25 Years of Data: Long-term Aging of Polyurethane Foam Used for Radioactive Materials Transportation Packages
Transportation packages for radioactive material containers must be made from durable materials to meet regulatory performance requirements, ensuring public health and safety during both normal and accident conditions. General Plastics’ LAST-A-FOAM® FR-3700 series rigid foam has been used for nearly 55 years to protect radioactive material containers by mitigating damage from impact and fire events.
To further understand the long-term durability of LAST-A-FOAM®, General Plastics has conducted ongoing studies. This paper presents data from samples manufactured 25 years ago as part of a longterm aging study initiated in December 1999. The study also compares LAST-A-FOAM® with multiple wood species exposed to the elements.
LONG-TERM AGING OF LAST-A-FOAM® FR-3700 SERIES
Both wood and polyurethane foams are used in the impact limiters and overpacks of radioactive material packages. These transportation packages are typically certified for decades of use, and it is critical that materials maintain their physical properties over the normal life span of the package. This study reviews the material aging characteristics LAST-A-FOAM® and various wood species and their fitness for use in long-term use applications.
In December 1999, General Plastics began an ongoing study of LAST-A-FOAM® and various wood species exposed to outdoor elements. This document summarizes recent results after 25 years of exposure.
Experimental Conditions:
Exposed Storage:
Samples were stored unprotected in a subterranean concrete vault with an open grate, exposing them year-round to Pacific Northwest weather, dust, dirt, insects, and plant debris (Figures 1, 2 & 3). After 25 years, the specimens were moved indoors, lightly brushed to remove debris, then conditioned at 70 ± 5 °F and 50 ± 5% RH for at least 40 hours before testing. Wood samples showed severe decay from fungal and insect attack; none remained suitable for cutting test specimens. Foam samples were cut from the 1.0″ thickness “as-is” aside from cleaning.
Figure 1 – Storage location and configuration
Figure #2- Exposed Wood Samples & Exposed Foam Samples
Figure #3 – Exposed Wood Samples
Figure #4 – Exposed Foam Samples
Data:
Foam specimens were tested parallel to the rise direction. Density was measured per ASTM D1622, compressive strength per ASTM D1621 using a Shimadzu AG-X Plus 100 kN universal tester. Five specimens were tested for each material and condition. Results (Table 1) show density and compressive strength remained within typical product variation, with no indication of degradation due to age or exposure.
LAST-A-FOAM® Test Samples and Testing Results :
Foam Density and Strength over 25 years:
Discussion:
These storage conditions subjected samples to far harsher environments than foam would typically see inside an essentially hermetically sealed impact limiter.
None of the LAST-A-FOAM® material showed any significant decrease in density or compression strength after 20 years. The FR-3704 appears to strength after 2004, but we guess the test apparatus was modernized and is the cause of the apparent shift upward in strength. The last 20 years have yield relatively stable strength in the FR-3704. The higher testing loads of the FR-3718 appear to have been less sensitive to test apparatus issues and have shown to be very stable over the last 25 years.
The exposed LAST-A-FOAM® products showed the expected yellowing, typical of polyurethane products due to UV exposure. The condition was limited to the surface only. The 21-year-old cut specimens revealed virgin material as one would see with a newly produced material. Any moss growth found on the LAST-A-FOAM® material was supported by the underlying organic matter of leaves, dust and dirt, and not the foam material. The foam condition was consistent with independent fungal tests previously conducted that showed LAST-A-FOAM® products do not support fungal growth. (Ref. ASTM G21 and MILSTD-810G1 Method 508.7)
Documented in the 20 year update (copies available upon request) the wood samples showed various instability in both density and compression strengths after 20 years of exposure. As expected and described in the 20 year study, non of the wood sample were in suitable condition to test.
As evident in the graphed performances, the foam exhibits exceptional resistance to harsh environmental and biological exposure and shows superior consistency in performance when compared to wood. While some darkening and soiling of the LAST-A-FOAM® exterior surfaces may occur, the closed-cell nature of the material ensures the environmental impacts do not encroach beyond the exposed surfaces. All foam samples exhibited virgin material when examined below the surface, and no negative effect on mechanical performance.
Conclusion:
In applications demanding long-term material stability, LAST-A-FOAM® has proven durable under both protected and harsh exposed conditions. Unlike wood, it does not rot or lose mechanical integrity, ensuring safe transport and storage of radioactive materials throughout the package’s certified life.