The versatile world of PU foam
The Wood Guide · Materials
MaterialPU foam
From the filling of your seat cushion to the insulation in your roof and the can of expanding foam in the cellar: it is always the same polymer, foamed in a different way.
- Polyol + isocyanate
- Soft to rock-hard
- Outdoors: polyester wins
PU foam
From the filling of your seat cushion to the insulation in your roof and the can of expanding foam in the cellar: it is always the same polymer, foamed in a different way.
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One reaction, a thousand forms
PU foam is formed by the reaction between polyols and isocyanates. By controlling that reaction, you get anything from down-soft to rock-hard.
The chemistry was discovered in 1937, and what is remarkable about it is the enormous range. The same basic reaction produces a soft seat cushion, a rigid insulation board and a hard structural element — the difference lies in the raw materials used, the blowing agents and whether the cells are open or closed.
Open cells let air through: that gives a soft, breathable foam for mattresses and cushions. Closed cells trap air: that gives a rigid, insulating foam that also keeps moisture out — the reason insulation boards are made from it.
In the context of garden furniture, that difference matters most. A soft seat cushion with PU foam is lovely to sit on indoors, but outdoors it holds water and becomes damp from the inside. For that, a polyester filling is the better choice.
Open cells soak up water like a sponge and release it slowly. A cushion with PU foam that gets caught in a shower stays damp for days and starts to go mouldy. For outdoor cushions there is special drainage foam with continuous channels, but ordinary furniture foam belongs indoors.
- Basis
- reaction of polyols with isocyanates
- Discovered
- 1937
- Open cell
- soft and breathable — cushions, mattresses
- Closed cell
- rigid and insulating — insulation boards
- Expanding foam
- cures with moisture from the air
- Outdoors
- ordinary furniture foam holds water
- Alternative outdoors
- polyester filling or drainage foam
To be fair: PU foam is a plastic made from crude oil and it is difficult to recycle, because it is chemically cross-linked and cannot simply be remelted. What does happen is reuse as flakes in, for example, carpet underlay. And expanding foam is the least innocent product in this family while you apply it: once cured it is stable, but you do not want to breathe in the fumes while spraying.
Which foam for what
| Species | Cell structure | What for |
|---|---|---|
| Soft furniture foam | open | seat and back cushions, mattresses |
| Cold-cure foam | open, finer cells | higher-quality mattresses; more resilient |
| Viscoelastic foam (memory foam) | open, temperature-sensitive | mattresses and pillows that follow your shape |
| Rigid foam | closed | insulation boards for roof, floor and walls |
| Expanding foam | closed, expanding | sealing gaps around windows and pipe penetrations |
| Drainage foam | open with channels | outdoor cushions that water needs to run through |
From liquid to foam
- MixingPolyol and isocyanate are brought together, along with catalysts, blowing agents and additives that control the properties.
- Reacting and risingThe reaction starts, gas is formed, and the mixture expands to many times its volume.
- CuringThe cell structure sets. From this moment on, the character of the foam is fixed for good.
- Cutting or mouldingBlock foam is cut to size for cushions and mattresses; moulded foam is poured straight into a mould.
Expanding foam from a can cures with the moisture in the air. That is why you dampen the surface before spraying, and why a thick layer cures slowly and unevenly in a dry environment — hard on the outside, still sticky inside.
The environmental side
From crude oil
The raw materials come largely from crude oil. Work is being done on bio-based polyols, but they are still a niche application.
Difficult to recycle
PU is chemically cross-linked and cannot be remelted. Reuse happens mainly as flakes in carpet underlay and filling material.
Insulation saves energy
The flip side: rigid foam insulates excellently, and the energy a well-insulated roof saves more than outweighs its production over its lifetime.
Using it longer helps most
A cushion you use for ten years beats any replacement round. A washable cover extends the life of the filling considerably.
Frequently asked questions about PU foam
What is PU foam?
Polyurethane foam: a plastic formed by the reaction between polyols and isocyanates. By controlling that reaction you get anything from soft cushion foam to rock-hard insulation boards.
Can PU foam be used outdoors?
Ordinary furniture foam cannot: open cells soak up water and stay damp for days. For outdoor cushions there is drainage foam with continuous channels, or you choose a polyester filling.
What is the difference between open and closed cells?
Open cells let air and moisture through, which gives a soft, breathable foam. Closed cells trap air, which produces a rigid, insulating foam.
Why do I need to dampen when using expanding foam?
Because it cures with the moisture in the air. Without moisture, a thick layer cures slowly and unevenly: hard on the outside, still sticky inside.
Is PU foam recyclable?
With difficulty. It is chemically cross-linked and cannot be remelted. What does happen is reuse as flakes in carpet underlay and filling material.
Which is better for an outdoor cushion: foam or polyester?
Polyester. The fibre absorbs virtually no water and dries quickly, whereas foam holds water and starts to go mouldy from the inside.
More about cushions and materials
What goes into an outdoor cushion and why it matters.
Have a question about your project? Just ask — we're happy to help.
Photos via Wikimedia Commons: Soft PU foam 2 cm thick with fine open cells — Johan, CC BY-SA 3.0 · PU foam close up: on the left with cell walls, on the right fully open — Silverchemist, CC0