Лако-красочные материалы — производство

This technique involves the polymerisation of acrylic monomer in the presence of an organic solvent. If this method is to succeed, both monomer and polymer must be soluble in the chosen solvent.

Monomer and initiator are heated in the presence of solvent. The solvent usually comprises 30-40% of the total weight and it may be removed by distillation and spray drying techniques, once polymerisation is complete, although it is more common for the copolymer to be used as a solution in surface coating formulations.

The presence of the solvent serves a number of functions, as discussed below:

a) it reduces the viscosity of the reaction mass allowing effective agitation.

b) it acts as a ‘heat’ sink for the heat of reaction (see calculation earlier). Also, when the polymerisation is carried out at the reflux temperature of the solvent the “latent heat of vaporisation” of the boiling solvent will effectively remove excess heat from the reaction mass, and will act to maintain the temperature at the reflux temperature of the solvent.

c) certain solvents can act as chain transfer agents, and act to control the molecular weight (as discussed in the section dealing with chain termination).

The molecular weight distribution is effected by the precise method employed to prepare the polymer. Two techniques used commercially are the “all-in” process and the “drip feed” process.

The “All-in” (or one shot) process involves charging all the monomer, plus solvent, initiator and modifier, into the reaction vessel and heating to reaction temperature.

As initiation and propagation occurs, the temperature of the reactants is allowed to rise to the reflux temperature of the solvent present. The reactants are held at reflux temperature under total reflux conditions, to condense all vapours and return the liquid to the reaction vessel. The latent heat of vaporisation of solvent and monomer helps to dissipate the heat of reaction. The course of the polymerisation is monitored by the measurement of viscosity and non-volatile content.