UV chamber tests outdoor sealants

When exposed to sunlight, things fall apart—paint fades, plastic pales and weakens, and cloth goes to pieces.

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When exposed to sunlight, things fall apart—paint fades, plastic pales and weakens, and cloth goes to pieces. The culprit is UV light, abetted by humidity and temperature extremes. Developing more-weather-resistant materials requires testing, which can be as easy as setting a specimen out in the sun and leaving it there for a couple of years. But weather does not stand still, even for researchers; ironically, testing for weather resistance is best done indoors.

A large (2-m-diameter) integrating sphere at the National Institute of Standards and technology (NIST; Gaithersburg, MD) has served since 2002 as a source of uniform high-intensity UV illumination, along with controllable temperature and humidity. The source, called simply the SPHERE (simulated photodegradation by high-energy radiant exposure), has 32 ports for experiments. Now, engineers at Labsphere (North Sutton, NH) have delivered to NIST a setup for one of these ports for testing sealants used in the building industry. The Labsphere device is a smaller integrating sphere that can simultaneously test eight two-sided samples (see figure). The samples are held in mounts that continually stretch and relax the sealant samples as they are bathed in heat and radiation. (Labsphere also built the integrating-sphere portion of SPHERE.)

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A sealant-test chamber is formed by an integrating sphere that opens in a clamshell style (left). Under test, eight mounted sealant samples are subjected to repetitive stretching as they are bombarded with heat, humidity, and UV radiation. The test chamber is mounted to the NIST SPHERE (right, test chamber seen at center), which provides the artificial outdoor conditions.
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Before the SPHERE existed, materials were typically tested in 26-month outdoor exposures, usually in Florida or Arizona, according to Chris White, a NIST researcher. In the SPHERE test chambers, cone concentrators channel light directly to test chambers for 2-D samples, or into integrating spheres for 3-D experimental setups within which the samples are suspended (such as the Labsphere setup). The SPHERE is powered by six microwave UV lamps, which generate 8400 W of UV radiation filtered to remove any nonterrestrial radiation (less than 295 nm) with quartz filters, as well as IR and much of the visible with dichroic filters. In the test chambers, sample surfaces receive about 20 to 25 "suns" of UV light, says White. The temperature and humidity in the sample areas is controlled to better than 0.1°C and 1% relative humidity (RH) from 25°C to 60°C and 1% to 95% RH.

In their 12-in.-diameter test sphere, the Labsphere engineers had to illuminate the eight samples efficiently and uniformly. The sphere's inner surface is made of Spectralon, a thermoplastic resin stable to 350°C and with a reflectance of greater than 95% from 250 to 2500 nm. The sealant samples—each held by two rods for stretching—are mounted in a circular array, an arrangement arrived at after extensive irradiance calculation. The coefficient of variation in UV flux for all the samples was experimentally determined to be 16%.

The completed Labsphere chamber is now in operation at NIST. Along with the sealants being tested in the chamber, other test chambers mounted to the SPHERE are testing or have tested fibers for bullet-resistant vests, coatings, fire-protective coatings, and bulk plastics containing fire-protective materials.

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