Critical Pigment Volume Concentration measurements ... a very fast method.

Because of the importance of knowing the Critical Pigment Volume Concentration (CPVC) for maximum performance of all pigmented coating systems, a large number of methods to measure CPVC have been proposed. All attempts to predict the critical point on theoretical grounds have been unsuccessful. Some tests for CPVC are quite accurate and reliable in showing where the break in the formulating ladder exists. Some require considerable judgment.

All tests for CPVC, unfortunately, require the preparation of the coating compositions, including pigments used, vehicles (both solution and latex type), and their grinds. The composition must then be deposited on some substrate and dried--either by air or bake--before the classical tests can be used. All facets of the coatings manufacture must be included in the test samples. The required sample may be produced relatively fast in the case of baked systems or very slow in the case of some air-dried systems.

All of the classical tests made on these PVC ladders are relatively time consuming and total testing time may require many hours. A new method of testing on the PVC ladder, which is both very fast and requires minimal instrumentation, is proposed here. For the most part, the tests on each sample can be made in less than one minute.

CRITICAL PIGMENT VOLUME CONCENTRATION MEASUREMENTS--A VERY FAST METHOD

Since its introduction over 50 years ago, (1) the concept of Critical Pigment Volume Concentration has been completely confirmed in coatings literature. It influences the behavior of coatings systems directly or indirectly. Performance, coatings application, production, and storage characteristics all are affected by CPVC.

Although the concept was developed for solution-type coatings, it works equally well for dispersion-type (latex) systems. The details of the two systems, however, are different. (2)

The CPVC has been defined (1) as: "the transition point above or below which substantial differences in the behavior and appearance of a paint film are encountered. It is that point in a paint system at which just sufficient binder is present to fill completely the voids left between the pigment particles incorporated in the film after volatilization of thinner or water, respectively. It represents the densest degree of packing of the pigment particles commensurate with their degree of dispersion."

Below the CPVC, the coating is coherent and relatively impermeable. Above the CPVC, the coating becomes progressively more porous.

Thus, most of the desirable properties of coatings systems deteriorate very rapidly above the critical point. Almost all coatings properties such as protective qualities, gloss, blistering, rusting, surface roughness, enamel holdout, hiding, tensile strength, elongation, stain removal, scrubbability, cleanability, dirt absorption, water sorption, electrochemical impedance, DC resistance, and many more change radically at this point. It has been found that many desirable properties are at a maximum at the CPVC.

As a consequence of this situation, measuring any or all of the coatings properties listed above can be used to determine the CPVC of a paint system. All measurements require testing the exact formulation, grind, and drying of a paint ladder spanning the expected value of the CPVC made and applied to a suitable substrate.

This coatings ladder preparation, then, is always the first step for any of these tests. The time required for this phase obviously depends on the methods of preparation and drying of the test coating. This may be relatively short in, for instance, the case of baked coatings, or very long for air-dried systems.

Table 1 lists some of the possible test methods employing the coatings failure characteristics previously listed. Others could be suggested. The order of the listing is arbitrary and includes the relative times required, limitations of substrates, and reliability of the tests to show substantial recognizable differences just below and above the CPVC. The characterization of each testing method is judgmental. Other researchers may differ on the conclusions that are presented.

A NEW TEST METHOD

Special attention is drawn to the Saline DC conductivity listing (number 15). The method was developed for use at a large coatings manufacturer. It is extremely fast compared with all of the other methods listed. It should take less than one minute to test each of the samples of the previously prepared PVC ladder. Furthermore, the testing method is nondestructive.

The method requires a conductive substrate. Any conductive, fairly smooth, solid surface can be employed. This would encompass all of the metals, including iron, aluminum, copper, zinc plate, tin plate, and others. Inherently conductive coatings cannot be measured. The equipment required is remarkably simple and consists of a small container such as a petri dish to hold a mild saline solution, a small piece of porous paper, and a small, ordinary analog multi-meter having resistance measuring capabilities. The time required for a measurement, as already mentioned, is minimal. It should take 45 seconds or less per sample for someone with minimal experience.

The exact procedure is as follows: (1) A piece of household paper towel about four inches square is folded four times into an approximately one-inch square "pad." (2) The pad is soaked briefly in a saline solution of about 50 ml of water containing about one to two grams of salt. A little more or less of the water or salt is acceptable. Also, other materials can be used for the pad, such as small sponges.

However, the pad must be sufficiently wet to saturate the paint coating but not so wet that the saline solution runs onto other test areas of the ladder.

The soaked "pad" electrode is laid onto the surface of the paint sample to be tested. The meter reading resistance circuit is then closed by touching one probe to the pad and the other to some uninsulated portion of the sample substrate. This could be, for instance, the uncoated back of the substrate. If this is not available, a scratch can be made through the coating to obtain a metallic contact. Although it is unimportant whether the positive or negative electrode is touched to the pad, slightly better excursions on the meter needle are found if the negative electrode is used. This is due to the enhanced migration of the salt ions through a porous coating under test as a result of the small potential applied.

The "pad" electrode area can be enhanced by placing a small, conductive object, like a copper penny, on it. However, this does not change the meter readings substantially. Also, clip type electrode probes can be used to hold and manipulate the pad.

The cohesive coating below the CPVC will show very high resistance and the meter needle will barely move. The 1K (1000) ohm setting on the meter is the preferred range. Above the CPVC, the reading will show large excursion of the meter indicating resistances in the order of 5 to 10 ohms. This, of course, shows that the coating is porous.

If for some reason the return electrode cannot be touched to a conductive portion of the substrate--for instance, where the sample is very large or a scratch cannot be tolerated--a second pad can be used to complete the circuit. However, attention must be paid to not allow the saline solution from the two pads to touch. This would indicate a porous coating when indeed it may not be.

CONCLUSIONS

The classical methods for measuring CPVC are discussed. Advantages and disadvantages are described as well as estimates of relative testing times.

A new, very fast, and simple method is described to measure the Critical Pigment Volume Concentration on a PVC ladder of the coating applied to a conductive substrate.

Table 1 -- Possible Test Methods

    Test Method                Substrate Limitations     Equipment

 1  Permeability                   Permeable          Humidity cabinet
 2  Rusting                        Iron               Salt cabinet
 3  Blistering                     Any                Humidity cabinet
 4  Gloss                          Any                Gloss meter
 5  Surface roughness              Any                Profile meter
 6  Enamel holdout                 Any                Gloss meter
 7  Hiding                         Any                Gloss meter
 8  Tensile strength               Pealable           Tensile tester
 9  Elongation                     Pealable           Tensile tester
10  Stain removal                  Any                Scrub tester
11  Scrubbability                  Any                Scrub tester
12  Cleanability                   Any                Scrub tester
13  Dirt absorption                Any                Scrub tester
14  Electrochemical impedance      Permeable          Impedance bridge
15  Saline DC conductivity         Conductive         Volt/ohm meter

    Test Method                Relative Time  Reliability

 1  Permeability                  Long           High
 2  Rusting                       Long           High
 3  Blistering                    Long           High
 4  Gloss                         Medium         Low
 5  Surface roughness             Medium         Low
 6  Enamel holdout                Medium         Low
 7  Hiding                        Medium         Low
 8  Tensile strength              Long           High
 9  Elongation                    Long           Low
10  Stain removal                 Long           Low
11  Scrubbability                 Long           Low
12  Cleanability                  Long           Low
13  Dirt absorption               Long           Low
14  Electrochemical impedance     Long           High
15  Saline DC conductivity        Short          High

References

(1) Asbeck, W.K. and Van Loo, M., Ind. Eng. Chem., 41, 1470 (1949).

(2) Asbeck, W.K., "Dispersion and Agglomeration: Effects on Coatings Performance," JOURNAL OF COATINGS TECHNOLOGY, 49, No. 635, 59 (1977).

by Walter K. Asbeck

Coatings Consultant