Q: The thought below from some of my colleagues is close but not perfect:
“I conclude that either DOI or gloss, the issue is the smoothness of the dried top coat. Which in turn depends on size and shape (of the filler particle) and how much resin is absorbed by each particle.”
What can you share either from an ASTM specification or what the big companies use to follow specific protocol for DOI (distinctness of image)? Any information would be greatly appreciated.
A: Distinctness of image is more than just a gloss measurement. It refers to the crispness or distinctness of a reflected image. Initially, this was assessed visually by projecting a series of different-sized images onto a coated surface. This was accomplished by using a “glow box” that works by passing focused light through a slide containing these images of varying sizes. It’s kind of like a projected eye chart. A little primitive, but it gets the job done.
DOI measurement has since evolved with an instrumental technique. This method is described in ASTM D-5767. Instrumental assessment eliminates operator influence and is therefore more precise and reproducible.
DOI can be affected by both long (i.e., orange peel) and short (i.e., low gloss) wavelength surface texture. It is important to know how DOI is being measured and what the target is. Both low gloss and orange peel will reduce DOI. Hence, large particles of filler in a powder formula that create low gloss will reduce DOI, as will small filler particles with high oil absorption.