The NIST Accelerated Weathering Laboratory provides researchers with well-quantified accelerated-aging environments, using a unique 2m SPHERE (Simulated Photodegradation via High Energy Radiant Exposure) device. The 2m SPHERE enables NIST to evaluate engineered infrastructural materials, which perform critical functions in infrastructure systems, such as structural support (composites), corrosion protection of structural elements (coatings), weatherproofing (roofing, siding) or sealing the building envelope. These materials experience changes in their properties when exposed to outdoor weathering, such as water, UV, elevated temperatures, thermal cycling, and mechanical fatigue. These property changes generally result in lower performance of the materials than assumed when the materials were selected and initially installed. The SPHERE can expose these materials to prescribed spectra and intensity of UV, under controlled temperature, relative humidity and strain. The development of reliable performance data requires accelerated weathering exposure equipment capable of conducting controlled, repeatable, and precise accelerated aging experiments. These experiments are essential to developing performance models for service life and durability. Developing measurement science tools for service life prediction, such as validation studies, material degradation databases, and advances in weathering and durability standards and codes are essential for producing innovative materials for resilient infrastructure. Currently, NIST/Industry efforts have been successful in accurately measuring and predicting the durability of materials exposed to outdoor weathering based upon scientific evidence using 2m NIST SPHERE. The long-term goal is to transfer SPHERE technology to industry - in support of this goal, a commercially viable, smaller 6-port SPHERE has been constructed and will be introduced to potential industrial adoptees.
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 (2nd) 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:
References
FY18 and FY19: For the NIST Accelerated Weathering Lab, accomplishments centered around the 2m SPHERE operation improvements and the installations of the small SPHERE devices. For SPHERE maintenance: formalized maintenance schedule for 2m SPHERE consumable components; upgraded to 2m SPHERE strain chambers to independent operation (previously daisy-chained operation); and documented, using CAD drawings, the components of the SPHERE devices and ducting/hardware for the facilities control systems in the laboratories for the 2m SPHERE and smaller SPHERE devices. For small SPHERE installation: completed the ducting for the smaller SPHERE devices; upgraded positioner for irradiance measurements for SPHERE devices using linear encoders for faster and more accurate position location; procured a commercial radiometer instrument, modified specifically for SPHERE high intensity lights; developed a plan for a SLP database to contain SPHERE operation and materials characterization data; and developed a SPHERE operation web-based program. Dow Chemical purchased a strain SPHERE from LabSphere, our partner in creating the SPHERE Devices. For the Accelerated Weathering of Engineering Polymeric Systems project: Measurement science tools, including traceable measurements, database, validated statistical model were completed on a variety of industrially relevant polymeric materials. A database of property changes (damage) vs. UV irradiance and exposure conditions was compiled for the legacy, model epoxy amine system (glassy, cross-linked polymer), polyethylene (thermoplastic polymers, and poly(ethylene terephthalate). Another polyester system was also studied in 2018. Mechanical properties for all polymer systems were used for the initial modeling and validation exercise. The chemical properties were also examined to better explain the progress in degradation.
FY20: For the 6-Port SPHERE Operation task, an experimental plan was executed to define capabilities and standard operating procedure for the commercial (LabSphere) radiometer and compare to NIST radiometer system. Two steps of the plan were completed: a positioner was fabricated for radiometer fiber assembly and software for operation and a standard operating procedure (SOP) was drafted for commercial radiometer. The irradiance SOP will be refined for the EL Budget Milestone. COVID19 has delayed the completion of the report on the irradiance results of commercial versus NIST radiometers as a set of irradiance measurements were scheduled for the end of March. The greater ambient operating temperature (50C versus 30C) of the 6-Port SPHERE was verified with a series of temperature measurements at sample surface and methods to mitigate sample surface temperature in 6-Port SPHERE, potentially via a cooling system or a modified sample holder arrangement has been investigated and will be summarized in a report. Standardized protocols for most laboratory accelerated experiments require 30C temperatures. The installation of one prototype environmental chamber on 6-Port SPHERE was completed. This included the hardware/electrical/electronics and software, the water system for humidity generation, and the safety control system. COVID19 delayed the completion of the operation parameters testing. A report will be completed to document the environmental chamber components and operation. A report describing the experimental plan, including irradiance measurements and well-characterized polymer system (epoxy amine) degradation experimental results, for the benchmarking the operation of 6-Port SPHERE compared to 2m SPHERE was completed. The preparation procedure was modified to successfully prepare the epoxy amine specimens for the exposure experiment. The reciprocity experiment with epoxy amine on 6-Port SPHERE, using 30C, dry conditions and 4 different UV levels and similar conditions on the 2m SPHERE was started. COVID19 has delayed the completion of the experiment.
For the SPHERE Engagement task, a white paper is in process on the SPHERE device impact to drive NIST research in accelerated aging. The paper will include a review of all weathering standards from a variety of SDOs and committees (ASTM, IEC, IEEE, UL) working with IMG staff and their current standard engagements, identify stakeholders whose applications provide impact on SPHERE device, examine overlap with Resilience Goal including Disaster and Failure programs, and solicit input from 4 defined communities (Scientific, Industrial, Standards Organizations (above already), and End Users). COVID19 has delayed a symposium on Service Life Prediction and other means to solicit input from the four communities. Plans for a survey will be developed for approval to be executed next year.
For the SPHERE Data Validation task, the results for the performance properties from PE, PET, and the polyester system from the former accelerated weathering of polymers project were compiled into a dataset for model validation. The work will be published in a peer reviewed journal. A real industrial relevant polymer system, a similar PE system with UV absorbers was selected. A design of experiment was drafted to include 2m SPHERE, 6-Port SPHERE (with key UV exposure parameters, time and chamber constraints) and outdoor exposure components to implement experimental design for fast service life prediction. COVID19 has delayed the completion of the data compilation and model validation.