Can long-oil and short-oil alkyds replace one another at equal solids?
Surface drying and through-film hardening are separate observations. Oil length, solvent composition, wet film thickness and environment can affect both.
Plot surface drying and subsequent hardness over time
Formulate current and candidate alkyds at equal solids. Control film thickness and drying conditions, then check hardness and appearance at surface dry and at fixed subsequent times. Base approval on the series rather than one touch-dry result.
Why can equal surface-dry times give different next-day hardness?
Follow through-drying and hardness development. Control thickness, ventilation and substrate. Air-drying coatings: With the actual drier package, compare touch-dry time, through-dry time and hardness development. Industrial topcoats and pigmented finishes: On the target substrate, assess gloss, adhesion, color retention and resistance to service fluids.
Checking Oil length and Acid value
Oil length (% by mass): Use the resin-solids basis and the supplier’s stated calculation. Acid value (mg KOH/g): State whether the value refers to resin solids or the as-supplied solution. Viscosity and solids content (mPa·s and % by mass): Specify solvent, temperature and drying method.
What to record
- Oil length
- Film thickness
- Drying environment
- Surface-dry time
- Subsequent hardness
Technical references
Products and processes
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