Water Absorption of Sandwich Core Materials: Test Methods & ASTM C272

What is the Water Absorption of Core Materials?

Water absorption testing determines the amount of water or moisture that a core material absorbs when immersed in water or exposed to humid environments over a defined period. Core materials — including structural foams (PVC, PET, PMI, SAN), honeycomb (aluminium, Nomex, PP), balsa wood, and syntactic foams — are used as the lightweight central layer in sandwich composite panels widely deployed in marine, aerospace, wind energy, and transportation structures.

Moisture ingress into core materials degrades their mechanical properties, increases the structural weight of the sandwich panel, and can lead to freeze-thaw damage, delamination, and galvanic corrosion in metal-core composites.

Why Water Absorption Testing Is Critical for Sandwich Structures

Sandwich composite panels derive their high specific stiffness and strength from the combination of stiff, strong face sheets and lightweight core materials. The core’s role is to separate the face sheets and carry shear loads between them. Any significant water absorption by the core:

• Adds dead weight to the structure without structural benefit
• Reduces core shear strength and stiffness (particularly for foam cores)
• Creates freeze-thaw damage cycles in cold climates as absorbed water expands during freezing
• Promotes delamination at the face sheet-to-core bond line through osmotic pressure
• Causes dimensional changes (swelling) that distort the panel geometry

Test Standards for Core Material Water Absorption

ASTM C272 — Water Absorption of Core Materials for Sandwich Constructions

ASTM C272 is the primary standard for water absorption testing of sandwich core materials. Specimens of defined dimensions are conditioned, weighed, completely immersed in distilled water at 23°C ± 1°C for a specified period (typically 24 hours, 2 days, 7 days, or as specified), then removed, surface water blotted, and reweighed. Water absorption is reported as:

• Percentage weight gain (mass of water absorbed / initial dry specimen mass × 100%)
• Water absorption per unit area (g/m²) — normalised for specimen thickness

ASTM D570 — Water Absorption of Plastics

For polymer foam core materials, ASTM D570 provides an alternative method with similar immersion conditions applicable to cellular plastic cores.

ISO 2896 — Rigid Cellular Plastics: Water Absorption

ISO 2896 provides an equivalent European standard for water absorption testing of rigid polymer foam materials used as core materials.

Core Materials and Their Typical Water Absorption

• Closed-cell PVC foam (Divinycell H): Very low water absorption (<1% by weight after 24h) due to closed-cell structure
• PET foam: Low absorption, good chemical resistance
• PMI foam (Rohacell): Very low absorption — excellent for aerospace applications
• Balsa wood: Higher water absorption — surface sealing is required for marine applications
• Nomex honeycomb: Low absorption, but node bonds can allow water ingress through cell walls
• Open-cell foams: High water absorption — generally unsuitable for structural sandwich applications

Long-Term Water Absorption Testing

Short-term 24-hour immersion tests identify gross water resistance differences. For critical structural applications, long-term immersion tests (28 days, 90 days, 1 year) and elevated temperature conditioning are used to characterise the equilibrium water absorption and any progressive degradation. ASTM C481 defines the environmental conditioning of sandwich constructions for long-term durability assessment.

Industrial Applications

In marine composite structures — boat hulls, offshore platforms, bulkheads — core water absorption directly determines the long-term structural performance after decades of seawater immersion. In aerospace sandwich panels, core water absorption from condensation in aircraft structure must remain negligible to avoid weight gain and freeze damage at altitude. In wind turbine blades, core moisture ingress in the leading edge and trailing edge bondlines is a primary driver of blade repair and replacement.

Conclusion

Water absorption is a critical durability parameter for core materials used in sandwich composite structures. Moisture ingress can significantly reduce core shear strength, increase structural weight, promote freeze-thaw damage, and weaken the bond line between face sheets and the core. Accurate testing is therefore essential to ensure long-term performance in demanding service environments.

Standards such as ASTM C272, ASTM D570, and ISO 2896 provide standardised methods to evaluate the water resistance of foam, honeycomb, and wood-based core materials. These test results are widely used for material selection, design validation, and quality control in marine, aerospace, wind energy, and transportation applications.

Why Choose Infinita Lab for Core Material Water Absorption Testing?

Infinita Lab provides ASTM C272, ASTM D570, and ISO 2896 water absorption testing for all sandwich core materials through our nationwide accredited composites and materials testing laboratory network, with both short-term and long-term immersion test capabilities.

Looking for a trusted partner to achieve your research goals? Schedule a meeting with us, send us a request, or call us at (888) 878-3090 to learn more about our services and how we can support you.

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