Continuous polymerisation

Polymer Production. Three processes are used to produce nylon-6,6. Two of these start with nylon-6,6 salt, a combination of adipic acid and hexamethylenediamine in water they are the batch or autoclave process and the continuous polymerisation process. The third, the soHd-phase polymerisation process, starts with low molecular weight pellets usually made via the autoclave process, and continues to build the molecular weight of the polymer in a heated inert gas, the temperature of which never reaches the melting point of the polymer. 

Fig. 12. Typical polymerisation flow diagram for the continuous polymerisation of vinyl acetate (246).

The Ticona materials are prepared by continuous polymerisation in solution using metallocene catalysts and a co-catalyst. The ethylene is dissolved in a solvent which may be the comonomer 2-norbomene itself or another hydrocarbon solvent. The comonomer ratio in the reactor is kept constant by continuous feeding of both monomers. After polymerisation the catalyst is deactivated and separated to give polymers of a low residual ash content and the filtration is followed by several degassing steps with monomers and solvents being recycled. 

During the experimental large scale continuous polymerisation of acrylic acid in aqueous solution in presence of an initiator and a moderator, failure of one of the feed pumps led to an unusually high concentration of monomer in solution. This led to runaway polymerisation which burst a glass vent line and the escaping contents ignited and led to an explosion and fire. 

Variables Affecting Conversion to Polystyrene. The conversion of styrene in dioxan (30% w/w) reaches a maximum at approximately 0.1M H2SO4 (Table 1). Beyond this acidity, the yield of hompolymer is not significantly increased under the radiation conditions used. At acid concentrations in excess of 0.7M there is a fall off in yield of polystyrene which may be attributed to the lower solubility of polystyrene at higher acid levels so that continued polymerisation is hampered by increasing immiscibility. The data in the same table show that the effect of water on the homopolymerisation is different from that of acid. The polymer conversion is also favored at the lower monomer concentrations (Table II) and is enhanced by the presence of acid at each monomer concentration studied. The conversion is also directly proportional to the total radiation dose (Table III) and inversely proportional to the dose rate (Table IV). The addition of acid in each case enhanced the resulting yields of homopolymer. ... 

The active centre continues polymerisation (growth of the chain), opening the following molecules of the cycles ... 

The continuous polymerisation of dimethylcyclosiloxanes is carried out in a screw apparatus (Fig. 60), which consists of three parts. Lower, horizontal part 3 has a blade agitator and a jacket used to maintain a temperature of 80-100 °C in the reaction zone. Middle, vertical part 2 is hollow and also has a jacket to be heated with hot water. Top, horizontal part 1 has a blade screw and two jackets one is filled with hot water (in the direction of the polymer flow), the other is filled with cold water (to cool the polymer before unloading). Horizontal part 3 is continuously filled with a blend of dimethylcyclosiloxanes and catalyst (6-7.5% of the quantity of the original dimethylcyclosiloxanes). 

Living polymerisations are processes that are virtually free of chain transfers and termination reactions. They permit the synthesis of homopolymers with controlled molecular weights, narrow polydispersities and well-defined terminal functionalities and also the synthesis of well-defined block copolymers. Living polymerisations proceed until all of the monomer has been consumed and further additions of monomer result in continued polymerisation. 

Batch polymerisations are often performed in screening experiments on the laboratory-scale level. However, batch polymerisations are used less often in large-scale, commercial produchon processes than semi-continuous polymerisations because of the inherent limitations in heat transfer and copolymer composition control. 

There are a wide variety of acryhc monomers available, each having specific properties. Styrene is often used as a comonomer in acrylic latexes because of its compatibility and wide availability. Auxiliary monomers can be used in small amounts to impart special properties to toe latex. Although acrylic monomers and styrene may be similar in reactivity, if toe monomer solubilities are significantly different, copolymerisation is less likely, possibly resulting in structured particle morphologies. The copolymer composition can be made more uniform by semi-continuous polymerisation. In some cases, a stmctured morphology is desired, and can be designed into toe process (225, 372). 

The effect of the drying process parameters (amount of latex fed in, air pressure for spraying the latex, consumption of heat transfer agent, temperature of heat transfer agent, latex concentration and surface tension of the latex) on the particle size of PVC latex obtained by continuous polymerisation of vinyl chloride using 2-3% sodium alkylmonosulphonate is studied. Articles from this journal can be requested for translation by subscribers to the Rapra produced International Polymer Science and Technology. 

Other polymerisation reactor designs include agitation without rotation in order to enhance heat transfer into and out of the reactants (Goebel, 1977). An agitator was used in the continuous polymerisation reactor proposed by Phillips Petroleum (Witt, 1986). This is claimed to have a significant positive effect upon mixing, reducing residue formation. 

Polyamide 6 can be produced both by batch or continuous polymerisation. Batch polymerisation is preferably used for changing polymer formulations with a wide range of molecular weight, mostly compounding grades. Continuous polymerisation reactors - VK columns (Vereinfacht Kontinuierlich) - have a lower range of product mix but higher productivity and are, i.e. used for the production of textile or industrial fibres. Continuous processes can be operated with one or two reactors in series. 

Both one-phase and two-phase polymerisation systems lend themselves to continuous polymerisation processes in which all the reactants are fed to the process continually and polymer is removed continually. Continuous processes are particularly useful for the manufacture of high volume products and, although initial capitalisation can be more expensive, operating costs are reduced in comparison to batch or semi-batch processes. 

Idemitsu Petrochemicals has developed a new production method based on continuous polymerisation and is now gearing up for the construction of a 10,000 tpa plant in Asia in 2003. 

Other types of surfactants are the polymeric (steric) stabilisers, such as partially hydrolysed polyvinyl acetate. Also oligomeric species formed in situ, when SO radicals react with some monomer units in the aqueous phase, will have surface active properties, and can even form a colloidally stable latex Electrosteric stabilisers combine steric and electrostatic functionalities for example, inclusion of acrylic acid in a recipe results in chains with blocks comprised largely of poly(acrylic acid) which for in the aqueous phase, then pick up enough hydrophobic monomer to enter the particle and continue polymerisation in the particle interior. The hydrophilic component remains in the aqueous phase and provides colloidal stability both sterically and, imder the appropriate conditions of pH, electrostatically. This mode of stabilisation is very common in surface coatings, because it gives excellent freeze-thaw stability. 

---