Polyethylene solution polymerisation process

The low vinyl acetate ethylene—vinyl acetate copolymers, ie, those containing 10—40 wt % vinyl acetate, are made by processes similar to those used to make low density polyethylene for which pressures are usually > 103 MPa (15,000 psi). A medium, ie, 45 wt % vinyl acetate copolymer with mbber-like properties is made by solution polymerisation in /-butyl alcohol at 34.5 MPa (5000 psi). The 70—95 wt % vinyl acetate emulsion copolymers are made in emulsion processes under ethylene pressures of 2.07—10.4 MPa (300—1500 psi). 

In contrast to polyethylene production, solution polymerisation at high temperature is rarely applied for isotactic polypropylene, but some special-purpose polypropylene grades are manufactured (Figure 3.57) [51]. However, the solution process, which yields isotactic polypropylene with a very low level of impurities, is characterised by high overall costs. The solution process is being used to make atactic polypropylene, to which it seems more suited [43],... 

In some processes, a diluent, like benzene or chlorobenzene are used as the solvent. At high pressure and temperature, both the polyethylene and the monomers dissolve in these solvents so that the reaction occur in a solution phase. In a typical process, 10-30 per cent of the monomer is converted to polymer per cycle. Rest of monomer is recycled. Extensive chain transfer reactions take place during polymerisation to yield a branched polyethylene. Apart from long branches it is also having a large number of short branches of unto 5 carbon atoms formed by intramolecular chain transfer reactions. A typical molecule of Low density polyethylene is having a short branch for about every 50 carbon atoms and one or two long branches per molecule. 

The solution process has been developed mainly for the manufacture of polyethylene. The term solution should not be taken literally since the polymer formed is often present in a molten state owing to maintenance of a high polymerisation temperature. In the solution process, a solvent such as... 

SAXSAVAXS/RAMAN is especially useful when dealing with chemically induced phase transitions. The example shown in Figure 2(e) is the polymerisation of solvent styrene into polystyrene in which polyethylene is in solution. Polyethylene is soluble in styrene but insoluble in polystyrene. RAMAN allows the determination of the reaction kinetics of polystyrene formation and monitors the crystallisation of the polyethylene. The SAXS monitors the liquid-liquid phase separation followed by the liquid-solid phase transition, whilst the WAXS also observes the liquid solid phase by monitoring the appearance of peaks due to the crystallisation of polyethylene. These are very valuable parameters when trying to define any new manufacturing process. ... 

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