Analysis of water in waterborne coatings.

ANALYSIS OF WATER IN WATER-BORNE COATINGS

Q: What is the most reliable method for measuring the water content of a water-borne paint or coating?

The issue here is that VOC regulations quote VOC limits as VOC lbs/gal, less water and exempt compounds. Whereas to calculate the emissions of solvents from a paint spraybooth operation you need to know the actual VOC of the mixed coating.

If you consider the two figures below you will see that the gallon can on the left contains water, VOC and coating, whereas the one on the right contains only VOC and coating. Therefore, Figure 1 represents the gallon of paint as mixed, and the VOC content of this can is used for the spraybooth emissions calculations. Figure 2, which is the same as Figure 1 except that the water has been excluded and the contents of Figure 1 have been expanded to fill a one gallon can, is used to calculate compliance in terms of VOC lbs/gal, less water.

The calculation for compliance (Figure 2) is very sensitive to the amount of water in the coating, and a reliable method is required to analytically measure this quantity.

Figure 2
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We spoke to a chemist at the South Coast Air Quality Management District (SCAQMD) who advised us that the agency follows their own in-house Method #304, which is the Karl Fisher Method. This method references ASTM Methods D-1475 - Density, D-2369 - Non Volatiles, D-4017 Karl Fisher, D-3792 Gas Chromatography. Although their method references the GC, the agency only uses the Karl Fischer Method.

In addition, they have set their own QC limit on the method. A single technician performs the test several times on the same day. The average is calculated as well as the spread of high value - low value. The relative repeatability is calculated by: (High value - Low value)/Average. The results are acceptable if the relative repeatability is <3.5%.

For instance, if the mean evaluation for the water content by Karl Fischer = 60.0% by weight of the coating; the high value is 62.0 and the low is 59.0, then the relative repeatability is (62.0-59.0)/60.0 = 5.0% and the analysis would be invalidated.

On the other hand, the results 59.5% (low) and 61.6% (high) would yield a relative repeatability of 3.5%, and the analyses would be acceptable.

How does this play out in calculating the VOC, (less water) of the water-borne coating, as required by the EPA and all state agencies for the determination of regulatory compliance?

Suppose the solids content of the coating as measured by ASTM D-2369 yielded a result of 29.6% by weight, then the VOC for the average water analyses would be (100 - 60.0 - 29.6) =10.4% by weight. Further, suppose that the density of the coating (weight per gallon - WPG) is 9.8 lbs/gal and the density of water is 8.3 lbs/gal, the VOC content, (less water) of the water-borne coating would be given per Table 1. The equation for calculating the VOC content, (less water) is as follows:

VOC lbs/gal (less water) = %Wt. VOC * WPG (lbs/gal)

(1 - { %Wt. Water*WPG lbs/gal})

8.3 lbs/gal

Table 1 - Results of VOC Content lbs/gal, (less water)

VOC lbs/gal, less water for 59.5% determination = 3.6 lb/gal

VOC lbs/gal, less water for 60.0% determination = 3.5 lbs/gal

VOC lbs/gal, less water for 61.6% determination = 3.2 lbs/gal

In other words, the first analysis would deem the coating to be out of compliance, the second would just be in compliance, and the third would be non-compliant

While the relative repeatability for the analysis of the water content of the coating was 3.5%, the relative difference of the VOC content, (less water) is considerably higher; namely (3.6-3.2)/3.5 = 11.4%

The example given above illustrates why the relative repeatability of the water content analysis is so critical to determining if a water-borne coating is compliant.

Calculations: VOC, HAP, Exempt Compounds