5 июля, 2015 
Malyar The “Drip-feed” (or continuous addition) process involves feeding monomer and initiator, separately, into solvent at reaction temperature. Ideally, as the monomer and initiator come into contact with the hot solvent, free radicals are formed which immediately initiate polymerisation of the monomer. The polymer formed is dissolved in the solvent and dispersed.
In practice, reaction does not occur immediately the monomer contacts the hot surface, and initially there is a slight build up of unreacted monomer. However, this can be minimised and maintained at a low concentration, by correct choice of reaction conditions, solvent and initiator.
At any time during the polymerisation, the relative amounts of monomer, initiator, modifier and solvent are all reasonably constant. This favours the formation of polymers with a narrow molecular weight distribution.
The feed rate is a critical factor in restricting the build-up of unreacted monomer, which otherwise may lead to an uncontrolled reaction and wide molecular weight distribution. It is normal to premix the monomers, and any chain transfer agent or modifier, and add the ‘premix’ to the reaction vessel as a separate feed to the initiator, although some processes employ a premix which contains initiator.
This latter technique is not generally recommended for safety reasons and great care is required to ensure that premature polymerisation does not occur in the premix vessel. The use of a separate initiator feed allows the use of a wider choice of initiator since premix stability in the presence of the initiator is no longer a factor to be considered by the formulator.
By controlling the relative rates of the monomer and the initiator feeds, it is possible to influence the molecular weight and molecular weight distribution. This is known as the ‘starved feed’ technique. However, care must be exercised to ensure there is no build up of unreacted monomer.
The use of the drip feed process allows more latitude in the preparation of constant composition copolymers. By constantly varying the composition of the monomer feed during the addition, differences in reactivity between monomer types can be partly overcome to produce copolymers of more consistent composition.
In practice, a constant composition copolymer is not required for use in surface coating applications. Since most acrylic polymers are manufactured by the batch process, the overall composition of the final solution is constant, and it is of little consequence that individual polymer chains may not have identical monomer composition.
The differences between a wide and a narrow molecular weight distribution is generally evident in solution rheology and film performance characteristics. In general, polydispersed systems are more flexible and have lower solution viscosities. This is because the low molecular weight fractions generally exert a greater overall influence on the properties of a polydispersed polymer than the high molecular weight fractions.
However, since these properties are also influenced by factors such as copolymer composition, solvent type and overall molecular weight, solution polymers made by both the all-in and drip-feed processes find use in surface coating applications. The latter process allows stricter control to be exercised over the process conditions, and this is often desirable when dealing with highly exothermic chemical reactions and where product consistency is required.
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