Accelerated Weathering Laboratory: Metrology and Technology Transfer

Summary

The NIST Accelerated Weathering Laboratory provides researchers with well-quantified accelerated aging environments, using a unique SPHERE (Simulated Photodegradation via High Energy Radiant Exposure) device. NIST has been studying engineered polymeric 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 (sealants).  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 result in lower performance of the materials than assumed when the materials were selected and initially installed.  The SPHERE can expose these materials to UV, under controlled temperature, relative humidity and strain.  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 new durable materials for resilient infrastructure. This technology has reached sufficient maturity with the possibility of being transferred to industry. ​

Description

Objective - To maintain, improve and expand capabilities at the NIST Accelerated Weathering Laboratory (AWL) to conduct safe, accurate and traceable aging experiments, develop materials degradation databases, and to transfer the SPHERE technology to industrial stakeholders.

What is the new technical idea?  

A NIST Accelerated 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. [“Buyers Guide: Accelerated Weathering.” Coatings World Magazine. Online. Accessed June 11, 2020;  https://www.coatingsworld.com/buyersguide/laboratory-equipment-/accelerated-weathering/; 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); “Laboratory Apparatus and Testing Equipment.” JCT Coatings Tech., November-December 2013; Atlas Material Testing Technology LLC (Chicago, Illinois); Q-Lab Corporation (Westlake, OH)]   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.[“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]  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 0.25 m sphere 6-port device, the 0.25 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 improving operational capabilities of these accelerated weathering devices.  The 2m SPHERE has been operating for over 15 years and is heralded 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. New data acquisition and access methods will be developed to improve the measurement and safety systems in the AWL.  Extensive SPHERE irradiance calibration using a new spectral radiometer system will be completed to maintain accurate irradiance values for any weathering device and provide a commercial irradiance measurement system. Statistical methods for uncertainty analysis and computational modeling will be used to systematically examine and improve the accuracy and control of all devices. Current NIST methodologies used for generating weathering data will be developed into standardized test methods and utilized to provide publicly available reference data for weathering experiments. Technology transfer consists of developing the capability of two smaller, economical SPHERE devices: 1) the 6-Port Multi-port Uniform Ultraviolet Solar Irradiance Chamber (MUUSIC) or SPHERE system and 2) the NIST strain SPHERE.  Both systems represent design improvements in SPHERE technology and support inter-laboratory testing of available weathering devices.  The project will focus on developing validation protocols to confirm these two new devices performance against the current 2m SPHERE.

What is the research plan? 

In FY18 and FY19, the research plan was divided into three goals: 1) maintain and improve operation of the NIST 2m SPHERE to support current research and to achieve 90 % operation time, 2) attain full operation of the 6-Port SPHERE system complete with functioning environmental chambers, and 3) attain full operation of NIST strain SPHERE. 

The AWL continued to improve operations by upgrading control systems and implementing a maintenance logging program for the NIST 2m SPHERE and hygrothermal chambers so that internal and external projects can efficiently continue. The SPHERE needs to operate at temperatures and relative humidities (RH) of up to 85 °C and 60 % RH, respectively. It was found that elements in these devices prematurely fail under these harsh conditions. Thus, solutions were developed to ensure reliability of all elements under all conditions.  A validated safety system for the AWL was also developed.  The safety system is a series of electronic interlocks to protect the facilities, samples, and users from over-temperature, chamber water overflow, and loss of experimental control.

Metrology and technical support for benchmarking new SPHERE devices were started for the 6-Port SPHERE system and the NIST strain SPHERE.  New instrumentation, data acquisition systems, and test fixtures were developed and maintained, including installation and functional testing of the weathering devices and safety systems.  Benchmarking and validation of the 6-Port SPHERE system by accurate irradiance measurements and comparing reciprocity data with the NIST 2m SPHERE at room temperature and dry conditions began.  This data is critical in validating the operation of these devices for industrial stakeholders.  Furthermore, a management system for capture and sharing of environmental conditions (temperature and relative humidity (RH)) and irradiance data was verified. Improved environmental chambers for 6-port SPHERE system began to be assembled and validated. A new system to control the temperature and RH for the strain SPHERE was added and validated.  

Finally, a database package for environmental (temperature, relative humidity and ultraviolet) data was developed for internal and eventually for future public use. In addition, a program for real-time sample irradiance data using the environmental database was designed. An inter-laboratory test program of current weathering devices will be planned for a well-defined construction material.

In FY20, this Accelerated Weathering Laboratory project and the Accelerated Weathering of Engineered Polymeric Materials project were reorganized due to several changes for the groups in the EMRI program and re-focused 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 project was organized into three tasks: 6-Port SPHERE Operation, SPHERE Engagement, and SPHERE Data Validation. 

For the 6-Port SPHERE Operation task, the objective is to complete and test a turnkey 6-Port SPHERE device.  The environmental chambers need to be finished; including hardware, electronics, and controlling software, and overcome challenges with the chamber control system as a 2m SPHERE control system cannot easily be replicated due to discontinued parts. Once the 6-Port SPHERE system is assembled, the performance of the new SPHERE system needs to be understood via a comparison to the legacy 2m SPHERE using UV irradiance, temperature, RH, and airflow device performance measurements.  Benchmarking the new SPHERE system will also be accomplished by degrading a well-characterized polymer system and validating against results from the 2m SPHERE. The 6-Port SPHERE capabilities will also be transferred in relation to current weathering devices, on which NIST stakeholders have decades of materials data.  Results from several polymer systems (including those that contain UV stabilizing components) will be linked for the 6 Port SPHERE, the 2m SPHERE, outdoor weathering, and potentially other commercial weathering devices- legacy data.

The focus of the SPHERE Engagement task is to generate awareness and demand for the NIST SPHERE scientific products.  Scientific peers (academia, consultants, workshop/conference attendees), industrial peers (e.g. consortium members, testing labs), standards organizations, end users (specifiers-architects, building engineers, inspectors) will be informed.  Opportunities to give invited talks and peer-reviewed papers to large audiences where we can drive the agenda will also be sought. This engagement will occur at multiple levels with each goal described below:

• Scientific:  To gain widespread acceptance of the technical approach and results as a foundation. 
• Industrial:  To use the accepted technical approach to develop the capability to adopt our methods for existing and new products internally.
• Standards Organizations:  To implement the technical data and acceptance from our industrial peers to develop new standards to enable our technologies to be used in commerce.
• End Users.  To engage and inform the end users about the benefits of our new approach using scientifically based standards. 

SPHERE Data Validation task will investigate the change in performance properties from “real” industrial samples with exposure on the 6-Port SPHERE to convince stakeholders of the SPHERE device utility. In addition to 6-Port SPHERE operation (controlled UV exposure conditions); accurate UV irradiance measurements with known uncertainties, a sensible DOE (time & chamber constraints), precise measurements of materials properties (mechanical, optical, and chemical) during the UV exposure will be conducted.  The team will focus on:

• Sample selection:  industrially relevant, potentially with existing outdoor data (working with industrial partners) or industrial weathering data. For example, PE with additives (UV absorber to start), and PET with absorber, PPE (PV backsheet)- has outdoor and 2 m SPHERE data
• Exposure experiment: based on selected samples; conduct 2m SPHERE or outdoor exposure (DOE is crucial – results for validating and comparison); when 6-port is operational, conduct exposure experiments with corresponding key UV exposure parameters. 
• Comparison and benchmarking: compare data with known uncertainties at various UV doses, UV exposure conditions from 6-port, 2 m SPHERE, outdoor, 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.

ction material.

Major Accomplishments

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.