Лако-красочные материалы — производство

The process of film formation is a crucial factor in achieving optimum performance from a water based emulsion.

Ideally, the polymeric, spherical particles should coalesce to form a fully coherent film, free from imperfections.

This coalescing process has been thoroughly studied by many workers over the years. Techniques such as scanning electron microscopes (SEM), small angle neutron scattering*1′ and luminescence spectroscopy*7′ have facilitated a greater understanding of the process.

The film forming process is very different from a solvent soluble resin and is much more difficult to control. From polymer solutions, the polymer chains are already present in a fully interpenetrated network with the molecules of solvent fully solvating the polymer species. On drying, the solvent merely evaporates, leaving the fully interpenetrated network of polymer chain as a concentrated uniform layer.

The particle coalescing process can be affected by a number of factors:

• particle size

• temperature of drying

• co-solvent addition

• reactivity/functionality

• glass transition temperatures (Tg) and minimum film forming temperature (MFFT)

The polymer particles dry down to a fully interpenetrated layer only if the MFFT is below the temperature of drying.

If, on the other hand, the MFFT is above the temperature of drying, then some other influence must be introduced. The most common influence is the use of a coalescent, i. e. a solvent for the polymer particles.

A diagrammatic representation of the coalescing process is shown in Figure 2-11.

(a)

Figure 2-11

In surface coating applications, an idealised polymer composition would be one in which the polymer exhibited a relatively high Tg but low MFFT. This combination is almost impossible to achieve at the present time, and other physical methods are used to achieve the best compromise.

Coalescing solvents are used to solvate the polymer particles to a degree where they adhere to one another and flow out to a continuous film. Coalescents are high boiling when compared with water, and evaporate from the film at a much slower rate than water.

Glycols such as ethylene and propylene are added to improve freeze-thaw stability, and rheological properties.

In addition they also help in forming a continuous film. Solvents such as ethylene glycol monobutyl ether acetate are frequently used to improve film forming properties.

The essential difference between a typical waterborne polymer and a solvent based system used, for example, as a decorative DIY paint, is the convertible nature of the resins used in solvent based coatings when compared with non-convertible water based emulsions.

Typical solvent based resins used for this type of coating are alkyd resins, which contain oxidisable unsaturation. During the drying process, the solvent evaporates and the unsaturation oxidises using atmospheric oxygen to cure and crosslink the film. This leads to solvent insolubility, film integrity and durability.

Conventional water based emulsions used for this market do not contain convertible groups. Chemistry does, however, exist which allows the incorporation of speciality functional monomers, which can be encouraged to post react during the drying process, either by heat application or the post addition of a suitable catalyst.

Such monomers were described earlier.