Hydrophilic/hydrophobic balance for cosolvent selection.
A major challenge faced by waterborne coatings formulators is the cosolvent choice. The cosolvent can have more influence on early hardness development and early water resistance than the resin, and this is often over-looked. Do you choose hydrophilic or hydrophobic? Is slow or fast evaporating better?
A recent study conducted by colleagues and I shows the effect of cosolvent balance on early water (blush) resistance and hardness development. A mid-range [T.sub.g] acrylic emulsion (40-45[degrees]C) was formulated into clear coatings. The final coating was 175 g/L VOC. The cosolvents used were: ethylene glycol monobutyl ether (EB, a fast hydrophilic); diethylene glycol mono butyl ether (DB, a slow hydrophilic); and dipropylene glycol normal butyl ether (DPnB, a slow hydrophobic). The vapor pressures, relative evaporation rates, and solubility for the cosolvents are shown in Table 1.
Table 1--Cosolvent Properties Cosolvent Vapor Pressure Relative Solubility in (mm Hg @ Evaporation water (g/100 g@ 25[degrees]C) Rate (n-Butyl 25[degrees]C) Acetate = 1) Ethylene 0.080 0.060 [infinity] glycol monobutyl ether (CAS #111-76-2) Diethylene 0.022 0.003 [infinity] glycol mono butyl ether (CAS #112-34-5) Dipropylene 0.045 0.006 4.5 glycol normal butyl ether (CAS #29111-28-2)
The coatings were measured for water resistance and pendulum hardness at 16 hr and pendulum hardness at 90 hr. Sixteen-hour blush resistance results are shown in Figure 1. The faster the evaporation, the better the early blush resistance. Higher hydrophobic cosolvent levels improved early blush resistance.
[FIGURE 1 OMITTED]
Sixteen and 90-hr hardness measurements are shown in Figure 2. Hydrophobic cosolvents had a detrimental effect on hardness development. This could be caused by the polymer-hydrophobic cosolvent affinity slowing evaporation of the solvent. Even at 90 hr, the hardness of the coatings with hydrophobic cosolvents was lower.
[FIGURE 2 OMITTED]
Hydrophobic cosolvents help early water resistance but hurt early hardness development. For hydrophilic cosolvents, the faster the evaporation rate the better both the hardness development and early water resistance. The results from the study are summarized in Table 2.
Table 2--Summary of Cosolvent Properties Cosolvent Type Open Time Early Water Early Hardness Resistance Development Fast hydrophilic - - + + Slow hydrophilic + - - - Fast hydrophobic - + 0 Slow hydrophobic 0 + - ++ Very positive effect + Positive effect 0 No noticeable effect - Detrimental effect--Strong detrimental effect
Mike Praw is senior applications chemist at Alberdingk-Boley, Inc.; email@example.com.
By Mike Praw
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|Title Annotation:||FORMULATOR'S CORNER|
|Date:||May 1, 2010|
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