Лако-красочные материалы — производство

This involves heating monomer mixed with initiator. The problems associated with heat transfer through a viscous reaction mass, result in poor temperature control. This means that long retention times are employed to reduce the chance of localised overheating. Bulk polymerisation can be carried out in disposable containers which are completely filled with the reactants, and held at constant temperature in an oven or water bath. When polymerisation is complete, the container is cut away and the product crushed and dissolved in solvent or other medium for use as a surface coating. Alternatively, a continuous or semi-continuous method of polymerisation can be used.

Polymers made in this way have a wide molecular weight distribution. Control of the reaction conditions is minimal, and batch to batch reproducibility is poor. This technique is rarely used for polymers intended for coating applications. Normally they are confined to polymers used in the packaging and moulding industries.

The advantages of the Bulk Polymerisation Technique are:

a) the resultant polymer is very “pure” since catalysts and monomer are normally the only species present.

b) high molecular weight polymers can be formed due to the absence of chain modifying species.

The disadvantages are:

a) high reaction temperatures are encountered, since there is little facility for the removal of the heat of reaction.

b) the viscosity of the reaction mass rises rapidly as the polymerisation proceeds. This gives the result that adequate agitation becomes very difficult, if not impossible. The formation of temperature gradients within the reaction mass takes place, and the rate coefficients of the initiation, propagation and termination reactions are affected. The result is a product with a wide molecular weight distribution.

c) the degree of conversion of monomer to polymer tends to be less than 100% with the resulting free monomer often proving difficult to remove.

d) if the polymer is insoluble in the monomer, it will be precipitated from solution as soon as it is formed, resulting in a two phase reaction mass.

Bulk polymerisation techniques are usually carried out as continuous or semi-continuous processes. Polymerisation is restricted to a low degree of conversion and is stopped while the viscosity of the reaction mass is still low enough to allow effective agitation. The unreacted monomer is removed and recycled.