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QUV/uvc Part IV: What Exposure Conditions Can Be Recommended for UVC Testing?

Posted by Sean Fowler on Mar 9, 2021 11:12:26 AM

The first three parts in this series discussed what the QUV/uvc is and why it was developed, outlined two published UVC exposure protocols, and provided background information for developing UVC exposure protocols. In this final post in the series, we’ll discuss a proposed QUV/uvc test cycle and ideas to standardize UVC exposures.

Controlled exposure conditions suitable to run in the QUV/uvc or other chambers are proposed as follows:
Irradiance:  6 mW/cm2 (60 W/m2) at 254 nm
Black Panel Temperature:  30 °C

This irradiance is chosen because it is a moderate level achieved by the QUV/uvc, and in the range of most low-pressure mercury lamps. Black panel thermometers are defined in ASTM G151 and ISO 4892-1, and the setpoint is chosen to approximate the highest expected service temperature of materials exposed to most ultraviolet germicidal irradiation (UVGI) protocols.

A test conducted at these conditions simulates one year of daily 1 J/cm2 UVGI cycles in about 17 hours. With this as a baseline exposure, it is simple to recalculate the test duration for any regular UVGI regimen. For example, the BIFMA standard discussed in Part 2 of this series prescribes and exposure that simulates weekly UVGI cycles performed over a product’s seven-year service lifetime. According to that standard, a UVGI cycle is 0.036 J/cm2, but it is easy to imagine this assumption changing given pandemic-era considerations. The 17-hour exposure referenced here could represent a revised seven-year simulation with 1 J/cm2 as the baseline UVGI dose.

Another consideration for exposures modeled on the BIFMA standard is that portable UVGI devices are being used multiple times per day on some items, such as patient chairs in medical or dental practices. Even if a single UVGI cycle from these portable units is less than 1 J/cm2, a product could experience much higher doses on a daily basis, given the high patient turnover in these facilities.

The IEC 60335-1 Annex T exposure, also discussed in Part 2, represents roughly 10 years of daily UVGI cycles. Although the test is intended for electrical insulation materials, it is unclear precisely what typical use this test is designed to simulate. The test temperature of 63 °C black panel is much hotter than experienced by materials in most UVGI systems. There is no information in Annex T to indicate the rationale for the temperature setting, but it appears to be copied from the referenced weathering standard, ISO 4892-2, for exposures of plastics in xenon arc test chambers. This may not be appropriate for the intended application. Xenon arc lamps operate at much higher power levels and produce significant radiant heat, unlike low-pressure mercury lamps for UVC exposures. In ISO 4892-2, a 63 °C black panel temperature is associated with a chamber air temperature of 38 °C. The difference between the black panel and chamber air temperatures is based on radiant heating of surfaces when exposed to xenon arc lamps, but this will not occur to the same degree when exposed to UVC lamps.

Another way of looking at this problem is to note that the irradiance setpoint in ISO 4892-2 is 60 W/m2 from 300-400 nm, and the recommended irradiance here is also 60 W/m2, concentrated at 254 nm. A xenon arc lamp produces a total irradiance (250-3000 nm) of over 1000 W/m2 to achieve the set point, while the UVC lamp total irradiance will be less than 70 W/m2 since about 90% of the lamp output is concentrated at 254 nm. For materials exposed to UVC lamps to be heated to 63 °C, the ambient air in that application would need to be at least 55 °C. Even if this is hotter than typical use, the standard covers safety, and of course it is better to err on the side of harshness in such cases.

Given the lack of UVC exposure standards, someone had to be first, so the writers of the BIFMA and IEC documents should be commended for taking on this task. The BIFMA standard is based on very sound rationale and provides a good template to work from. The IEC method recognizes that some materials may be exposed to quite high levels of UVC radiation and sets the exposure accordingly.  Given what has been learned and published over the last year, we are in a good position to develop new UVC exposure methods for a broad array of applications or improve on these initial standardized tests.

This concludes the series on the QUV/uvc, the latest member of the QUV accelerated weathering tester family. We will provide updates on standardization efforts, case studies, products, or any other developments in UVC exposure testing in this space and elsewhere on our website or at public events. Thanks for reading, and don’t hesitate to contact us with questions or to let us know about new applications for UVC exposures.

A Series: Testing Durability of Materials Exposed to UVC Light
Read Part I.
Read Part II.
Read Part III.
You are reading Part IV.

Topics: Weathering, QUV