Pigment to Binder Ratio Calculation

Q. I am a paint chemist. I work in a lab at a coatings company and am currently working on the pigment to binder (p/b) ratio of one our paints. But when I come to the formula to make my calculation, I’m having a little bit of trouble. For my percent ash value the formula reads: % ash value = ((100% -% moisture of sample)/%NV) × 100. The problem I encounter is that I’m unable to figure out the percent moisture of sample. Is there a formula I’m missing? R. H.

 

A. I was a paint chemist once, but I haven’t done those calculations for 40 years. I still feel your pain, though. If I understand your question, the answer is, you must measure the percentage of moisture in your pigment to make your calculation. The formula you are missing is: The weight of the untreated sample - the weight of the dried sample = the weight of moisture lost during drying. Use this weight to calculate the percentage of moisture and use that in your formula.

I also suggest you look up ASTM D 280, Hygroscopic Moisture (and Other Matter Volatile Under the Test Conditions) in Pigments. The D 280 standard gives two methods. Method A is for pigments that do not decompose at 110°C. It involves weighing a sample, driving off the moisture by heating and then re-weighing. Method B is for pigments that decompose at 110°C. It involves weighing, desiccation and re-weighing. From the difference in weights of the treated and untreated samples you can calculate the value for percentage of moisture you need for your formula. I would like to give you more details, but my ASTM books are a bit out-dated. I’m sure you have access to later editions.