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ASC: Sphere tells all

Author: Lou Reade


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Accurate service life prediction of polymeric materials, including adhesives and sealants, will soon be a step closer, says Christopher White of NIST

Predicting the lifetime of anything – be it a human being, a coat of paint or a polymeric sealant – is notoriously difficult, as there are so many factors to take into account.

“It’s really difficult to determine how things last,” says Christopher White, a research chemist at the National Institute for Standards and Technology (NIST), and ­acting group leader of the polymeric materials group. “Much of it is done by real time ­exposure: you put it in the weather and 
you wait.”

 Sphere – a commercial version is close

Copyright: Earl Zubkoff

Of course, accelerated testing is widespread and its results form the basis of many warranties and lifetime guarantees. However, White says there is still no real correlation between laboratory tests and the real world.

“All we have is a series of industry consensus threshold standards,” he says. “They are our best guess at durability. We need to develop the science to link our experiments with the exposures we predict in the field.”

NIST is helping to bring this about. Later this year, it expects to introduce a commercial version of Sphere, its exposure and testing system that can control and monitor the four most critical parameters of outdoor exposure: temperature, relative humidity, ultraviolet (UV) radiation and mechanical deformation.

“It’s the first device that allows you to do very precise exposures with independent control of all four of these properties,” he says. With it, White says service life prediction will become far more accurate.

Sphere, which stands for Simulated Photodegradation via High Energy Radiant Exposure, is not new. It has been used within NIST for around a decade, but this is the first time it will be available to everybody.

“It’s no good if just a government agency has this,” says White.

NIST is “taking a lot of the risk” to develop the commercial version of Sphere and is being supported by commercial companies, he says. NIST will soon be evaluating the first “commercial” version of Sphere, prior to launch.

“Service life prediction is the Holy Grail that people worry about; we’ve actually demonstrated that it’s possible,” says White.

White and several NIST colleagues have recently published technical papers explaining the basis behind Sphere testing. In an article published in the Review of Scientific Instruments, they detailed how Sphere was used in the accelerated testing of building joint sealants.

Test specimens are held in place. Measured force is applied to the sample by the movement of automatically controlled stepper ­motors. The samples can also be exposed to carefully controlled levels of heat, UV light and moisture. Simultaneous control of temperature and relative humidity is difficult: because humidity relies strongly on temperature, the thermal stability is the first condition to fix. Once this has been achieved, control of relative humidity can begin.

The system will be used to test polymeric substances – meaning that plastics, adhesives and sealants will be among the first materials to be characterised. NIST says that US homeowners spend $60bn each year repairing their homes. Some of this will be caused by the failure of sealants and other polymers.

“Plastics and polymers have very useful properties: elasticity, adhesion, paintability. All of these are affected by weathering and this will happen much faster than it would for concrete, steel or glass,” he says.

Sphere helps to determine the change in modulus (mechanical performance) according to a wide range of exposure conditions – from hot and dry to cold and wet, and everything in between.

This experimental data is then used to create a statistical model, which can predict the physical effect of specific conditions on a material. This can even be linked with real weather data to create accurate predictions for real products “in the field”.

“You compare your lab prediction with the one that you get outside. Once you can make them match, you have a validated predictive model,” he says, adding that it gives an answer to the question of “How long will it last”.

White says that Sphere will help to speed up the approvals process, but it must do this without compromising safety.

“I’m grateful that the people responsible for our safety are very cautious. Convincing them to do something new is meant to be difficult,” he says.

One of the issues to resolve is the “chicken question” – an egg left out will go bad, he says, while one that is kept warm underneath a hen turns into a chick.

“Same egg, different rates of exposure, different outcomes,” he says. “We need to be sure that, by accelerating a test, we do not change the mechanism.”