* Solvent Evaporation: Nail polish (including top coat) is essentially pigments and resins dissolved in solvents. These solvents, like ethyl acetate and butyl acetate, are designed to evaporate quickly when applied to the nail, allowing the polish to dry and harden. However, even when the bottle is closed, a small amount of solvent can still escape over time, especially if the seal isn't perfect or if the bottle is frequently opened and closed. As the solvent evaporates, the remaining solids (pigments and resins) become more concentrated, leading to thickening.
* Air Exposure: Every time you open the top coat bottle, the liquid inside is exposed to air. This accelerates solvent evaporation. Additionally, the air can introduce moisture, which can affect the chemical balance of the formula.
* Contamination:
* Dust and Debris: Small particles of dust, lint, or other debris can get into the bottle, especially from the brush as it is dipped back in. These contaminants add to the solid content and contribute to thickening.
* Nail Polish Backflow: As you apply the top coat, some nail polish from your nails can get onto the brush and then back into the bottle. The different chemical composition of the nail polish can alter the top coat's formula and cause it to thicken.
* Temperature Fluctuations: Extreme temperatures or frequent changes in temperature can affect the stability of the polish formula and potentially accelerate evaporation or chemical changes leading to thickening.
In summary, the most common cause is solvent evaporation. The other factors exacerbate this issue.