Accelerated Weathering Laboratory: Metrology and Technology Transfer

Objective
By 2027, validate and implement the NIST Accelerated Weathering Laboratory (AWL) technology for conducting accurate and traceable aging experiments. By 2028, transfer the SPHERE technology to industrial stakeholders.

Technical Idea
A NIST Accelerated Weathering Laboratory (AWL) has been developed to deliver the best in the world, NIST traceable weathering measurement technology to stakeholders. Current commercial technology for accelerated weathering includes design and engineering innovations to improve irradiance light exposure uniformity and intensity, airflow, temperature, and relative humidity control [1-3]. Although commercial weathering devices are dominated by xenon arc lamp systems and traditional mercury lamps, the integrating sphere technology used in NIST weathering devices provides the highest uniformity and intensity of UV irradiation and the electrode-less microwave-energized mercury arc lamp provides more stability.  Furthermore, the SPHERE has novel environmental chambers that have precise control of temperature and relative humidity. Some NIST environmental chambers also offer mechanical stress in tension or compression.

The AWL can be used to conduct accelerated weathering experiments on material systems used in construction and other infrastructure applications, support advances in weathering metrology capabilities, facilitate the transfer of UV integrating sphere exposure technology, enable service life prediction model validation and promote advances in standards and codes. The AWL is composed of the NIST 2m SPHERE, the 0.5 m sphere 6-port device, the 0.5 m strain SPHERE, and hygrothermal test chambers (no UV component). 

To meet the needs of researchers using the AWL (both NIST staff and external collaborators), a focus is placed on (1) development of a commercially viable version of NIST SPHERE - the 0.5 m (6-port) SPHERE and (2) developing validation protocols to confirm the 0.5 m (6-port) SPHERE performance against the current 2m SPHERE. The 2m SPHERE has been operating for more 20 years and is heralded [3] as a benchmark for the SPHERE technology such that its well-documented environmental conditions (temperature, UV, and relative humidity) data will be used to validate any new SPHERE devices. Currently, the design and construction of the first prototype 6-port SPHERE with 6-enviromental chambers are completed and operation conditions are under testing to compare the performance of 2m SPHERE. A vendor has been contracted to build six environmental chambers using NIST designs and expected to deliver by July 2024.  With success in installation and operation of the commercially viable (2^nd) 6-port SPHERE, NIST can demonstrate the possibility of a turn-key 6-port SPHERE working with Labsphere and the chamber contract vendor. Additionally, an extensive SPHERE irradiance calibration using a commercially spectral radiometer system will be completed to maintain accurate irradiance values for any weathering device and provide a commercial irradiance measurement system. 

Research Plan
In FY25, this project will focus to achieve operation of a NIST-traceable weathering device, the 6-Port SPHERE system for the development of complementary standard test methods to improve accuracy of accelerated aging for our stakeholders.  The project products will provide specifiers (architects, engineers, inspectors, and builders) the knowledge needed to select materials for new construction and estimate of the remaining service life of materials already in place for a most resilient structure.  The research plan consists of the following tasks:

• Engage industry partners to advance and transfer measurement science to stakeholders in an effective, timely manner. FY25 encouragement will include working with scientific peers (academia, consultants, workshop/conference attendees), industrial peers (ex. former consortium members, testing labs), standards organizations, end users (specifiers-architects, building engineers, inspectors) to generate awareness and demand for the NIST SPHERE scientific products. In addition to attending standards organizations (ASTM), PI will work with American Coatings Association (ACA) to organize a Service Life Prediction symposium at 2025 CoatingsTech to showcase and recruit early adopters of the new 6-port SPHERE.
• Achieve full operation of the prototype (1^st) 6-port SPHERE and complete testing and validate the capabilities of environmental chambers of the commercially viable (2^nd) 6-port SPHERE. In FY25, the chamber control system (hardware, electronics, air flow, water supply, and controlling software/sensors) of the 1^st 6-port will be fully debugged and expected to run robustly. In the meantime, 6 environmental chambers delivered by the contractor for the 2^nd 6-port SPHERE will be tested and validated with the capabilities (operation limit of temperature/relative humidity).
• Continue to establish an absolute UV-vis Spectra Irradiance Measurement System for measuring the irradiance of the SPHERE devices.   Infrastructure Materials Group (IMG) owns and operates a NIST-calibrated spectroradiometer (CAS 140C) with an irradiance probe for automating the irradiance measurements of the existing 2m SPHERE and provides NIST with accurate NIST-calibrated irradiance data. Due to a higher UV intensity of 6-Port (0.5m) SPHERE, the current CAS140C cannot accurately measure the irradiance (the intensity of the 6-Port SPHERE is 4 times higher than the 2m SPHERE). IMG acquired a CDS-2600 UV-vis spectrometer from Labsphere, Inc. in 2017 for measuring the higher irradiance of the 0.5 m SPHERE. This spectrometer can measure the higher spectra irradiance but provides insufficient irradiance data below 280nm. Furthermore, it is difficult to attain accurate calibration files (including stay light correction) from the system to integrate into the IMG-established automated irradiance measurement program.  In FY 2024, a new spectroradiometer (CAS140D-157U2B) with compatible API (application programming interface) was purchased. Upon the arrival of the new spectrometer, we will configure and incorporate the existing SPHERE irradiance measurement program into the new spectroradiometer and compare the irradiance measurement results of the 2m SPHERE wall and one port with the results from the existing NIST radiometer. Action items for FY 25: (i) Improving the SPHERE irradiance calibration process for route and manual acquisition, (ii) Streamlining the SPHERE irradiance measurement for USERs. 
• Continue to validate the performance of 6-port SPHERE to 2m SPHERE, including the effect of lower UV wavelength (< 300 nm) on the reciprocity law.  Benchmarking the 6-port SPHERE system will also be accomplished by degrading a well-characterized polymer system and validating against results from the 2m SPHERE. The materials selection will also include (not limited to) several polymer systems containing UV stabilizing components, commercial products (PVC siding), and multi-layer photovoltaic back sheet systems. Compare data with known uncertainties at various UV doses, UV exposure conditions from 6-Port, 2 m SPHERE, outdoor, and other weathering results to demonstrate the UV exposure results from 6-Port are reliable, repeatable, and can be used for service life prediction (SLP) for shorter accelerating period.

References

1. Buyers Guide: Accelerated Weathering.” Coatings World Magazine. Online. Accessed June 11, 2020;  https://www.coatingsworld.com/buyersguide/laboratory-equipment-/accelerated-weathering/
2. Wu “Highly Accelerated UV Weathering: When and How to Use it”, in Chapter 6, “Service Life Prediction of Polymers and Plastics Exposed to Outdoor Weathering,” Dec 4, 2017. Elsevier, (2017)
3. “Laboratory Apparatus and Testing Equipment.” JCT Coatings Tech., November-December 2013; Atlas Material Testing Technology LLC (Chicago, Illinois); Q-Lab Corporation (Westlake, OH)
4. “Ultraviolet Chambers Based on Integrating SPHERES for Use in Artificial Weathering,” J. W. Chin, E. Byrd, N. Embree, J. Martin, J.D. Tate, J. Coatings Tech., 2002, 74(929), 39.