Introduction: Solvents — the “Invisible Skeleton” of Coatings: They Evaporate Yet Determine Coating Quality
Solvent accounts for 20-60% (by weight) in liquid coatings
——After the coating is cured, it all volatilizes (>99%)
——It leaves no “trace” in the final coating, but the solvent’s type/evaporation rate/solvency/surface tension
——have a “ghostly” decisive influence on the coating’s application properties (viscosity/atomization/leveling)——storage stability (resin precipitation/phase separation)——coating appearance (orange peel/gloss/pinholes)
——“Solvent is the ‘ghost’ of coatings—it evaporates—but it determines the lifetime of the coating”
. Scientifically selecting the solvent system—based on Hansen Solubility Parameters (to ensure complete resin dissolution—stable solution—no precipitation), evaporation rate gradient (fast→slow—to ensure sufficient leveling time for the coating—and final rapid drying), and a comprehensive consideration of cost/safety/environmental protection—is a “required course” for coating formulation engineers.
I. Hansen Parameters and Evaporation Rates of the Five Major Solvent Categories
| Solvent Type | Representative Solvents | δD | δP | δH | RER | TLV(ppm) | Flash Point(°C) |
|---|---|---|---|---|---|---|---|
| Hydrocarbons | Xylene (mixed)/Solvent gasoline No. 200 | 17.8/15.5 | 1.0/0 | 3.1/0 | 0.7/0.1 (slow drying) | 100/300 | 27/40 |
| Alcohols | n-Butanol/Isopropanol IPA | 16.0/15.8 | 5.7/6.1 | 15.8/16.4 | 0.5/2.0 (medium) | 20/200 | 35/12 |
| Ketones | Acetone/MEK/Cyclohexanone | 15.5/16.0/17.8 | 10.4/9.0/6.3 | 7.0/5.1/5.1 | >5/3.8/0.3 (slow) | 500/200/20 | -18/-6/44 |
| Esters | Butyl acetate (BuAc)/Ethyl acetate (EtAc) | 15.8/15.8 | 3.7/5.3 | 6.3/7.2 | 1.0 (baseline)/4.0 | 150/400 | 22/-4 |
| Ether alcohols | Ethylene glycol monobutyl ether (BC/Butyl Cellosolve)/Propylene glycol methyl ether acetate (PMA) | 16.0/15.6 | 5.1/5.6 | 12.3/9.7 | 0.08/0.3 (slow drying) | 20/50 | 67/42 |
FAQ
Q1: How to scientifically combine “true solvent” and “diluent”?
True Solvent——can dissolve resin alone
(resin in this solvent——HSP distance Ra<RED×R₀——completely dissolved——transparent solution——no particles/microgels). Diluent cannot dissolve resin alone
——but in the presence of true solvent——can reduce system viscosity + lower cost + adjust evaporation rate
——no adverse effects (will not cause resin precipitation). For example——epoxy resin——ketones (MEK——true solvent) + hydrocarbons (xylene——diluent)——MEK dissolves epoxy——xylene reduces viscosity——their HSP are compatible——mixed system——epoxy resin stable without precipitation. Excessive diluent (>70% of mixed solvent)——may push the overall HSP of the system beyond the resin’s dissolution R₀——resin suddenly precipitates (high molecular weight fraction precipitates first——solution becomes “turbid” and gels)
.
Q2: What is the “blush prevention” mechanism of blush retarder (ethylene glycol butyl ether/BC)?
High humidity environment (RH>85%)——solvent evaporates rapidly during spraying——coating surface temperature drops sharply below the dew point
——water vapor in the air condenses on the coating surface——forming micro water droplets
——micro water droplets scatter light——coating “turns white” (Blushing). Blush retarder (BC/ethylene glycol butyl ether)——(1) evaporates extremely slowly (RER several hours——high boiling point); (2) BC and water are completely miscible
(BC’s Hansen δH=12.3——close to that of water)——micro water droplets condensed on the coating surface——are “absorbed” by BC and mixed with BC——no scattering water droplets form——coating remains transparent——this is the physicochemical essence of “blush prevention”.
Q3: Why is the ratio of “fast-drying/medium-drying/slow-drying” in the solvent formulation the key to the “gradient design” of RER (Relative Evaporation Rate)?
After spraying, the >wet film——(1)Fast-drying solvent (RER>3)——evaporates first——wet film volume shrinks——resin concentration rises——viscosity rises——leveling stops
; (2) Medium-drying solvent (RER 0.5-2)——continues to evaporate——coating further cures; (3) Slow-drying solvent (RERseveral hoursprevents “sealing” inside the coating——tiny residue of slow-drying solvent——helps the coating’s “post-leveling” to finally form a smooth surface
. The ratio of fast-drying/medium-drying/slow-drying——needs to be based on the resin’s Tg/drying conditions (room temperature/baking)——precisely controlled——improper ratio——too much fast-drying——orange peel; too much slow-drying——sagging; insufficient medium-drying——uneven gloss
.
Related Reading
Summary
Coating solvents fall into five major categories—hydrocarbons/alcohols/ketones/esters/ether-alcohols—and ensure resin dissolution and solution stability via Hansen solubility parameters (δD/δP/δH—Ra < RED × R₀), while controlling film leveling and drying through RER (relative evaporation rate—fast-to-medium-to-slow drying gradient). A rational blend of true solvents and diluents balances solvency and cost. Kexin New Materials provides customers with full-spectrum solvent system optimization and VOC compliance technical support.