Bulk polymerisation processes

Bulk polymerisation processes have been known for many years but until the mid-1960s the only commercial process was one operated by Pechiney-St Gobain in France. This process was a one-stage process and according to one patent example vinyl chloride was polymerised with 0.8%% of its own weight of benzoyl peroxide in a rotating cylinder containing steel balls for 17 hours at 58°C. 

High molecular weight polymers are produced by an adiabatic bulk polymerisation process ° using di-tert-butyl peroxide (0.02%) and 2,2 -azo-bisdi-isobutyronitrile (0.01%) as initiators and pressurised with N2. Heating to 80-90°C causes an onset of polymerisation and a rapid increase in temperature. After the maximum temperature has been reached the mass is allowed to cool under pressure. A typical current commercial material (Luvican M.170) has a A -value of about 70 (as assessed in a 1% tetrahydrofuran solution). 

Caprolactam can also be prepared by bulk polymerisation process using anionic catalysts like strong bases and metal hybrides. 

Two series of polyether polyurethanes (PU) based on hydroquinone bis (P-hydroxyethyl) ether (HQEE) or 1,4-butanediol (BDO) as a chain extender were prepared by the one step bulk polymerisation process. By varying the mole ratio of poly tetra methylene oxide (PTMO) extender (with Mn = 1000 and Mn = 2000) and 4,4 -diphenylene methane diisocyanate (MDI) the two series of HQEE (PUlOOOHj, PU 1000H2, PU2000Hj,... 

Figure 3.54 Flow scheme of polypropylene production using the bulk polymerisation process (with the removal of catalyst residues)...

The system involved in the bulk polymerisation process is the simplest from the point of view of composition and is used for large-scale radical polymerisation. In this process, the initiator is mixed with the monomer, usually under pressure of an inert gas, and the mixture is heated to induce generation of free radicals by thermal decomposition of the initiator. Depending on whether the monomer is a gas or a liquid, the system can be homogenous or heterogeneous... 

In bulk polymerisation, the polymer is produced in a reactor where only the monomer and a small amoxmt of an initiator are present. Bulk polymerisation processes are characterised by high product purity, high reactor performances and low separation costs, but also by high viscosities in the reactors. Bulk processes cause reactor fouling, and in the case of polycondensation products, a high vacuum is required. 

Solution Polymerization. In this process an inert solvent is added to the reaction mass. The solvent adds its heat capacity and reduces the viscosity, faciUtating convective heat transfer. The solvent can also be refluxed to remove heat. On the other hand, the solvent wastes reactor space and reduces both rate and molecular weight as compared to bulk polymerisation. Additional technology is needed to separate the polymer product and to recover and store the solvent. Both batch and continuous processes are used. 

By rolling on a two-roll mill the molecular weight of the polymer can be greatly reduced by mechanical scission, analogous to that involved in the mastication of natural rubber, and so mouldable materials may be obtained. However, bulk polymerisation is expensive and the additional milling and grinding processes necessary make this process uneconomic in addition to increasing the risk of contamination. 

For both types of polymerisation mechanisms, different polymerisation processes can be used ranging from simple bulk and solution polymerisation processes to more sophisticated ones such as suspension, emulsion, interfacial, plasma,... polymerisation processes. 

Bulk and suspension polymerisation are the most commonly used techniques. In bulk polymerisation styrene is heated to 80°C for about 2 days to get a viscous solution of polymer in styrene. The solution is then fed to a tower wherein polymerisation is completed at 100°C, 150C° and 180C° stagewise. In suspension process, styrene is suspended in dimineralised water in presence of suspending agent and initiator like benzoyl peroxide and heated to 20°C. The product is washed with acid, water and dried. 

Investigations into the effect of ultrasound upon these polymerisation processes began in the mid 1980 s when Akbulut and Toppare [81] examined the potentiostatic control of a number of copolymerisations. In such copolymerisations initiation takes place once a potential in excess of the oxidation potential of either monomer has been applied. However, often potentials even higher than these are required due to the formation at the electrode of a polymer film. These films create a resistance to the passage of current in the bulk medium with consequent reductions in the possible electrochemical reactions and therefore reductions in the rate and the yield. The use of ultrasound has been rationalised in terms of its removal of this layer in a... 

It seems that increasing the surfactant concentration causes thinning of the films between adjacent droplets of dispersed phase. Above a certain level, the films become so thin that on polymerisation, holes appear in the material at the points of closest droplet contact. A satisfactory explanation for this phenomenon has not yet been postulated [132], It is evident, however, that the films must be intact until polymerisation has occurred to such an extent as to lend some structural stability to the monomer phase if not, large-scale coalescence of emulsion droplets would occur yielding a poor quality foam. In general, vinyl monomers undergo a volume contraction on polymerisation (i.e. the bulk density increases) and in the limits of a thin film, this effect may play a role in hole formation, especially at higher conversions in the polymerisation process. 

The technologies that have been developed for the production of polyolefins, olefin homopolymers and copolymers are slurry, solution and gas-phase polymerisation bulk polymerisation of propylene in the liquid monomer as a special case of the slurry process has also emerged. The fundamental differences in the various olefin polymerisation processes reflect the different approaches that have been devised to remove the substantial heat of polymerisation. In addition, processes can be operated in a batch or a continuous mode. In the batch process the reagents are loaded into a polymerisation vessel, the polymer forms and the vessel is emptied before a new charge of reagents is introduced. In the continuous process, the catalyst precursor, activator and other necessary... 

Possible Effect of Charged End-groups. A possible reason for differences between samples might be the process used for polymerising the original polyvinyl acetate. Emulsion polymerisation is likely to introduce a proportion of ionic (sulphate or carboxyl) end-groups which would not be expected if bulk polymerisation with benzoyl peroxide had been used. An Antweller Microelectrophoresis apparatus was used to measure rates of electrophoresis of polyvinyl alcohols in solution in a pH 7.8 phosphate buffer. No significant difference was observed between... 

The MIP is usually prepared as a highly cross-linked, rigid bulk polymer and the polymerisation reaction is initiated by photo- or thermo-labile free radical initiators such as 2,2 -azobis(isobutyronitrile). For molecular imprint-based CEC systems, the introduction of the imprinted polymer into the capillary column has been focused on and several approaches have been developed (see below). The polymerisation process can be performed in between 1 and 24 h. It has been shown that the temperature during the polymerisation process is important. A lower temperature leads to imprinted polymers with higher selectivity [47] or better chromatographic performance [39]. 

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