Handbook of Solvents - George Wypych - ChemTech - Ventech!

970 David Randall
14.19.2.2 Water based coatings
Until recent times, most paints were solvent based. Since many polymers were produced in
solution, it was natural that the coating system, derived from these polymers, would use the
solvent of reaction as the diluent for the polymer. These were almost invariably organic species.
The objections to the use of organic solvents in paint formulations were at first confined
to those with strong views regarding the loss of significant quantities of organic
species to the environment. Latterly, these opinions have been reinforced by the role of
some organic species in damage to the ozone layer, or to the production of smog in the lower
atmosphere. This is as a result of the reaction between organic species present catalyzed by
sunlight and exacerbated by the presence of other pollutants associated with motor vehicles,
etc. Flammability of paints also poses a significant problem in storage and use.
All this tended to reinforce the need to develop aqueous based systems, for which
these problems would be eliminated.
14.19.2.3 Emulsion polymers
Of course, the manufacture of polymers need not be carried out in solution; emulsion polymerization
has a long and honorable tradition in the field of macromolecular chemistry.
This polymerization technique is performed using water-insoluble monomers, which are
caused to form an emulsion with the aqueous phase by the addition of a surfactant. Polymerization
may be effected by the use of a variety of radical generating initiating species, and
the molecular weight of the polymer is controlled by the use of chain transfer agents and the
concentration of initiator employed. The final polymer dispersion is often described as a latex,
named after natural rubber, which is also an emulsion polymer!
This polymerization technique allows for the formation of copolymers in which the
addition of relatively small quantities of comonomer may have a significant effect on the final
properties of the polymer. This is particularly the case with the glass transition temperature,
Tg, of the polymer. This is a physical transition, which occurs in the polymer when the
amorphous structure of that polymer begins to change from a glassy to a rubbery state. At
temperatures below a polymer’s Tg, it will be relatively brittle, and will be unlikely to form a
coherent film.
This effect has a major impact on the use of polymers in aqueous systems. In the case
of a polymer in solution, the presence of the solvent plasticizes the polymer during film formation.
A polymer with a high Tg, i.e., one greater than ambient temperature, can, when in
solution be applied at temperatures below its Tg. In the case of an emulsion polymer, water
is a non-solvent in the system and film formation below the polymer Tg is unlikely. The temperature
at which a coherent film may be formed from a solution or emulsion based system
is known as the Minimum Film Forming Temperature (MFFT or MFT).
Figures 14.19.2.1-14.19.2.4 describe the formation of a discrete film from an emulsion
system.
When the film is applied onto the substrate, discrete polymer particles are dispersed in
the aqueous phase. The system is stabilized by the presence of the surfactant at the water-particle
interface. The particles are spherical, with an average diameter of 0.1-0.2 μm.
As water is lost from the system, the mutual repulsive forces associated with the surfactants
present inhibit the close packing of the particles and a cubic arrangement of the particles
is formed.