Лако-красочные материалы — производство

Acrylic resins for use in industrial surface coating applications are not normally man­ufactured by this technique. However, the high molecular weights that can be obtained, coupled with the freedom of choice of solvent in the application formulation, result in suspension polymerised acrylic resins finding uses in areas such as solvent based inks.

The technique is a simple but effective one, although in practice, obtaining stable particles within the required size range, can prove demanding of both formulator and manufacturing equipment. Initiator is dissolved in monomer, and the solution dispersed in water to form discrete droplets. On heating, polymerisation occurs within the droplet resulting in the formation of “beads” or “pearls” of polymer.

In effect, the process is a bulk polymerisation in which small droplets of monomer are polymerised in an environment where they are surrounded by water. The water acts as a “heat sink” to absorb the heat of reaction, whilst the relatively low viscosity of the dispersion allows effective agitation. Hence, efficient heat transfer and cooling can be achieved.

In practice, commercial suspension polymerisation is brought about as follows:

A monomer phase, consisting of a mixture of monomer, monomer soluble initiator and monomer soluble modifiers, is suspended in an aqueous phase. It is essential that the initiator and any modifier used (chain transfer agent etc) are completely soluble in the monomers used.

As the name implies, the aqueous phase is composed largely of water. However, also present are dispersing agents (such as gelatine and polyvinyl alcohol), and various salts (such as sodium carbonate and di-ammonium hydrogen phosphate) which can act to increase the interfacial tension between the aqueous and monomer phases, hence preventing coalescence of the monomer droplets during polymerisation.

Initiation occurs within each monomer droplet and propagation proceeds (termination and modifying reactions occurring as previously described) until all the monomer is consumed (or until there is no free radical or free radical generating species remaining).

The agitation, generated by the reactor stirrer, has a major influence on the product formed. Not only does the agitation act to keep the polymer beads suspended in the aqueous phase, the shear induced by the stirrer also acts to control the bead size. A fast stirring rate will result in the monomer drops being “shattered” into smaller droplets, and hence resulting in the formation of smaller polymer beads, whilst a slow stirring rate results in the monomer droplets coalescing to form large beads.

When the reaction is completed, agitation is continued, whilst the reaction product is discharged to holding tanks, where, in the absence of agitation, the polymer beads are allowed to settle out of the aqueous phase. The beads are washed free of contaminating salts and suspending agents, and are dried and bagged. If the polymer is required as a solution, then dissolving the bead in solvent would be carried out as a second and separate operation following the manufacture of the bead.

The suspension polymerisation technique places certain restrictions on the choice of initiator. The mechanism involves initiation, propagation and termination within the monomer droplet, thus the initiator used must be monomer soluble, and water insoluble. Furthermore, the polymerisation temperatures are, of necessity, below the boiling point of the aqueous phase, and for practical reasons commercial polymerisations are carried out in the range 60-85°C.

The relatively low temperatures employed, coupled with the solubility considerations, mean that a thermal initiator, with a very short half life at temperatures below 80°C is required.

Perpivalates, perdicarbonates, peroctoates and azo bis isobutyryl nitryl are a few of the more commonly encountered suspension polymerisation initiators.

When 2,2’azo di-isobutyronitrile (AZDN) or 2,2’azo bis-isobutyronitrile (AIBN) as it is known in the USA (they are the same initiator) is used, there is die possibility of nitrogen from the initiator decomposition becoming trapped inside the bead giving rise to hollow beads. Whilst hollow beads are not deleterious in terms of the polymer application, the reduction in density experienced with hollow beads leaves them liable to float in the settling tanks, resulting in practical difficulties during the downstream stages of manufacture.

Suspension polymerised acrylics are not widely used in surface coating applications, although there is a large specialised use area in printing ink formulations which are intolerant to the presence of solvent types normally encountered with solution processed acrylics.

The advantages of the suspension polymerisation technique is that the product:

a) has a very narrow molecular weight distribution.

b) can be produced as a solid, allowing the surface coating formulator a free choice of solvents when compounding the coating.

c) can be produced with both high or low molecular weight as required.

The disadvantages are that:

a) the polymer has to undergo a lengthy washing and drying procedure to ensure that all traces of surfactant and salts from the aqueous phase are removed. These contaminants, even in trace quantities, would substantially affect the performance of the end use product.

b) temperature control of this highly exothermic reaction can only be maintained by utilisation of the aqueous phase as a “heat sink”. This means that a ratio of 35:65 monomer phase:aqueous phase represents the practical limit of the monomer phase concentration. This, coupled with the losses during the washing and drying operations, results in typical product yields of 30% for this type of polymerisation. This results in high manufacturing costs relative to acrylic polymers manufactured by emulsion or solution polymerisation techniques.

c) temperature control and control of the shear due to agitation, must be precise to ensure coalescence of the beads does not occur during the course of the polymerisation.