Materials and Processes for Foam Filling in PE Buoys
The shell breaks. Foam is floating on the water, yet the buoy may be losing flotation. What has gone wrong?
One user reported foam scattered across the water after a collision damaged a buoy, making cleanup difficult. Filling has two connected jobs: provide buoyancy when the shell floods, and remain sufficiently intact inside the float. The names EPS and PU do not establish either outcome.
Floating foam must still support the buoy
An intact, sealed float displaces water and provides buoyancy. When a breach admits water, part of the original air-filled cavity loses that role. Foam remaining inside can still exclude water and contribute residual buoyancy: flotation available after damage. [1]
Water uptake, crushing and escape of filling reduce available flotation. Beads drifting away may float on their own, but no longer support their original buoy. Continued flotation depends on whether enough effective volume remains to support the float, equipment, ballast and other loads.
EPS performance depends on a reliably formed core
EPS means expanded polystyrene. Beads are normally pre-expanded, conditioned and fused using steam in a block or shape mould. Their outlines remain visible in the finished core. Seeing beads does not establish that the filling is loose. [1][2]
Unfused beads remain separate, with water-accessible spaces between them. Without reliable restraint, they can escape through a sufficiently large breach. A formed core reduces this release of free beads, but poor fusion, compression or tearing can still cause fragmentation. Density, fusion quality, fit and restraint matter.
Low weight and economy are advantages of EPS. Sound formed cores can provide flotation. Loose beads, casually placed offcuts and reliably moulded cores are different engineering arrangements. [1][6]
Loose EPS beads
Beads remain separate
Formed EPS
Beads fused into a core
Marine closed-cell PU
Expanded in place as a core
PU density changes the engineering tradeoff
The material discussed here is marine closed-cell rigid polyurethane foam, distinct from soft sponge and an elastomer skin. Density in kg/m³ describes the mass of a cubic metre of cured foam, not the liquid components or PE shell.
Within comparable formulations and cell structures, higher density generally increases compression resistance, while adding weight. Consider a hypothetical 1 m³ dry core: increasing density from 32 to 64 kg/m³ adds 32 kg. At the same displaced volume, that reduces the mass-equivalent flotation available for other parts by 32 kg. Higher density therefore involves a tradeoff.
A published marine PU data sheet illustrates the difference. The two grades below have the same listed closed-cell content, while the denser grade has a higher compression value. “PU” is not one fixed performance level; density is not established as the sole cause of the difference. [3]
Data from one published material series [3]
| Cured foam density | Compression parallel to rise | Closed-cell content |
|---|---|---|
| 2 lb/ft³ ≈ 32.0 kg/m³ | 31 psi ≈ 214 kPa | 92–96% |
| 6 lb/ft³ ≈ 96.1 kg/m³ | 80 psi ≈ 552 kPa | 92–96% |
Compression: ASTM D1621. The source specifies proper mixing and curing under optimal environmental conditions. These are material examples, not recommended densities, minimum buoy requirements or collision results.
Cell closure, water uptake and strength are separate
Closed-cell content describes how much of the microscopic cellular structure is sealed. Density describes weight per volume. Compression data describes resistance under a particular test. None substitutes for the others, and higher compression resistance alone does not establish resistance to fragmentation during impact or tearing.
Closed cells limit water uptake; they do not mean permanent zero absorption. Formulation, cell size, walls, cut surfaces and damage affect ingress. In one water-blown PU study, decreasing density accompanied larger cells, thinner walls and increased absorption. That finding does not establish a universal density threshold for all PU. [1][4]
Absorption figures also need a time and exposure condition. ASTM D2842 normally uses 96 hours of immersion under a specified water head. Its guidance notes that some foams take up more water over longer periods or at higher pressure. A short test value cannot simply be read as years of zero absorption at sea. [5]
The same material name can produce different cores
PU can be preformed and installed, or mixed from two components and expanded and cured inside the float. Expansion in place can follow cavity geometry without using loose beads to fill the space. The component ratio, mixing, liquid and cavity temperatures, charge quantity and cure all affect the result. Poor mixing or unsuitable conditions may produce insufficient expansion, shrinkage or incomplete local cure. [3][6]
Free-rise density should be distinguished from actual density after constrained moulding; published marine material data lists these separately. [8] Following the cavity shape does not automatically establish a strong bond to PE. Polyethylene is a difficult-to-bond, low-surface-energy substrate. Core retention still needs reliable structural restraint or a validated bonding system. [7]
Damage performance belongs to the complete buoy
The shell’s impact resistance and ageing condition, core restraint and the load paths for lifting and mooring also matter. Foam cannot be assumed to carry every structural load merely because it fills the float. Effective installed volume and distribution need to work with equipment, ballast and stability. [1][6]
Separate sealed modules may limit the flotation lost when one is damaged. An enlarging breach or fragmentation and escape of the core can reduce residual buoyancy further. Filling-hole seals also affect water ingress in normal service. Remaining afloat does not automatically mean adequate freeboard, stability or normal AtoN performance. [1]
The conclusion is effective flotation after damage
- EPS can be suitable: Use sound formation, appropriate density and reliable restraint. Unrestrained loose beads are particularly vulnerable to escape through a breach.
- PU needs a defined grade: Marine closed-cell PU is an option when an in-place core and reduced free-bead release are priorities. Density, weight, strength and absorption still need balancing.
- Finished construction determines the result: Sufficient effective foam volume, sound manufacture, core retention and shell protection must work together after damage.
References and scope
Sources checked on 14 October 2026. Conversions: 1 lb/ft³ = 16.01846 kg/m³; 1 psi = 6.89476 kPa. The 32 and 64 kg/m³ mass comparison is a hypothetical illustration of the principle.
[2] BASF Styropor P 26 Technical Data Sheet
[3] TotalBoat Flotation Foam Instruction Sheet and TDS
[4] Thirumal et al. Effect of foam density on the properties of water blown rigid polyurethane foam
[5] ASTM D2842-25 Water Absorption of Rigid Cellular Plastics
[6] U.S. Coast Guard BoatBuilder’s Handbook Flotation Section
[7] 3M Science of Adhesion Categorizing Surface Energy
[8] Carpenter Polyurethane Systems for Marine Flotation
uestion is how much effective flotation remains after local shell damage, and whether the core stays sufficiently intact inside the float. That is more informative than whether the specification says EPS or PU.
