Long-term hygrothermal aging test results (70°C/95%RH, 1000 hours):
- PET (hydrolysis-resistant grade): flexural strength retention 85%, compressive strength 90%
- PVC (crosslinked closed-cell): flexural strength 78%, compressive strength 82%
- XPS (high density): flexural strength 70%, compressive strength 75% (but surface powdering occurs)
- PU (ordinary rigid foam): flexural strength 45%, compressive strength 50% (hydrolysis occurs)
Therefore, for high-humidity applications, hydrolysis-resistant PET or crosslinked PVC is recommended. If budget is limited, PU cores can be fully encapsulated. Data source: Composite Structures Durability Handbook.
