March 8, 2010

Running the Permeability Tests

by Bob Murphy

Running the Tests

Wet Cup

The majority of products tested n are done via D1653, Wet Cup method. The paints are drawn down at approximately 10 mil wet film thickness on release paper and allowed to dry overnight at 25ºC and 50% RH. A second approximately 10 mil wet film is drawn down over and perpendicular to the first. This 2-layer system is then allowed to dry for an additional week at 25ºC and 50% RH. Layering the paint tends to minimize the number of pinholes or other defects that can artificially raise the water vapor permeability. It also creates a film that is sufficiently thick to handle as a free film, about 5-7 mils dry film thickness. Once dry, the films are examined to locate areas with the least number (preferably zero) defects. The quality of the film can have a large bearing on the final results. A film covered in pinholes or other defects is going to produce an erroneously high perm rating. Pieces large enough to cover the mouth of a 2 ounce glass jar are then cut out of the film. The jars are filled with water and the films are glued to the mouth of the jar using a permanent glue stick. Three samples are prepared for each paint to be tested.

The permeability samples are then placed in the environmental chamber and allowed to equilibriate for at least 24 hours prior to the initial weighing. The whole apparatus is then weighed twice a day for 3 to 4 days. Once weighing is completed, the film is removed from the mouth of the jar and the thickness of the dry film directly above the mouth of the jar is measured with a digital micrometer. The adhesive used to affix the film to the jar produces an regular area, so cross sectional area of the mouth of each jar is used as area of permeance.

Dry Cup

Samples for the dry cup method are typically spray applied over drywall and are tested as received (usually spray applied at 4-6 mils thick and allowed to dry for one week). The drywall is cut to fit over the lip in a Rubbermaid Ice Storage tray. The tray is filled with approximately 1 lb of indicating desiccant (Drierite) and the coating drywall is placed in the tray and sealed in place with caulk. Testing has shown that the caulk used to seal the perm cups is impermeable to water. Two to three test cups for each paint tested are typically prepared. The test cups are then placed in the environmental chamber at 25ºC and 50% RH for a day to equilibrate and allow the caulk to dry/cure. The test cups are then weighed daily for at least 4 days. Generally, the desiccant saturates with water fairly quickly so extended testing times are not possible. The area of permeance is calculated by taking the average of 5 measurements in both the width and length of exposed sample between the caulk. Multiplying the average width by the average length produces the area. Since the coating is applied to a substrate, it is difficult, if not impossible, to measure the dry film thickness.

Next up: Calculations and results

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