Handbook of Solvents - George Wypych - ChemTech - Ventech!

14.19.2 Recent advances in coalescing solvents 971
Figure 14.19.2.1. Latex in contact with substrate.
Figure 14.19.2.2. Initial water evaporation
and film formation.
Figure 14.19.2.3. Close packing of latex particles.
Figure 14.19.2.4. Coalescence to form a
polymeric film.
As the water continues to evaporate, the particles
become close packed with a solids volume of
around 70%. Capillary forces continue to force the
particles together.
The final stage is achieved when most of the
water is lost from the system. Here, the
interparticular repulsive forces are overcome by increasing
surface tension and the particles coalesce
into a discrete film. This will only occur at temperatures
in excess of the MFFT.
14.19.2.4 Role of a coalescing solvent
Most coatings based on emulsion polymers are used
in environments where they will be expected to form
a coherent film at temperatures as low as 0°C. However,
other physical properties besides film forming
capability are required from the polymer. These include;
abrasion resistance, hardness, chemical resistance,
impact performance, etc. These can often be
impossible to achieve with a polymer of low T g. The
polymers, which most clearly meet these criteria are
acrylics or copolymers of vinyl acetate or styrene.
These would, without additions of coalescing species,
be brittle, forming incoherent films with little
adhesion to the substrate at normal application temperatures.
Coalescing solvents allow these polymeric systems
to form films at ambient or sub-ambient temperatures.
The presence of the coalescing solvent has
the following effects.
• It reduces the total surface energy of the system
by reducing polymer surface area.
• It increases the capillary forces by the controlled
evaporation of the water.
• It reduces the repulsive forces between the
polymeric particles.
• It allows deformation of the particles in contact
with each other by effectively lowering the T g of
the polymer.
An emulsion polymer consists of a dispersion
of polymeric particles varying in size from 0.05-1
μm, dispersed in an aqueous environment. The coalescing
solvent may be found in several different locations
depending on their nature. They are
classified according to their preferred location in the
aqueous system.