PNNL-26070 PNNL-26070 Prepared for the U.S. Department of Homeland Security Domestic Nuclear Detection Office under U.S. Department of Energy Contract DE-AC05-76RL01830 Water Vapor Permeation in Plastics Paul E. Keller Richard Kouzes January 2017 Revision 1 DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor Battelle Memorial Institute, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. 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Box 62, Oak Ridge, TN 37831-0062; ph: (865) 576-8401 fax: (865) 576-5728 email: [email protected] Available to the public from the National Technical Information Service, U.S. Department of Commerce, 5285 Port Royal Rd., Springfield, VA 22161 ph: (800) 553-6847 fax: (703) 605-6900 email: [email protected] online ordering: http://www.ntis.gov/ordering.htm This document was printed on recycled paper. (9/2003) PNNL-26070 Water Vapor Permeation in Plastics Paul E. Keller Richard Kouzes January 2017 Revision 1 Prepared for the U.S. Department of Energy under U.S. Department of Energy Contract DE-AC05-76RL01830 Pacific Northwest National Laboratory Richland, Washington 99352 Executive Summary Polyvinyl toluene (PVT) and polystyrene (PS) (referred to as “plastic scintillator”) are used for gamma ray detectors. A significant change in radiation detection performance has been observed in some plastic scintillator-based gamma-ray detectors in systems in outdoor environments as they age. Recent studies have revealed that plastic scintillator can undergo an environmentally related temporary or permanent material degradation that adversely affects gamma ray detection performance under certain conditions and histories. The degradation of plastic scintillator over time is due to a variety of factors, and the term “aging” is used to encompass all factors. Some plastic scintillator samples show no aging effects (no significant change in sensitivity over more than 10 years), while others show severe aging (significant change in less than 5 years). Aging effects arise from weather (variations in heat and humidity), chemical exposure, mechanical stress, light exposure, and loss of volatile components. The damage produced by these various causes can be cumulative, causing observable damage to increase over time. Damage may be reversible up to some point, but becomes permanent under some conditions. The objective of this report is to document the phenomenon of permeability of plastic scintillator to water vapor and to derive the relationship between time, temperature, humidity and degree of water penetration in plastic. An overview of diffusion and the properties of materials is also presented. Several conclusions are documented about the properties of water permeability of plastic scintillator. Page iv of vii Acronyms and Abbreviations AAT Accelerated Aging Test C Concentration D diffusion coefficient HDPE high-density polyethylene J diffusion flux LDPE low-density polyethylene MVTR moisture vapor transmission rate PNNL Pacific Northwest National Laboratory P permeability PS polystyrene PVT polyvinyl toluene S solubility STP standard temperature and pressure (exactly 273.15 K and 100 kPa) VTR vapor transmission rate WVTR water vapor transmission rate Page v of vii Contents Executive Summary ..................................................................................................................................... iv Acronyms and Abbreviations ........................................................................................................................v 1. Introduction ............................................................................................................................................1 1.1 Unit Conversions and Consistency ..................................................................................................2 1.2 Polymers ...........................................................................................................................................2 2. Fickian Diffusion ....................................................................................................................................3 3. Permeability ............................................................................................................................................7 3.1 Gas Permeability .................................................................................................................................7 3.2 Water Vapor Permeability ...................................................................................................................7 4. Moisture (Water) Vapor Transmission Rate ..........................................................................................9 5. Permeation of Water into Thick Plastic ................................................................................................13 6. MVTR for PVT ....................................................................................................................................16 6.1 MVTR Measurement .....................................................................................................................16 6.2 Constants Derived From MVTR Measurement .............................................................................17 6.3 Saturation Time ..............................................................................................................................18 7. Humidity and Equilibrium ....................................................................................................................19 8. Diffusion in Thin Plastic ......................................................................................................................21 9. Mitigation Approaches .........................................................................................................................23 10. Conclusions ........................................................................................................................................24 11. References ..........................................................................................................................................25 A. Appendix – Properties of Plastics ........................................................................................................28 Figures Figure 1.1. Internal fogging of PVT (left) after rapid aging compared to unexposed PVT (right). .............1 Figure 2.1. Change in concentration of a liquid, gas, or vapor diffusing through a material of thickness (x1 – x0) in the steady state. .........................................................................................................................3 Figure 2.2. Plot of molecular weight versus kinetic diameter for various permeant gases and vapors. ........6 Figure 3.1. Relative degrees of permeation from [Bhadha 1999]. .................................................................7 Figure 5.1. Non-steady state for diffusion from the left and right boundaries into the center of the material [Karimi 2011]. .....................................................................................................................................14 Figure 5.2. Concentration as a function of time. ..........................................................................................15 Page vi of vii Figure 6.1. Thin sample of PVT provided by Saint Gobain. .......................................................................16 Figure 6.2. WVTR results, with trendline. ...................................................................................................17 Tables Table 2.1. Diffusion coefficients for water and oxygen for various polymers from the literature. ...............4 Table 2.2. Kinetic diameter of various permeant molecules .........................................................................5 Table 3.1. Conversion between volume permeability units commonly used with gases at STP. ..................8 Table 4.1. Conversion between various gas transmission rates (left side) and water vapor transmission rights (right side). ................................................................................................................................10 Table 4.2. Permeability coefficient (P0) of oxygen and water vapor for various polymers from the literature. .............................................................................................................................................10 Table 4.3. Some