

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