PIR and PUR

Polyurethane (abbreviated PUR) is a family of plastic polymers formed by a chemical reaction between two liquid base components:

1. A polyol — an alcohol with several hydroxyl groups (-OH)
2. An isocyanate — a chemical with reactive (-N=C=O) groups, typically MDI (methylene diphenyl diisocyanate) or TDI (toluene diisocyanate)

When the two components are mixed, the -OH group reacts with the -N=C=O group and forms a urethane bond (-NH-CO-O-). The reaction is exothermic (releases heat), it occurs over seconds to minutes, and the result is a solid plastic polymer. It is this urethane bond that has given the whole family its name.

The interesting thing is that you can control the process and thereby the product properties by adjusting:

• The ratio between polyol and isocyanate
• The type of polyol (short chain vs. long chain → soft vs. rigid)
• Addition of a blowing agent (blowing gas) → produces a porous foam
• Catalysts → control the reaction rate and the bond type
• Additives such as flame retardants, UV stabilisers, pigments

The result is a remarkably flexible family of materials. The same base chemistry can yield:

• Rigid foam (refrigerator, DH pipes, roof insulation for building boards)
• Soft foam (sofa cushions, mattresses, car seats)
• Adhesive and sealant (PUR adhesive, shoe-sole adhesive)
• Paint and varnish (PUR varnish for wood and metal)
• Elastomers (skate wheels, conveyor belts, sealing rings)
• Fibres and textiles (Spandex / Lycra)

In technical insulation only the rigid, closed-cell foam form is used — it is the stiff, yellow or off-white foam you see in a cut-through DH pipe section.

PUR was discovered in Germany in 1937 by the chemist Otto Bayer (1902-1982) at IG Farben in Leverkusen — the company that later became Bayer AG. The discovery was originally an alternative to American nylon (Du Pont's recently patented polyamide fibre). The Bayer team arrived at diisocyanate polyaddition as an entirely new polymerisation method, and that opened the door to a whole family of materials.

Industrial production of PUR foam did not really begin until the 1950s. It was in the 1960s-70s that PUR foam truly broke through as an insulation material — first in refrigerators, then in district heating pipes and building structures. In the 1970s-80s PIR (polyisocyanurate) was developed as a more fire-safe variant for the same application, and this shift coincided with stricter fire requirements in northern European building regulations.

The difference between PUR and PIR is not a new base material, but an adjustment of the reaction recipe:

• PUR: roughly equal molar ratio of isocyanate and polyol → urethane bonds dominate → "ordinary" polyurethane foam
• PIR: large excess of isocyanate (typically 1.5-3 times the polyol amount) + a catalyst that promotes the isocyanate-isocyanate reaction → some of the isocyanates react with one another in a trimerisation and form isocyanurate rings — a dense six-membered ring structure with three nitrogen atoms

The trimerisation is the key. The isocyanurate ring is thermally and chemically much more stable than the linear urethane bond. This is due to several things:

1. Aromatic-like stability — the ring structure is energetically favourable, just like the benzene ring
2. Higher nitrogen content — nitrogen-rich polymers burn less readily than a pure hydrocarbon chain
3. More cross-linked network — harder to melt and deform

Concrete consequences:

On the λ value the difference is small — the cell structure and the blowing gas are largely the same. PIR typically lies 1-3 hundredths lower than PUR, but not by much. The really big difference lies in the fire and temperature range.

Rule of thumb: PUR for concealed and low-temperature applications (typically DH in the ground). PIR where fire is a real risk or the temperature rises above 100 °C.

The low lambda value in PUR/PIR is not because the polymer material itself is particularly good at insulating — the polymers' "own" thermal conductivity is actually around 0.20-0.25 W/(m·K), i.e. 10 times worse than the finished foam. The secret is the structure.

A piece of rigid PUR foam consists of billions of microscopic closed cells — typically 0.1-0.5 mm in diameter — separated by thin polymer walls. Each cell contains a gas-filled bubble. When heat is to be transported through the foam, it happens via four mechanisms:

1. Conduction through the polymer walls (~30-40 % of total)
2. Conduction through the gas (~40-50 % of total)
3. Convection within the cells (almost 0 — the cells are too small for the gas to circulate)
4. Radiation between cell walls (~10-20 %)

It is the blowing gas that makes the difference. Whereas air's lambda is about 0.026 W/(m·K), fluorocarbons (the old CFCs) sit at 0.008-0.010, and cyclopentane — the modern replacement — at 0.012-0.015 W/(m·K). A cell filled with cyclopentane thus conducts 40-50 % less heat than a cell filled with ordinary air.

This is why you get an overall lambda down to 0.022 W/(m·K) for fresh PIR sections — it is a combination of closed cells, a dense polymer net and a heavy gas with low thermal conductivity.

For comparison: mineral wool normally has λ ~0.033-0.040. PUR/PIR thus insulates about 20-35 % better per millimetre — the one area where PIR/PUR beats mineral wool on physical performance.

Here it becomes professionally important. The blowing gas in the cells does not remain in the cells forever. It diffuses out slowly through the polymer walls, and at the same time the N₂ and O₂ from the air diffuse in. It is an irreversible process — over time the cells' contents change from "pure cyclopentane" to "a mixture with an increasing air fraction".

The consequence: lambda rises.

The diffusion rate depends on:

• The thickness and density of the cell walls — good production quality → slower ageing
• Protective barrier — aluminium foil is near diffusion-tight and slows ageing drastically
• Temperature — heat accelerates diffusion
• Foam thickness — thicker foam means a longer diffusion path for the gas

Manufacturers are obliged to state a declared λ ("declared" or "aged" λ) — a value corresponding to what can be expected after about 25 years under simulated operating conditions. The measurement method is specified in EN ISO 8497 (for pipe sections) and EN 253 (specifically for DH pipes).

KEY PROFESSIONAL RULE: In sizing you must ALWAYS use the declared, aged λ. Never the fresh initial value that is often shown in marketing material. If you calculate with 0.022 W/(m·K), and 25 years later the product sits at 0.028, you have underestimated the heat loss by ~27 % — and the customer gets a heating bill that is much higher than expected.

For Logstor DH pipes with diffusion-tight alu foil, ageing can be so slow that λ rises only a few percent over 30 years [logstor.com]. For a simple PUR section without a barrier, ageing can be significant within 10-15 years.