Handbook of Solvents - George Wypych - ChemTech - Ventech!

972 David Randall
Hydrophobic substances such as hydrocarbons will prefer to be within the polymer
particle; these are described as A Group coalescents. These tend to be inefficient coalescing
agents. Molecules, which are more hydrophilic than hydrocarbons tend to be preferentially
sited at the particle - aqueous interface, along with the surfactant system. These are described
as AB Group coalescents. These exhibit the best efficiency as coalescing solvents.
In the third group are the hydrophobic glycol ethers and similar species. These exhibit
good coalescing power, but are partitioned between the polymer particle, the boundary
layer, and the aqueous phase. More of these are required than AB Group coalescents, they
are called ABC Group coalescents. Finally, hydrophilic species such as glycols and the
more polar glycol ethers are inefficient coalescing agents, and are more commonly used as
freeze thaw stabilizers. These are described as C Group coalescents.
14.19.2.5 Properties of coalescing agents
14.19.2.5.1 Hydrolytic stability
A coalescing agent should have good hydrolytic stability (a high degree of resistance to hydrolysis)
so that it can be used successfully in both low and high pH latex systems.
14.19.2.5.2 Water solubility
It is desirable for a coalescing aid to have low water miscibility for the following reasons:
• When added to a coating formulation, a coalescing aid with low water miscibility
partitions into the polymer phase and softens the polymer; this improves pigment
binding and polymer fusion.
• There is less tendency for the evaporation of the coalescing aid to be accelerated by
evaporation of the water during the early stages of drying.
• The early water resistance of the coating is not adversely affected.
• When a latex coating is applied to a porous substrate, water immiscible coalescing
aids are not lost into the porous substrate along with the water; i.e., more coalescing
aid will be available to coalesce the coating.
14.19.2.5.3 Freezing point
The freezing point of a coalescing agent should be low (below -20°C) as materials with a
high freezing point may require specialized (therefore more expensive) handling techniques
in transport and storage.
14.19.2.5.4 Evaporation rate
The evaporation rate of a coalescing aid should be slow enough to ensure good film formation
of the emulsion coating under a wide range of humidity and temperature conditions;
however, it should be fast enough to leave the coating
film in a reasonable length of time and not cause
excessive film softness. The evaporation rate of the
coalescing aid should be less than that of water but
not so slow that it remains in the film for an extended
period of time causing dirt pickup.
14.19.2.5.5 Odor
Figure 14.19.2.5. Location of species in aqueous
dispersions.
The odor of a coalescing agent should be minimal.
This is especially important for interior coatings applications.