Scope

1.1 This test method covers the determination of the percent volume nonvolatile matter of a variety of clear and pigmented coatings. The approach used should provide faster and more accurate results than the use of the liquid displacement technique in Test Method D2697, particularly for coatings that are difficult to wet or that contain voids, cracks or other defects. The improvement in accuracy stems from the superior ability of helium gas under pressure to penetrate very small pores and surface irregularities in dried films. This provides a more accurate determination of void volumes than can be obtained via liquid displacement.

1.2 The technique will provide results under the following constraints:

1.2.1 The stability of the helium gas pycnometer is greater than ±0.005 cm3

1.2.2 Test specimen weights are greater than 1 g.

1.3 The values stated in inch-pound units are to be regarded as standard. The values given in parentheses are mathematical conversions to SI units that are provided for information only and are not considered standard.

3. Summary of Test Method

3.1 This procedure measures the volume of nonvolatile material in a dried or baked coating film. A helium gas pycnometer is used to determine the volume occupied by a film by measuring the reduction of gas capacity in the pycnometer sample chamber caused by the presence of the test specimen. (The actual measurement is accomplished with a pressure transducer that measures the difference in pressure between the empty sample compartment and when loaded. The volume occupied by the coating sample is then calculated from the Ideal Gas Law.) The weight of the specimen is also measured and the two values are used to calculate the dry film density.

3.2 The percent volume nonvolatile content of a coating is calculated using the dry film density, liquid coating density, and the weight percent nonvolatile content of the coating.

Significance and Use

4.1 This test method measures the volume of dry coating obtainable from a given volume of liquid coating. This value is useful for calculating the volatile organic content (VOC) of a coating and could be used to estimate the coverage (square feet of surface covered at a specified dry film thickness per unit volume) obtainable with different coating products.

NOTE 1: In Practice D3960 paragraph 10.3.1, the equation for calculating the VOC content using the percent volume nonvolatile is given. Prior to this method a satisfactory procedure for measuring percent volume nonvolatile did not exist (see Note 11 in Practice D3960).

NOTE 2: Since the actual coverage of a coating includes the void volume and the porosity of the film, the coverage value calculated from this method will be inaccurate by that amount, that is, the actual coverage will be greater. The higher the pigment to binder ratio (P/B) of a coating or the higher content of void containing material (latices, hollow beads, etc.) or both, the greater will be the deviation of the coverage calculation (This is also true to a lesser degree with Test Method D2697).

4.2 For various reasons the volume nonvolatile value obtained for a coating is often not equal to that predicted from simple linear addition of the weights and volumes of the raw materials in a formulation. One reason is that the volume occupied by a solution of resin in solvent may be the same, greater, or less than the total volume of the separate ingredients. Such contraction or expansion of resin solutions is governed by a number of factors, one of which is the extent and direction of spread between solubility parameters of the resin and solvent.

4.3 The spatial configuration of the pigment particles and the degree to which the pigment particles are filled with the binder also affect the volume of a dry coating film. Above the critical pigment volume concentration, the apparent volume of the dry film is significantly greater than theoretical due to the increase in unfilled voids between pigment particles. The use of volume nonvolatile matter values in such instances should be carefully considered as the increased volume is largely due to air trapped in these voids.

4.4 For thin films, the issue of critical pigment volume effects is usually not a concern. With high poly(vinyl chloride) (PVC) films, however, liquid displacement of air voids takes place with difficulty even under high pressures. Helium solves this problem since, as a gas, it readily penetrates and displaces air, water, and volatile solvents even at low pressures. Purging the gas pycnometer flushes these materials from the film.

Apparatus and Reagents

5.1 Gas Pycnometer, equipped with a suitably sized cup.

NOTE 3—The data from the round robin was obtained using a 5-mL cup instrument. 

5.2 Panels, steel or aluminum, 4 in. by 12 in. (102 mm by 305 mm).

5.3 Sheeting, approximately 1.5-mils (38-μm) thick.

5.4 Doctor Blades, 5 to 8 mils (127 to 203 μm), appropriate to give 1.0 to 1.8 mils (25 to 46 μm) dry film thickness). A 3-in. (76-mm) wide, multiple clearance applicator is recommended.

5.5 Standard Spray Equipment, capable of obtaining a uniform film of 1.0 to 1.8 mil (25 to 46 μm) dry film thickness after baking.

5.6 Forced Draft Oven, capable of maintaining 110 6 5°C.

5.7 Single Edge Razor Blades orScalpels,

5.8 Anti-Static Instrument.

5.9 Analytical Balance, capable of weighing to 60.0001 g.

5.10 A Paper/Thin Film Cutter, equipped with a rolling blade, available from most office supply centers.

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