The Hole Truth - and nothing but the Hole

Microporous coating architecture has enjoyed a respected place in the pantheon of coating philosophies, particularly in recent generations. The ability of a membrane to transmit species of one kind and not to transmit species of another is a well-known technique in nature. The extension of the principle (and the understanding of the principle) to surface coatings took a long time in gestation. However, the measurement of the microporous nature of coatings has not been a well-developed science, and the measurement of microporosity and the correlation of this property to performance has not yet been fully accepted by coating scientists.

That situation is about to change. The first in a short series of international standards has been published1 Other parts of the standard are still in preparation by the Technical Committee to whom the work has been entrusted.2,3 The published standard is of general applicability, and derives from academic work published 50 years ago.4,5 The types of material which can be addressed by the method are varied, ranging from controlled drug release, through environmental protection, to polymers and ceramics. However, the standard notes that materials can take other forms including extrudates and sheets as well as monoliths and compacts, so, along with the academic basis of the method, it seems likely that the testing regime will be appropriate for surface coatings.

In essence, the method involves submitting a test specimen to extreme pressures (up to 400Mpa) in the presence of mercury. The uptake of mercury liquid into the test specimen is measured through a sensitive apparatus as the pressure is increased. As the pressure is increased, mercury is forced into the pores in the material. This is not a linear process, and will depend on factors such as the surface tension between mercury and the test specimen. Other correction factors include the compressibility of mercury and the specimen. However, in general, the uptake of mercury will be very slow initially as the pressure increases. At a point determined by pore dimensions and geometry, the mercury will be forced into the pores, causing a large increase in the mercury volume uptake, with a relatively small increase in the applied pressure. Once the pores have been filled, very little further mercury can be taken up, no matter how high the pressure.

It seems clear that while the method has academic credentials, it is best thought of as a comparator method. The method is clearly unsuitable for any material capable of forming an amalgam, or which has mechanical properties affected by contact with mercury. Many of the more obvious correction factors are duly noted in the standard; pre-treatment of the sample to remove included gases, as well as thermal analysis of the test material are highlighted as areas which could repay initial study. Adiabetic heating of the test chamber and specimen needs to be considered and guarded against - because the voids are initially filled with a gas, the application of even low pressure will cause an apparent uptake of mercury. Isothermal operation renders this error linear and predictable. Changes in the pore geometry, particularly of thermoplastic or otherwise deformable materials, need to be guarded against when the sample is degassed by evacuation prior to testing, and during the testing cycle. Although the standard text permits measurement of the mercury volume introduced to the specimen during pressure decreases as well as pressure increases, this procedure gives rise to caveats, notably through the possibilities of an unclosed hysteresis loop in included volumes. No reproducibility or precision data are published.

The text also recommends that a blank determination is carried out, using as a test specimen a material known to be free of pores, and with thermal properties similar to those on which the porosity data is being sought.

The mathematics, for the calculation of the pore diameter using the Washburn equation, and the calculation of the specific surface area, assumes a cylindrical pore. However, valuable qualitative information may also arise from pores of the 'ink bottle' variety, as well as pores of a non-cylindrical or even open-ended habit. Use of the pore dimensions calculated from this test do not necessarily correlate from those arising from gas absorption methods.

Conclusion

This standard could represent a significant step in understanding film behaviour, particularly where films predicted to contain pores are examined. On the other hand, the method demands robust and time-critical ways of capturing data. In addition, the method cannot be left in the hands of untrained junior members of staff; interpretation of the results is still a high-level activity. The standard also does not specifically include paints as likely candidate materials for the test.

Nevertheless, as a candidate for inclusion in the battery of analytical and predictive tests, this method has value. It is just a bit unfortunate that paint technology has had to wait 50 years for the academic work to be acknowledged by industry.

References

1. ISO 15901-1:2005 Evaluation of pore size distribution and porosimetry of solid materials by mercury porosimetry and gas absorption. Part 1. Mercury porosity[Return ]

2. ISO 15901-2 (in preparation) Evaluation of pore size distribution and porosimetry of solid materials by mercury porosimetry and gas absorption. Part 2. Analysis of mesopores and macropores by gas absorption[Return ]

3. ISO 15901-3 (in preparation) Evaluation of pore size distribution and porosimetry of solid materials by mercury porosimetry and gas absorption. Part 3. Analysis of micropores by gas absorption[Return ]

4. Ritter HL and Drake LC Pore size distribution in porous materials. Pressure porosimetry and determination of complete macropore size distributions. Ind Eng Chem Anal ed, 17, (1945) pp 782-786[Return ]

5. Ritter HL and Drake LC Pore size distribution in porous materials. Macropore size distributions in some typical porous substances. Ind Eng Chem Anal ed, 17, (1945) pp 787-791[Return ]

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